Consumer Homes.

Question:

Hi All, We are in the process of having a house designed and want to take advantage of passive solar gain.  It will be built on a south facing knoll near Ottawa, Ontario.  It will be slab-on-grade with an exposed cement floor (~ 6" thick with 2" insulation under) with radiant floor heating.  The house is ~1700 sq ft – 50′ by 34′ with a central loft area.  There is a green-house built on to the south east corner. Essentially, all the south side of the house is window -almost 500 sq ft.  We’ve had a solar assessment of the lot – it is pretty clear – very few obstructions.  The windows will have overhangs of about 3′. We hope to use triple glazed low-e windows. I am very naive about passive solar design.  I would like to get practical advice from people who have experience with passive solar heating. Do you think we have too much south facing window?  Will the cement slab be able to sink all the heat gained throughout the day? Any advice would be greatly appreciated. -Doug

Response:

A search of Altavista.com for "passive solar design" came up with" http://www.sundancesupply.com/SolarPrimer.html http://www.greenbuilder.com/sourcebook/PassiveSol.html among many other links. I am sure the people on this group can give you some practical information as well.

– Hide quoted text — Show quoted text -> Hi All, > We are in the process of having a house designed and want to take > advantage of passive solar gain.  It will be built on a south facing > knoll near Ottawa, Ontario.  It will be slab-on-grade with an exposed > cement floor (~ 6" thick with 2" insulation under) with radiant floor > heating.  The house is ~1700 sq ft – 50′ by 34′ with a central loft > area.  There is a green-house built on to the south east corner. > Essentially, all the south side of the house is window -almost 500 sq > ft.  We’ve had a solar assessment of the lot – it is pretty clear – > very few obstructions.  The windows will have overhangs of about 3′. > We hope to use triple glazed low-e windows. > I am very naive about passive solar design.  I would like to get > practical advice from people who have experience with passive solar > heating. > Do you think we have too much south facing window?  Will the cement > slab be able to sink all the heat gained throughout the day? > Any advice would be greatly appreciated. > -Doug

Response:

>We are in the process of having a house designed and want to take >advantage of passive solar gain.  It will be built on a south facing >knoll near Ottawa, Ontario.

The NRC Solarium Workbook indicates that December is the worst-case month for solar house heating in Ottowa, with 2257 Wh/m^2 (715 Btu/ft^2) of sun on a south wall on an average -8 C (17.6 F) day. >It will be slab-on-grade with an exposed cement floor (~ 6" thick with >2" insulation under) with radiant floor heating.  The house is ~1700 sq ft >- 50′ by 34′ with a central loft area.  There is a green-house built on >to the south east corner.

You might enclose plants in a smaller structure within the greenhouse and circulate warm dry air between the greenhouse and house during the day… The slab has about 1700×6/12×25 = 21.3K Btu/F of heat capacity, enough to store 213,000 Btu of overnight heat with a 10 F daily temp swing. You might use part of the roof as a trickle collector. >Essentially, all the south side of the house is window -almost 500 sq ft. >We’ve had a solar assessment of the lot – it is pretty clear – >very few obstructions… We hope to use triple glazed low-e windows.

How about making the house squarer, say 36′x48′, with US R48 12" SIPs with 0.1 ACH and A ft^2 of U0.2 fiberglass windows with 50% solar transmission the house thermal conductance G would be 1728ft^2/R48 = 36 Btu/h-F for the ceiling plus 0.2A for the windows plus (1344-A)/R48 for the walls plus about 0.1×1728x8/60 = 23 for 23 cfm of air leakage, ie G = 87+0.179A. With a 300 kWh/mo (34K Btu/day) electric bill, 0.5×715A+34K = 24h(65-17.6)G, so A = 422 ft^2, with 84 Btu/h-F (52%) of the 162 Btu/h-F house conductance as south windows collecting 151K Btu of sun on an average December day. >Do you think we have too much south facing window?

Maybe. They account for more than half of the house thermal conductance. Fewer windows would lower the need for backup heat on cloudy days. On an average day, the greenhouse (or a sunspace, or passive air heaters built into the south wall) could provide house heat for about 6 hours, making 0.5×715A+34K = 18h(65-17.6)G, so A = 196 ft^2. A square foot of greenhouse or sunspace or air heater R1 south glazing with 90% solar transmission might gain 644 Btu on an average day and lose about 6h(80-17.6)1ft^2/R1 374 Btu, for a net gain of 270 Btu (or maybe 400, with whitewashed stone on the ground to the south). G = 122 Btu/h-F means the house needs about 6h(65-17.6)G-6hx34K/24h = 26.2K Btu over 6 hours, which might come from 26.2K/270 = 97 ft^2 of greenhouse glazing. >Will the cement slab be able to sink all the heat gained throughout the day?

Seems likely, if the sun shines on the dark colored slab. Nick

Response:

Thanks for your response, Nick.  See below for concerns… >We are in the process of having a house designed and want to take >advantage of passive solar gain.  It will be built on a south facing >knoll near Ottawa, Ontario. > The slab has about 1700×6/12×25 = 21.3K Btu/F of heat capacity, enough > to store 213,000 Btu of overnight heat with a 10 F daily temp swing.

