[Strawbale] PEX, Concrete Slab, Fear of Death (by contractor)

[Sherwood Botsford]

Ilan Ungar wrote:
> Yes, thats what I thought, as I have read here a long time ago about
> extremely thick floor slabs, in climates similar to yours, charged with
> solar heat during August-September and slowly releasing it, into the
> super-insulated house, during the winter. (the relevant point being, the
> house will overcome the inevitable heat loss any enclosed space would have,
> and keep going for the winter months)
>
> Thanks.
>
> Ilan.
>   
A 1 foot slab will not hold a winter's heat.  You're now talking PAHS or 
passive annual heat storage.  (google it) If you are in a mild climate 
(NOT my 10,000 degree day heating climate) this is workable.  Even in my 
climate it may be workable if I built enough solar collectors and got a 
house sized volume of dirt up to about 130 F or so.

Consider a square foot of house:
8' of dirt under the floor has a thermal storage of about 30 
btu/F/cuft.  So if you heat it from 75 F to 155F you have 80 F * 30 
btu/F/cuft * 8 cuft = 19200 btu/sq ft.  If that square foot of house has 
1 square foot of ceiling and 1 square foot of wall, and a 10,000 degree 
day heating season:

10,000 degree day = 240,000 degree hour.  2 square feet of R40 = 1/20 
btu/F/Hr/
So our 1 square foot of house needs 240,000/20 = 12,000 btu to heat it.

So in principle it's doable.

Caveats:  Your floor must be insulated MORE than your ceiling.  
Otherwise you will cook in the fall, when the subslab space is at it's 
maximum, and your heating needs are at a minimum. (You can make up for 
this by keeping the windows open.)

You must have a means other than conduction to move heat from the sub 
slab to the house.  E.g. Circulate water between the subslab and the 
slab.  Otherwise you can't control the rate of heating in the house. (Of 
that 10,000 degree day heating, typically 1/3 of it happens in under 4 
weeks.)

Modeling the subslab as a block 8' thick is over simplification.  In 
actual fact, you are heating a large cone, and as you go further down, 
you are heating it less and less.  It's an interesting math problem. 
Near as I can figure, it doesn't come to equilibrium, but once you get 
more than about 12' from the source, the fluctuations are small.  
However this may mean that it won't work well the first year or so, 
unless suplemented with auxilary heat, or by having oversized solar 
collectors.  (Going oversize may mean that you could solar heat a 
greenhouse later...)