[Strawbale] Pressure-treated Wood Foundations ( PWF) (was re: Frost heave)

[Speireag Alden]

Sgrìobh Shody Ryon:

>Assuming dry earth has about ½

     Your fraction didn't survive transmission; it comes through on my 
side as the Greek letter pi.  However, stone has about one quarter 
the thermal storage of an equivalent volume of water, if memory 
serves.  Highly compacted earth would have a bit less than stone. 
(Someone correct me if I'm misremembering.)

>  the thermal storage
>capacity of water and some degree of R value,
>all-be-it rather low, perimeter rigid insulation
>buried horizontally underground out side the
>structure, as opposed to installing it vertically on
>the exterior of the foundation, will yield more
>thermal mass, if I understand this concept.

     If you put insulation down with an "umbrella" on top, to shed 
water, yes, you will be keeping a large thermal mass dry.  However, 
you will no longer be enclosing your house in an envelope of 
insulation.  Before you earth-couple, you want to make sure that you 
understand all the implications, and you want to make sure that your 
heat is coming from renewable resources, because you're going to lose 
some of it to the earth.  This is more true in colder climates, and 
less true in warmer ones.

     Now, I think there are circumstances where earth-coupling is a 
good thing.  I'm earth-coupling right now, and because it's taking 
longer than I thought, I'm having to burn fossil fuel to keep the 
house warm and therefore heat the earth, which is annoying me.  (I'm 
fixing it as fast as I can, I promise.  Just spend all day working on 
that.)

>So I propose back filling the excavated trench dug to
>erect the foundation walls with 3/4” drain gravel in
>case of membrane failure and covering it with rigid
>insulation then rubber then soil for flowers. So the
>perimeter is underground exterior to the insulation
>envelope.

     You can use EPDM for that, but I used 6-mil poly, laid carefully, 
lapped with the uphill over the downhill, and with salvaged used 
carpet on top to protect it, pile side down.  A bit more work, but a 
lot less rubber, and some carpet diverted from the waste stream.

>  >      But that's a fantasy wall.  They almost all
>  > have some small amount of air movement, and I would venture to say
>  > that insulated stud wall *all* have some small amount of air
>  > movement.
>
>That is the common conception, I am sure. Lets take a
>look at the vaulted ceilings that have rigid
>insulation without any air space. Why don't they have
>an air space? For some reason they are exempt from the
>rule other ceilings have of needing to be vented.

     Actually, this rule is nowhere near universal, and apparently 
much in dispute among the foremost building experts.

>  >      If you're building a wall in a cold climate,
>  > put a moisture
>  > barrier on the inside, and seal all penetrations
>>  obsessively.  The
>>  main way that vapor gets into a wall and condenses
>>  is not vapor
>>  diffusion through solids, but rather, carried on the
>>  back of air
>>  movement through very small cracks and pinholes.
>
>This is what really gets me confused. Isn't gypsum
>board  air tight? That is, if the wall penetrations
>like electrical boxes are sealed on the inside of the
>box as well as on the outside?

     Not my area of experience, so I'll defer to someone who knows better.

>They used to call the
>first coat of mud and tape, “fire taping” because fire
>was positively slowed by this process. Assuming the
>wall is sealed with either the gypsum board or the
>vapor barrier, am I to believe moisture will not enter
>the joist bay from the exterior where there is no
>vapor barrier?

     It will, but it will stay as vapor, because there is no point in 
the wall which is colder than the outside atmosphere.

>If there is a vapor barrier on both the
>interior and exterior, does that mean the inside of
>the wall will have less vapor inside than the climate
>supplies to the general area?

     No.  You don't want a vapor barrier on both sides.  Just the 
inside (for heating climates) or the outside (for air conditioning 
climates).

-Speireag.

-- 
A wise man will make haste to forgive, because he knows the true 
value of time, and will not suffer it to pass away in unnecessary 
pain.
--Samuel Johnson, lexicographer (1709-1784)