I’m wondering if all of the slab can be considered as the solar mass. From what I’ve read, only the mass that is exposed to the sun’s rays should be considered.  Of course, this doesn’t make perfect sense since cement will conduct the heat away from the sunny areas and into the recesses of the house.  I’m just wondering how effective this will be.  I was considering designing the radiant floor heating so that the pumps can run even when the heater is off.  That way the water in the pex tubing can distribute the heat throughout the slab.  Do you think this is a good idea?  I don’t think conventional radiant floor heating allows the pump to be turned on without heating the water so I’m not sure how complex this would be. There seems to be a fair amount of contradicting info out there which is why I’m asking for people with real experience to comment.  One thing I was wondering about is the amount of insulation I should put under the cement slab.  I’ve read that the insulation should only be from the perimeter walls in towards the centre but the centre of the slab should rest on the ground.  Perhaps this is the case for more southern climes – it’s -28 degrees C as I type.  Also the house will be sitting on an escarpment which seems to be solid rock so perhaps this connection to the ground isn’t such a good idea. – Hide quoted text — Show quoted text ->Essentially, all the south side of the house is window -almost 500 sq ft. >We’ve had a solar assessment of the lot – it is pretty clear – >very few obstructions… We hope to use triple glazed low-e windows. > How about making the house squarer, say 36′x48′, with US R48 12" SIPs with > 0.1 ACH and A ft^2 of U0.2 fiberglass windows with 50% solar transmission > the house thermal conductance G would be 1728ft^2/R48 = 36 Btu/h-F for the > ceiling plus 0.2A for the windows plus (1344-A)/R48 for the walls plus about > 0.1×1728x8/60 = 23 for 23 cfm of air leakage, ie G = 87+0.179A. wow… > With a 300 kWh/mo (34K Btu/day) electric bill, 0.5×715A+34K = 24h(65-17.6)G, > so A = 422 ft^2, with 84 Btu/h-F (52%) of the 162 Btu/h-F house conductance > as south windows collecting 151K Btu of sun on an average December day.

the trick would be to collect all that energy without raising the temperature too much.  In the winter time we can always open windows, I suppose.  In the summer time we’ll have to hope that the overhangs will be effective.  Perhaps the low-e windows can reflect the far infra red back outside. >Do you think we have too much south facing window? > Maybe. They account for more than half of the house thermal conductance. > Fewer windows would lower the need for backup heat on cloudy days. On an > average day, the greenhouse (or a sunspace, or passive air heaters built > into the south wall) could provide house heat for about 6 hours, making > 0.5×715A+34K = 18h(65-17.6)G, so A = 196 ft^2. A square foot of greenhouse > or sunspace or air heater R1 south glazing with 90% solar transmission > might gain 644 Btu on an average day and lose about 6h(80-17.6)1ft^2/R1 > 374 Btu, for a net gain of 270 Btu (or maybe 400, with whitewashed stone > on the ground to the south). G = 122 Btu/h-F means the house needs about > 6h(65-17.6)G-6hx34K/24h = 26.2K Btu over 6 hours, which might come from > 26.2K/270 = 97 ft^2 of greenhouse glazing.

In our design, the greenhouse accounts for about 140 sq ft of windows. >Will the cement slab be able to sink all the heat gained throughout the day? > Seems likely, if the sun shines on the dark colored slab.

thanks again, Nick -Doug

Response:

>> The slab has about 1700×6/12×25 = 21.3K Btu/F of heat capacity, enough > to store 213,000 Btu of overnight heat with a 10 F daily temp swing. >I’m wondering if all of the slab can be considered as the solar mass.

That’s a first approximation. >From what I’ve read, only the mass that is exposed to the sun’s rays >should be considered.  Of course, this doesn’t make perfect sense >since cement will conduct the heat away from the sunny areas and into >the recesses of the house.

k = 0.54 Btu/h-ft-F and C = 0.156 Btu/lb-F and rho = 144 lb/ft^3 make alpha = k/(CRho) = 0.024 ft^2/h for concrete, so a slice x feet into an infinite slab would change halfway from an initial temp to a new surface temp in time t = x^2/alpha = 41.6x^2… t = 6 hours makes x = 0.38′, ie 4.6"… t = 24h makes x = 0.79′, ie 9". >I’m just wondering how effective this will be.

You might just count the mass exposed to the sun… >I was considering designing the radiant floor heating so that the pumps >can run even when the heater is off.  That way the water in the pex >tubing can distribute the heat throughout the slab.  Do you think >this is a good idea?

