[Strawbale] insulation and thermal recovery

[Sherwood Botsford]

Shody Ryon wrote:
> Hi Sherewood,
> Thanks for that answer. Since I am fast approaching
> the stage in my project were this concept my have
> design consequences (fast as in negitive 2 MPH!) I am
> wondering if I were to actually build this... has
> anyone build this type of system? and have any tips? I
> wounder if I were to remove the soil about 14 feet
> (depth to be determined) and install a french drain,
> then a waterproof membrane (wpm), then R 50 rigid
> insulation, then back fill with soil and PEX, cap it
> with wpm, R50 and a slab.

Unless you have underground movement of water where you are 
building, this is all way overkill.  Look up Passive Annual Heat 
Storage.  Typically what they do is put down 2-3" of foam 
insulation at a 45 degree angle to the to foundation.  So the 
foundation is the tip of a rectangular cone or truncated pyramid.
As I recall they only insulate for about 4' down.  This isn't the 
ultimate efficiency, but you don't care.

Even in our climate (Edmonton, alberta) with it's two months of 
near dark.(I have to use my sunvisor driving at noon -- sun is 13 
degrees above horizon) and 30-40 below zero temperatures, once 
you are down 10' the temperature is 50F and only varies about 2 
degrees between summer and winter.  So with passive annual heat 
storage, you're heating the top of of an effectively infinitely 
block.

Compacted soil is effectively about R1-2 per foot. Heat capacity 
for dirt/stone/sand is about .2 BTU/lb/F  1/5 that of water.  But 
the density is typically 2.5 times that of water, so it's about 
1/2 the heat capacity of the same VOLUME of water.

R20 is probably enough for your under slab layer.

You need to track down an engineer to verify the numbers.

If I were in your shoes, doing my first building, I'd build the 
garage/shop first.  It's simpler.  Your mistakes don't cost as 
much.  It may save your marriage.



> I was thinking of a commercial grade waterproof
> membrane that would be strong enough to withstand
> heavy equipment and not tear.
> How do people deal with radiation that comes from
> concrete??
> Shody 

Radiation from the concrete is insignificant.  Or rather it's 
about the same as the radiation from the walls.  Radiation
and absorbtion depend on only two factors:  The emisivity of the 
wall. (Perfect white = 1, perfect black = 0) and the temperature.
At room temperature a black body (e=0) will radiate about 75 
watts/square meter.  But the wall also radiates 75 watts/square 
meter, which is absorbed by another wall or the floor.  Back and 
forth.  Since they are equal, you ignore them.

What if the wall is white, you ask:  If the wall is white, with 
say e=.90 then it only radiates 10% of a black wall.  But it also 
only absorbs 10%, the rest reflects.  So for walls/floors of 
equal temp, it makes no difference what color they are.

But when you have different temps, then it gets more interesting.
Consider a window:  Inside it's 70F outside it's -30 F.  Inside 
each square meter may be radiating 75 watts, outside, only 40 
watts. So the window has a net loss of 35 watts/square meter.

Turn it around:  A sunny window is exposed to about 800 w/m^2. 
Inside is radiating out at 75.  Net heat gain of over 700 w/m^2

Windows are about the only place where radiation makes a 
significant effect in a reasonably insulated house.

Picture this:  Normal insulation.  70 F inside.  Heat moves by 
conduction through the wall, warming the outside a bit above the 
surrounding temp.  It will gradually warm up until heat is being 
carried off by convection. (Warms the air next to the wall, which 
then rises and runs away.) and by radiation.  Since the heat flow 
through the wall is small, the outside surface is only a few 
degrees above the outside air temp.  So radiation out is 
ballanced by radiation in.