[Greenbuilding] Trying to maximize passive solar gain...

Sat Apr 21 10:32:11 CDT 2007

Hi Nick,

Thanks for replying to my questions.  Wow there certainly are allot of 
different ways to look at an issue aren't there?  I have some more questions 
and some clarifications for you.  I hope you don't mind.

You said - "Sounds good, with dark mesh curtains behind the windows to 
collect hot air which heats mass in the basement ceiling?"

No, the thermal mass I was hoping to use is already in the insulated slab 
and foundation walls.  I hadn't planned to put much mass in the ceiling. 
Would you recommend an extra layer of gyproc there?

You also said "> A little more thermal mass upstairs seems good, enough to 
allow  the living space temp to drop from 70 F at dusk to 50 at dawn
> on an average December day? Basement mass wastes energy.
> It would slow family room warmup before occupation and delay cooloff at 
> night, when stored solar heat disappears to the outdoors through windows. 
> Insulation and carpet over the concrete can reduce this loss."

Does this mean you wouldn't bother insulating the slab and foundation walls 
on the outside, but would insulate them from the inside instead?  Even 
though, the basement is exposed to the sun with lots of windows?  This just 
seems o contrary to any of my readings about solar gain and mass.

I said "The home will be insulated to the Energuide 80 spec - here in New 
Brunswick, Canada." and you asked the question " I wonder what that means, 
numerically. I suppose it includes good airsealing, with how many cfm of 
natural air leakage? "

I'm not sure what the numbers are.  Maybe Corwyn is familiar.  If so, I 
would appreciate it if she could provide them.

What do you calculations below suggest Nick?  I have no idea how to 
interpret them.

Thanks again,

Christa C.

----- Original Message ----- 
From: "Nick Pine" <nick at early.com>
To: <greenbuilding at listserv.repp.org>
Sent: Wednesday, April 18, 2007 4:59 PM
Subject: [Greenbuilding] Re: Trying to maximize passive solar gain...