Yes. The pump might not have to run full time. You might bury temp sensors all over the floor and run the pump until the floor temps are close to each other, or put a fast temp sensor in the loop output and run the pump for a minute every hour and note the water temp changes during that minute and run the pump more frequently if they are large, or put a few floor temp sensors near south windows and run the pump if any sense more than 80 F, or use a differential thermostat between the warmest and coldest room, or start the pump with a thermostat in the coldest room, or just run the pump on a timer. >…One thing I was wondering about is the amount of insulation I should put >under the cement slab.  I’ve read that the insulation should only be >from the perimeter walls in towards the centre but the centre of the >slab should rest on the ground…

I’d vote for insulation under the whole slab. Kreider and Rabl’s "Heating and Cooling of Buildings (McGraw Hill, 1994) has a good explanation of how to calculate heat loss from slabs. You might also look at HUD’s 1994 "Design Guide for Frost-Protected Shallow Foundations," which you can download from www.huduser.org/publications/destech/desguide.html as two Wordperfect files by clicking on the full-text executable link to the right. You can see a copy with drawings that do not print well at http://cs.arizona.edu/people/jcropper/desguide.html. >…it’s -28 degrees C as I type.

One simple ASHRAE method assumes a basement floor slab loses heat to the deep ground temp (about 41 F in Ottowa) through an R10 ground resistance. An 80 F with no insulation might lose (80-41)1700ft^2/R10 = 6630 Btu/h. R10 (2") Styrofoam might halve that. > How about making the house squarer, say 36′x48′, with US R48 12" SIPs with > 0.1 ACH and A ft^2 of U0.2 fiberglass windows with 50% solar transmission > the house thermal conductance G would be 1728ft^2/R48 = 36 Btu/h-F for the > ceiling plus 0.2A for the windows plus (1344-A)/R48 for the walls plus about > 0.1×1728x8/60 = 23 for 23 cfm of air leakage, ie G = 87+0.179A. >wow…

Simple, huh? > >Do you think we have too much south facing window? > Maybe. They account for more than half of the house thermal conductance. > Fewer windows would lower the need for backup heat on cloudy days. On an > average day, the greenhouse (or a sunspace, or passive air heaters built > into the south wall) could provide house heat for about 6 hours, making > 0.5×715A+34K = 18h(65-17.6)G, so A = 196 ft^2. A square foot of greenhouse > or sunspace or air heater R1 south glazing with 90% solar transmission > might gain 644 Btu on an average day and lose about 6h(80-17.6)1ft^2/R1 > 374 Btu, for a net gain of 270 Btu (or maybe 400, with whitewashed stone > on the ground to the south). G = 122 Btu/h-F means the house needs about > 6h(65-17.6)G-6hx34K/24h = 26.2K Btu over 6 hours, which might come from > 26.2K/270 = 97 ft^2 of greenhouse glazing. >In our design, the greenhouse accounts for about 140 sq ft of windows.

Good, especially if most of the greenhouse gets cold at night… Canadians bathe, right? You might make the roof a trickle collector… Nick

Response:

> >From what I’ve read, only the mass that is exposed to the sun’s rays >should be considered.  Of course, this doesn’t make perfect sense >since cement will conduct the heat away from the sunny areas and into >the recesses of the house. > k = 0.54 Btu/h-ft-F and C = 0.156 Btu/lb-F and rho = 144 lb/ft^3 make > alpha = k/(CRho) = 0.024 ft^2/h for concrete, so a slice x feet into an > infinite slab would change halfway from an initial temp to a new surface > temp in time t = x^2/alpha = 41.6x^2… t = 6 hours makes x = 0.38′, ie > 4.6"… t = 24h makes x = 0.79′, ie 9".

How am I to interpret the above? – Hide quoted text — Show quoted text ->I’m just wondering how effective this will be. > You might just count the mass exposed to the sun… >I was considering designing the radiant floor heating so that the pumps >can run even when the heater is off.  That way the water in the pex >tubing can distribute the heat throughout the slab.  Do you think >this is a good idea? > Yes. The pump might not have to run full time. You might bury temp sensors > all over the floor and run the pump until the floor temps are close to each > other, or put a fast temp sensor in the loop output and run the pump for > a minute every hour and note the water temp changes during that minute and > run the pump more frequently if they are large, or put a few floor temp > sensors near south windows and run the pump if any sense more than 80 F, > or use a differential thermostat between the warmest and coldest room, > or start the pump with a thermostat in the coldest room, or just run the > pump on a timer.

I’ll look into this.  Another poster suggested that at the least, i should consider having the insolated area on a different zone than the shady areas. >…One thing I was wondering about is the amount of insulation I should put >under the cement slab.  I’ve read that the insulation should only be >from the perimeter walls in towards the centre but the centre of the >slab should rest on the ground… > I’d vote for insulation under the whole slab. Kreider and Rabl’s "Heating > and Cooling of Buildings (McGraw Hill, 1994) has a good explanation of > how to calculate heat loss from slabs. You might also look at HUD’s 1994 > "Design Guide for Frost-Protected Shallow Foundations," which you can > download from www.huduser.org/publications/destech/desguide.html as two > Wordperfect files by clicking on the full-text executable link to the right.

