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

[Ilan Ungar]

Thanks for all the information Sherwood.

I definitely wont need any of these strategies as my climate has a mere 800
degree days Celsius...!
I am wondering whether my planned house (renovation includes wrapping with
SB) will need any auxiliary heat at all.
If it wasn't for a group of trees overshadowing the house for most of the
effective solar hours (in Winter), I would feel confident in designing the
house with no heating system. The other problem is, there is no insulation
under the existing concrete floor slab. These houses are terribly
uncomfortable in their current condition (seen in photos, following link).
This link describes the situation (has been posted here already, just in
case it was missed).
http://theungarresidence.googlepages.com/home

Thanks.

Ilan.


On Jan 8, 2008 4:49 PM, Sherwood Botsford <sgbotsford at gmail.com> wrote:

> 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...)
>



-- 
Ilan Ungar, Architect
Lehavot Habashan 12125
Israel
972 4 6953429
972 52 2431398