> Christa writes:
>
>>... the main living space is on the top floor.  A large, open kitchen,
> dining, living room space is the full depth of the house.  Two bedrooms
> are on the south side and one bedroom and two baths are on the north.
> The basement will have a large family room across entire front section
> (south facing with large windows) with a coldroom, laundry room and
> storage room along the back (north)...
>
> Sounds good, with dark mesh curtains behind the windows to collect
> hot air which heats mass in the basement ceiling?
>
>>We also plan to finish the basement floor with dark polished concrete
> and are considering doubling up on gyproc in some of the upstairs rooms
> so that there is extra thermal mass up there too.
>
> A little more thermal mass upstairs seems good, enough to allow
> the living space temp to drop from 70 F at dusk to 50 at dawn
> on an average December day? Basement mass wastes energy.
> It would slow family room warmup before occupation and delay cooloff
> at night, when stored solar heat disappears to the outdoors through
> windows. Insulation and carpet over the concrete can reduce this loss.
>
>> The home will be insulated to the Energuide 80 spec - here in New 
>> Brunswick, Canada.
>
> I wonder what that means, numerically. I suppose it includes good
> airsealing, with how many cfm of natural air leakage? December looks like
> the worst-case month for solar house heating in Fredericton NB (Lat 45.9 
> degrees),
> when 2127 Wh/m^2 of sun (674 Btu/ft^2) falls on a south wall on an 
> average -6C day,
> according to the 1981 NRCC Solarium Workbook. The Energy Plus CWEC hourly
> weather data file for a typical year in Fredericton could be used for a 
> simple solar house
> simulation. They say 1893 and 701 and 1229 Wh/m^2 of direct and diffuse 
> and global
> sun fall on the ground on an average -6.6C (20 F) December day with 
> a -3.6C daily max,
> for an average daytime temp of about 5.1C (23 F) and an average dawn temp 
> of about
> -9.6C (15 F.)
>
>> Can you have too much thermal mass?
>
> Yes.
>
>>  I have a book that gives minimum numbers corresponding to the area of 
>> southfacing
> glazing, but they don't mention whether or not you can have too much.
>
> Those books (eg Chiras) are often inspired by masonry salesmen who
> gave those recommendations unstintingly and enthusiastically :-) Sunspace
> mass kills solar collection efficiency by making it lukewarm all day 
> instead of
> hot during the day and cold at night and storing solar heat which leaves 
> via
> sunspace windows at night. Living space mass can limit night setbacks
> which save energy and prolong warmup times after setbacks.
>
>> Do you think it is worth it to do this for a conventional house that
> we will try to sell to conventional people in a few years time or am I
> banging my head against a wall unnecessarily?
>
> Perhaps you can build a house that's conventional and energy-efficient
> and live in it for a long time, if you wish. Airsealing and some other 
> things
> won't change the "conventional" look and feel of the house...
>
>> Do you have any suggestions for how to heat this house in addition to
> the passive solar gain?
>
> Avoid direct gain mass and glass.
>
>> Is there any other advice you might give?
>
> If your house were (say) 32' square and 16' tall with 80 ft^2 of US R4
> windows upstairs with 50% solar transmission and (say) an R40 ceiling
> and R32 walls, eg 8" SIPs or foamboard and fiberglass, the upstairs
> would have a thermal conductance of 1024ft^2/R40 = 25.6 Btu/h-F
> for the ceiling + 20 for windows + 944/32 = 29.5 for walls. Adding
> 15 cfm for air leaks would make the conductance about 90 Btu/h-F.
>
> With a 60 F average temp (70 day and 50 at night), the house would need
> about 24h(60-20)90 = 86.4K Btu on an average December day. A frugal
> 600 kWh/mo of indoor electrical use could provide 68.2K Btu/day of that,
> leaving 18.2K from other sources, eg the windows, which might contribute
> 0.5x100ft^2x400 Btu/ft^2 = 20K Btu, enough on an average day.
>
> Meanwhilst, 32'x8' of U0.51 basement sliding glass doors with 63% solar
> transmission (US$261 for 6'x7' doors at Home Depot) might collect about
> 0.63x210ft^2x674 = 89.2K Btu. of sun and lose 6h(T-23)210x0.51. If
> gain equals loss on an average December day, T = 162 F :-) If the house
> cools from 70 F at dusk to 50 at dawn and 50 = 20+(70-20)e^(-18h/RC)
> makes RC = -18/50-20)/(70-20) = 35 hours, which makes C = RCxG
> = 3171 Btu/F upstairs, or less, with some help from basement ceiling mass
> or electrical usage at night.
>
> Making the house 70 F on a 15 F morning requires (70-15)90 = 4950 Btu/h,
> which might come from A ft^2 of Tmin (F) surface with a 1.5 Btu/h-F 
> airfilm
> conductance if 4950 = (Tmin-70)1.5A. A = 1024 ft^2 makes the min usable
> mass surface temp Tmin = 73 F. We might store heat for 5 cloudy days in
> C = 5x18.2K/(162-73) = 1022 pounds of water cooling from 162 to 73 F
> between the basement ceiling joists, with some holes and foil below it and
> insulation above. it. The water might be 0.2" deep in flat polyethylene 
> film
> greenhouse ducts. As an alternative, we might have more water in 20+ 
> 4"x10'
> thinwall PVC pipes with less surface, with 13 gallons of 300' of 1" 
> pressurized
> PE  pipe ($60) inside to act as a heat exchanger for showers.
>
> Harry Thomason used inside walls as convection chimneys for 
> thermosyphoning
> basement air with supply outlets near the first floor ceiling and air 
> return grills in
> the basement ceiling near outside walls to reduce heat lost to outside 
> walls and
> take advantage of natural room air circulation in wintertime. Room air 
> falls near
> outside walls, slides across the floor and rises near inside walls and 
> slides under
> the first floor ceiling and back down the outside walls...
>
> All this could be hidden without changing the look of a conventional 
> house,
> or proudly exposed, as a solar house :-)
>
> Nick
>
>
> 