I felt the same.  thanks for the pointers. > You can see a copy with drawings that do not print well at > http://cs.arizona.edu/people/jcropper/desguide.html. >…it’s -28 degrees C as I type. > One simple ASHRAE method assumes a basement floor slab loses heat to the > deep ground temp (about 41 F in Ottowa) through an R10 ground resistance. > An 80 F with no insulation might lose (80-41)1700ft^2/R10 = 6630 Btu/h. > R10 (2") Styrofoam might halve that. >In our design, the greenhouse accounts for about 140 sq ft of windows. > Good, especially if most of the greenhouse gets cold at night… > Canadians bathe, right? You might make the roof a trickle collector…

Once a year whether we need to or not. At this stage (sketches on paper, er, auto-cad), I wanted to ensure that we did not waste an opportunity to use passive solar heating within the design of the house.  I will ensure that the radiant floor heating system that we choose allows future alterations for solar heating as well.  But i want to keep things very simple.  Collect the lowest hanging fruit. -Doug – Hide quoted text — Show quoted text -> Nick

Response:

Although I see that Mr. Pine, who is far more knowledgeable than I, has replied there appears to be one item missed. You specified ‘low e’ windows which will cut the absorption. I had the pleasure of being connected with a company years ago that specialized in ’sunspace’ design.  In your application they would have probably suggested the low e windows but then put a single glazed "green house" the entire length of the south wall with dark colored tile on the floor to capture the heat.  A couple of small fans to move the warmed air around and you are in business. I have to agree with Nick about insulating the whole slab though some contractors may fight you on it.  I had it explained to me that I was wasting my money because once you get about 36" from the weather exposed edge of the slab it shouldn’t be an issue because of something they called ‘cave effect’.  They also stated that the ground will transmit minimal heat that far in just as caves rarely have huge temp. swings.  They did it MY way ’cause I was footing the bill!  I put four inches under the floor and around the sides of the foundation and six inches in the frame walls that sat on top of the (basement) foundation.  Extensive weather stripping, airlock entries, and some other common tricks gave me a house that used only 40% of the energy that other similar sized homes in the area. I like Nicks’ ideas of the sensors in the slab etc.  Very elegant!  The low tech answer would be to put a much smaller circulating pump in parallel with the main pump and a relay that would shut it down when the main pump is running.  This would assure the whole slab would stay about the same temp. BUT, if I understand the way radiant floor heating works, doesn’t the ‘heating unit’ turn the flame up and down on demand for more/less cooling? My house used gas/forced air so I am probably extremely ignorant on this issue. Good luck!!!!  I was once told that efficient and successful solar design begins with knowledge, is furthered by mega-insulation, and ends up feeding on sweat. Dutchman

– Hide quoted text — Show quoted text -> Hi All, > We are in the process of having a house designed and want to take > advantage of passive solar gain.  It will be built on a south facing > knoll near Ottawa, Ontario.  It will be slab-on-grade with an exposed > cement floor (~ 6" thick with 2" insulation under) with radiant floor > heating.  The house is ~1700 sq ft – 50′ by 34′ with a central loft > area.  There is a green-house built on to the south east corner. > Essentially, all the south side of the house is window -almost 500 sq > ft.  We’ve had a solar assessment of the lot – it is pretty clear – > very few obstructions.  The windows will have overhangs of about 3′. > We hope to use triple glazed low-e windows. > I am very naive about passive solar design.  I would like to get > practical advice from people who have experience with passive solar > heating. > Do you think we have too much south facing window?  Will the cement > slab be able to sink all the heat gained throughout the day? > Any advice would be greatly appreciated. > -Doug

Response:

>> >From what I’ve read, only the mass that is exposed to the sun’s rays > >should be considered… > k = 0.54 Btu/h-ft-F and C = 0.156 Btu/lb-F and rho = 144 lb/ft^3 make > alpha = k/(CRho) = 0.024 ft^2/h for concrete, so a slice x feet into an > infinite slab would change halfway from an initial temp to a new surface > temp in time t = x^2/alpha = 41.6x^2… t = 6 hours makes x = 0.38′, ie > 4.6"… t = 24h makes x = 0.79′, ie 9". >How am I to interpret the above?

If most of the slab is 60 F in the morning and the sun makes one part of the surface say, 80 F, you might find the temp 4.6" beneath 70 F after 6 hours. After 24 hours, you might find the temp 9" from the edge of the heated surface 70 F… 9" is not very far, compared to the size of the slab, so, unless you turn on the pump… > You might just count the mass exposed to the sun… > >I was considering designing the radiant floor heating so that the pumps > >can run even when the heater is off.  That way the water in the pex > >tubing can distribute the heat throughout the slab.  Do you think > >this is a good idea? > Yes. The pump might not have to run full time…

OTOH, it might… 400 ft^2 of windows in full sun with 50% solar transmission might put 50K Btu/h into part of the slab. A 10 gpm flow could move that heat to other parts of the slab with a 10 F temp diff. > Canadians bathe, right? You might make the roof a trickle collector… >Once a year whether we need to or not. >At this stage (sketches on paper, er, auto-cad), I wanted to ensure >that we did not waste an opportunity to use passive solar heating >within the design of the house.  I will ensure that the radiant floor >heating system that we choose allows future alterations for solar >heating as well.  But i want to keep things very simple.  Collect the >lowest hanging fruit.

A large dark colored steep-sloped south metal roof would make a future trickle collector easier. Trickle collectors and hydronic slabs and lots of insulation and airtightness and very few windows seem to go well together. The slab can provide overnight heat storage, and an unpressurized insulated box full of hotter water can store heat for a few cloudy days. A bare galvanized tank in the cloudy day store can provide pressurized hot water for showers. Nick

Response:

>…You specified ‘low e’ windows which will cut the absorption.

Perhaps you mean "reduce the heat loss." >I had the pleasure of being connected with a company years ago that >specialized in ’sunspace’ design.  In your application they would have >probably suggested the low e windows but then put a single glazed >"green house" the entire length of the south wall with dark colored >tile on the floor to capture the heat…

Adding thermal mass to a sunspace is a mistake promoted by the Passive Solar Industry Council and their brick and masonry sponsors. It increases cost and cripples performance by storing heat that is lost through the glazing overnight. This tends to make the sunspace lukewarm for 24 hours vs hot for 6 hours (so it can provide heat to the house) and cold for 18 (so it loses little heat to the outdoors.) Then again, it’s hard to store much overnight heat in a typical house, if the source is warm air from a sunspace. An extra layer of drywall or some thermal mass in a wall or ceiling can help. Nick

Response:

- Hide quoted text — Show quoted text ->> >From what I’ve read, only the mass that is exposed to the sun’s rays >> >should be considered… >> k = 0.54 Btu/h-ft-F and C = 0.156 Btu/lb-F and rho = 144 lb/ft^3 make >> alpha = k/(CRho) = 0.024 ft^2/h for concrete, so a slice x feet into an >> infinite slab would change halfway from an initial temp to a new surface >> temp in time t = x^2/alpha = 41.6x^2… t = 6 hours makes x = 0.38′, ie >> 4.6"… t = 24h makes x = 0.79′, ie 9". >How am I to interpret the above? > If most of the slab is 60 F in the morning and the sun makes one part of > the surface say, 80 F, you might find the temp 4.6" beneath 70 F after 6 > hours. After 24 hours, you might find the temp 9" from the edge of the > heated surface 70 F… 9" is not very far, compared to the size of the > slab, so, unless you turn on the pump…

This is very surprising.  I would have thought that cement would radiant heat outwards much faster.  I believe your calculations were predicated on an infinite depth slab, no?  Then if the slab is say 6" deep with 3" of insulation under, it would be reasonable to assume faster heat dispersal within the slab. >> You might just count the mass exposed to the sun… >> Yes. The pump might not have to run full time… > OTOH, it might… 400 ft^2 of windows in full sun with 50% solar transmission > might put 50K Btu/h into part of the slab. A 10 gpm flow could move that heat > to other parts of the slab with a 10 F temp diff.

The maximum length for a 7/8" pex tubing loop is 400′.  You can have many loops within a zone.  You need a single pump per zone. It’s an interesting problem to design a system with multiple loops and zones that will take care of heating and at the same time disperse the heat caused by sunshine.  I’ll have to think about this. >> Canadians bathe, right? You might make the roof a trickle collector… > A large dark colored steep-sloped south metal roof would make a future > trickle collector easier. Trickle collectors and hydronic slabs and > lots of insulation and airtightness and very few windows seem to go well > together. The slab can provide overnight heat storage, and an unpressurized > insulated box full of hotter water can store heat for a few cloudy days. > A bare galvanized tank in the cloudy day store can provide pressurized > hot water for showers.

I’ll ensure that our design won’t preclude this as a future enhancement. <<< i’ve copied the following from the other post within this thread…>>> – Hide quoted text — Show quoted text ->Adding thermal mass to a sunspace is a mistake promoted by the Passive >Solar Industry Council and their brick and masonry sponsors. It increases >cost and cripples performance by storing heat that is lost through the >glazing overnight. This tends to make the sunspace lukewarm for 24 hours >vs hot for 6 hours (so it can provide heat to the house) and cold for 18 >(so it loses little heat to the outdoors.) >Then again, it’s hard to store much overnight heat in a typical house, >if the source is warm air from a sunspace. An extra layer of drywall or >some thermal mass in a wall or ceiling can help.

Or a cement slab on grade? This is very interesting.  So it would be best to design a system that sucks the heat away from this area during the day and to store it in the slab within the house proper. Nick, I want to thankyou for this information.  It is very helpful. -Doug

Response:

– Hide quoted text — Show quoted text ->…You specified ‘low e’ windows which will cut the absorption. > Perhaps you mean "reduce the heat loss." >I had the pleasure of being connected with a company years ago that >specialized in ’sunspace’ design.  In your application they would have >probably suggested the low e windows but then put a single glazed >"green house" the entire length of the south wall with dark colored >tile on the floor to capture the heat… > Adding thermal mass to a sunspace is a mistake promoted by the Passive > Solar Industry Council and their brick and masonry sponsors. It increases > cost and cripples performance by storing heat that is lost through the > glazing overnight. This tends to make the sunspace lukewarm for 24 hours > vs hot for 6 hours (so it can provide heat to the house) and cold for 18 > (so it loses little heat to the outdoors.) > Then again, it’s hard to store much overnight heat in a typical house, > if the source is warm air from a sunspace. An extra layer of drywall or > some thermal mass in a wall or ceiling can help.

Or direct the warm air through a hollow block wall, as per Kachadorian.

Response:

>…One thing I was wondering about is > the amount of insulation I should put > under the cement slab.  I’ve read that > the insulation should only be from the > perimeter walls in towards the centre > but the centre of the slab should rest > on the ground… > I’d vote for insulation under the whole slab.

In the UK insulation has to under the whole slab.  I have just watched 2" of the stuff being put in before the re-bar was laid in a new build near me. You can loose heat to the ground in any part of the slab, not just the outer rim.

Response:

– Hide quoted text — Show quoted text ->> Then again, it’s hard to store much overnight heat in a typical house, >> if the source is warm air from a sunspace. An extra layer of drywall or >> some thermal mass in a wall or ceiling can help. >Or direct the warm air through a hollow block wall, as per Kachadorian. > No. He uses a fan to store heat in the floor. Only a very small fraction > of the wall’s surface is exposed to warm air. Storing heat in the wall > would work much better, IMO. Air could move vertically through a hollow > block wall or column with no fans, charging and discharging the heat > battery, the wall could support the roof, and the house could have > a basement. Chiras says: >    …some designers are concerned that moisture can build up in the >    passageways. The cool moist environment, they assert, could serve >    as a breeding ground for mold and mildew. Spores could become >    entrained in the air flowing through the system, contaminating >    indoor air and causing potentially serious health problems. Fans >    require a fair amount of energy, too. >    Concerned about problems such as these, passive solar designer and >    builder Bruce Brownell of Adirondack Alternative Energy in Edinburg, >    New York, circulates warm air through a slab, a 70- 100-ton mass >    storage system [a foot or two of dry sand, as I recall], under the >    lowest floor. [mostly woodstove heated] Warm air is delivered to >    the slab via air shafts that syphon heat from the ceilings [using >    fans.] The pipes are easier to clean and less prone to mold and >    mildew than the cement block labyrinth… > But the pipes have a lot less > heat transfer area,

How about earthenware pipes?  better heat transfer. > and the sand has > a large thermal resistance.

As it has been determined  But it may always be moist reducing the resistance.

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>> Then again, it’s hard to store much overnight heat in a typical house, > if the source is warm air from a sunspace. An extra layer of drywall or > some thermal mass in a wall or ceiling can help. >Or direct the warm air through a hollow block wall, as per Kachadorian.

No. He uses a fan to store heat in the floor. Only a very small fraction of the wall’s surface is exposed to warm air. Storing heat in the wall would work much better, IMO. Air could move vertically through a hollow block wall or column with no fans, charging and discharging the heat battery, the wall could support the roof, and the house could have a basement. Chiras says:    …some designers are concerned that moisture can build up in the    passageways. The cool moist environment, they assert, could serve    as a breeding ground for mold and mildew. Spores could become    entrained in the air flowing through the system, contaminating    indoor air and causing potentially serious health problems. Fans    require a fair amount of energy, too.    Concerned about problems such as these, passive solar designer and    builder Bruce Brownell of Adirondack Alternative Energy in Edinburg,    New York, circulates warm air through a slab, a 70- 100-ton mass    storage system [a foot or two of dry sand, as I recall], under the    lowest floor. [mostly woodstove heated] Warm air is delivered to    the slab via air shafts that syphon heat from the ceilings [using    fans.] The pipes are easier to clean and less prone to mold and    mildew than the cement block labyrinth… But the pipes have a lot less heat transfer area, and the sand has a large thermal resistance. Nick

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> The maximum length for a 7/8" pex > tubing loop is 400′.  You can have > many loops within a zone.  You need a > single pump per zone.  It’s an interesting > problem to design a system with multiple > loops and zones that will take care of heating > and at the same time disperse the > heat caused by sunshine.  I’ll have to think about this.

The control system may be quite complex.  It is knowing when to start distributing heat around the slab from hot to cool areas.  You don’t want to use purchased heat until all situations are exhausted. This web site will give you some ideas on solar hot water and heating systems.  There are completed research test cases and some nice diagrams too.  Well worth a study. http://www.iea-shc.org/ >Then again, it’s hard to store much > overnight heat in a typical house, > if the source is warm air from a sunspace. > An extra layer of drywall or > some thermal mass in a wall or ceiling can help. > Or a cement slab on grade? > This is very interesting.  So it would be > best to design a system that > sucks the heat away from this area during > the day and to store it in > the slab within the house proper.

This all sound like an air-core slab may be the answer.  Also use some dense concrete block internal walls for thermal mass. Another approach is to have 1-2" of sand above the ceiling. This is also "excellent" sound insulation if having two floors.  the joists and ceiling drywall have to be man enough to hold the weight of the sand.

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>Adding thermal mass to a sunspace is a mistake promoted by the Passive >Solar Industry Council and their brick and masonry sponsors. It increases >cost and cripples performance by storing heat that is lost through the >glazing overnight. This tends to make the sunspace lukewarm for 24 hours >vs hot for 6 hours (so it can provide heat to the house) and cold for 18 >(so it loses little heat to the outdoors.) >Nick

Nick,   Would you say that a solar sunspace is best utilized when the heat from the space is taken via ductwork from the space into the living areas as actively as possible during the best heating hours of the day? I have seen mention of the thermal mass in a sunspace being wasteful before. It would seem to me that actively dispersing the heat to the living area during the day, and closing off the sunpace at night would be the most effective use of the space, other than growing some nice plantlife.  Just my thoughts. M Russon

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>  Would you say that a solar sunspace is best utilized when the heat >from the space is taken via ductwork from the space into the living >areas as actively as possible during the best heating hours of the day?

Sure, altho a higher sunspace temp can make air flow passively thru plastic film one-way dampers over vents at the top and bottom with less fan power and less thermal efficiency. If A ft^2 of R1 south glazing with 90% solar transmission in full sun loses (100-30)A/R1 Btu/h in a 100 F sunspace with 30 F air outdoors, we need Av ft^2 of vents 16′ below Av ft^2 of upper vents, where 225A-(100-30)A = 16.6Avsqrt(16′)(100-70)^1.5, so Av = 0.014A, eg 7.3 ft^2 vents in a 32′x16′ sunspace. A mesh or an inner glazing near the north wall can collect higher temp heat while keeping a sunspace cool and more comfortable for people. >It would seem to me that actively dispersing the heat to the living area >during the day, and closing off the sunpace at night would be the most >effective use of the space, other than growing some nice plantlife.

The average Jan temp is 30 F where I live in Phila. Leaking a little warm house air into a sunspace to keep it from freezing on a cold night isn’t a large thermal penalty. Nick

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> A mesh or an inner glazing near the north wall can collect higher temp > heat while keeping a sunspace cool and more comfortable for people.

Fixing a sheet, or sheets of glass over the main house wall inside a sunspace:  This will heat up the house wall that is certain, especially if the wall is a dark colour.  Does the glass have to sealed?  Would just a sheet in a frame a few inches off the wall with the edges open do?

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>> A mesh or an inner glazing near the north wall can collect higher temp > heat while keeping a sunspace cool and more comfortable for people. >Fixing a sheet, or sheets of glass over the main house wall inside a >sunspace:  This will heat up the house wall that is certain, especially if >the wall is a dark colour.  Does the glass have to sealed?  Would just a >sheet in a frame a few inches off the wall with the edges open do?

I suppose that would work. With a dark mesh, house air might come into the bulk of the sunspace through a hole in the north house wall near the bottom and warm as it moves from south to north through the mesh and go back into the house through a larger hole near the top, north of the mesh. Nick

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- Hide quoted text — Show quoted text ->  Would you say that a solar sunspace is best utilized when the heat >from the space is taken via ductwork from the space into the living >areas as actively as possible during the best heating hours of the day? > Sure, altho a higher sunspace temp can make air flow passively thru plastic > film one-way dampers over vents at the top and bottom with less fan power > and less thermal efficiency. If A ft^2 of R1 south glazing with 90% solar > transmission in full sun loses (100-30)A/R1 Btu/h in a 100 F sunspace with > 30 F air outdoors, we need Av ft^2 of vents 16′ below Av ft^2 of upper vents, > where 225A-(100-30)A = 16.6Avsqrt(16′)(100-70)^1.5, so Av = 0.014A, eg 7.3 > ft^2 vents in a 32′x16′ sunspace. > A mesh or an inner glazing near the north wall can collect higher temp > heat while keeping a sunspace cool and more comfortable for people. >It would seem to me that actively dispersing the heat to the living area >during the day, and closing off the sunpace at night would be the most >effective use of the space, other than growing some nice plantlife. > The average Jan temp is 30 F where I live in Phila. Leaking a little warm > house air into a sunspace to keep it from freezing on a cold night isn’t > a large thermal penalty. > Nick

One idea was to have the greenhouse made in two sections – the south glass section would have its own cement slab thermally broken from the main house slab and this small slab would have its own radiant floor zone to keep the plants from freezing at night.  The northern section will have a lot of thermal mass especially in the back wall. A roll out insulated barrier would separate the two sections at night. During the day, both sections are quite open to the rest of the house. The other idea was to keep the greenhouse as one section and use roll down thermal blinds to insulate at night.  It seems easier to construct.  Plants would probably be happier too.

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>…greenhouse made in two sections – the south glass section would have >its own cement slab thermally broken from the main house slab and this >small slab would have its own radiant floor zone to keep the plants from >freezing at night.

This might work better if a) the outer section has greater temp extremes than the inner one, and b) the "slab" is vertical, since winter sun has a low elevation and the    plants might like to grow in the soil floor), and c) the mass wall contains water vs masonry, with 2-3X more specific    heat by volume and a higher thermal conductance. >The northern section will have a lot of thermal mass especially in >the back wall. A roll out insulated barrier would separate the two >sections at night. During the day, both sections are quite open to >the rest of the house.

Sounds complicated. How about making a box on part of the back wall with an R1 cover with 90% solar transmission and a few vertical 55 gallon drums inside? It might have a passive Thermofor temperature- sensitive vent or $11 Leslie-Locke automatic foundation vent at the top that opens when the air temp dips to 32 F (or slightly higher.) On an average January day in Phila, 1000 Btu/ft^2 of sun falls on a south wall. The 24-hour average temp is 30.4 F, with a 22.8 average min and 37.9 average max temp, ie the daily temp is a sine wave with an average value of 30.4 and a (37.9-22.8)/2 = 7.55 F peak amplitude: T = 30.4+7.55sin(2Pit/24) = 32 when t = sin^-1(32-30.4/7.55) = 0.82 h, with the argument in radians. The number of "freezing degree hours" is the integral (gasp) of 32-T from t1 = 12-0.82 = 11.18 to t2 = 24.82 h, ie (32-30.4)(t2-t1) – 2×7.55×24/(2Pi)cos(2Pix11.18/24) = 78.16 FDD. The difference between the 24-hour average temp and average daily min is 7.6 F. We might want our box to keep the greenhouse at 32 F on a colder night, say 22.8-7.6 = 15.2 F. With 16′x32′ of R1 glazing and a conductance of 16×32/R1 = 512 Btu/h-F, we need to supply 78.16×512 = 40K Btu on an average night. On a 15.2 F night, we need to supply heat at a peak  rate  of (32-15.2)512 = 8602 Btu/h. If a 2′ diameter x 3′ tall vertical 55 gallon drum sits behind 6 ft^2 of glazing, it will collect 0.9×0.9×1000x6 = 4860 Btu on an average day. If the sunspace is 80 F (no more, because any excess heat goes to heating the attached house) for 6 hours per day and 32 F for 13, the average daytime greenhouse temp is about 56 F. Drum water at temp T would lose about 10h(T-56)6ft^2/R1 = 60(T-56) Btu through the glazing during the day. 4860N = 60N(T-56)+40K makes T = 137 – 667/N. Each drum has about 25 ft^2 of surface with a 1.5 Btu/h-F conductance to slow-moving air. With free airflow, 8602 = (T-32)25×1.5N makes N = 8.5. We might stack 8 drums with T = 53.6 F and an average nightly temp swing of 40K/(8×55x8) = 11 F 4-wide and 2-high behind an 8′x8′ layer of glazing with 2′ of smaller water containers on top. Nick

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- Hide quoted text — Show quoted text -> This might work better if > a) the outer section has greater temp extremes than the inner one, and > b) the "slab" is vertical, since winter sun has a low elevation and the >    plants might like to grow in the soil floor), and > c) the mass wall contains water vs masonry, with 2-3X more specific >    heat by volume and a higher thermal conductance. >The northern section will have a lot of thermal mass especially in >the back wall. A roll out insulated barrier would separate the two >sections at night. During the day, both sections are quite open to >the rest of the house. > Sounds complicated. How about making a box on part of the back wall > with an R1 cover with 90% solar transmission and a few vertical 55 > gallon drums inside? It might have a passive Thermofor temperature- > sensitive vent or $11 Leslie-Locke automatic foundation vent at the > top that opens when the air temp dips to 32 F (or slightly higher.) > On an average January day in Phila, 1000 Btu/ft^2 of sun falls on a > south wall. The 24-hour average temp is 30.4 F, with a 22.8 average > min and 37.9 average max temp, ie the daily temp is a sine wave with > an average value of 30.4 and a (37.9-22.8)/2 = 7.55 F peak amplitude: > T = 30.4+7.55sin(2Pit/24) = 32 when t = sin^-1(32-30.4/7.55) = 0.82 h, > with the argument in radians. The number of "freezing degree hours" is > the integral (gasp) of 32-T from t1 = 12-0.82 = 11.18 to t2 = 24.82 h, > ie (32-30.4)(t2-t1) – 2×7.55×24/(2Pi)cos(2Pix11.18/24) = 78.16 FDD.

you love this stuff don’t you So let me summarize where my thinking is… I’m not looking for the most effective solution from a solar perspective – I’m looking at gaining from solar while at the same time staying comfortable for people and also be esthetically pleasing.  In my opinion, 55 gallon drums are not esthetically pleasing nor is 32 degrees F very comfortable for people (or plants). I’ve learned a lot though.  The idea is to allow the sunroom/greenhouse to heat up fairly quickly and to somehow get that heat into the rest of the house – fans, vents, or even tubing in the cement slab.  Then allow the sunroom to cool (within reason) at night so that the temperature differential with the outside is reduced.  The design allows the sunroom to be separated at night with pull out barrier or we can simply use insulated blinds to increase the R factor of the windows. The rest of the south side of the house may have an excess of windows but I’m not too concerned with this since in the winter time we can just open some windows to cool the house down.  In the summer, the deciduous trees and the overhangs will shade the windows fairly well. And being on a knoll, we should have breezes as well to cool the house.  I may try to incorporate an extra separate loop of pex tubing in the slab whose job it is to drag the heat away from the sunny areas and dump this heat into the slab further from the sun.  I’ll see what the cost/complexity of this is. In the future (and probably not too far away) we’ll look into trickle down collectors for the roof of the house to augment the radiant floor heating and our domestic hot water. I want to thank everyone (and especially Nick) for helping with their ideas. -Doug

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