[Strawbale] Frost Heave-basement/cob

[Speireag Alden]

Sgrìobh kim thompson:

>I have been following the discussion of frost heave with 
>considerable interest, hoping that it might align with a project I 
>am about to embark upon, but the thread is not quite there, so I 
>would like to add another twist to the conversation and would 
>appreciate all comments anyone might have on it.

     Hi, Kim!  We've never met, but I've seen your house.  Your 
friends were very pleasant when we dropped in without warning, and 
showed us around.  We visited it on the way home from staying in 
Baddeck many years ago.  Your home convinced me that strawbale would 
be possible in our climate, and we live in our straw bale house now! 
So, thank you.

>Basically am trying to wrap my head around the building science side of
>what the pros and cons are of installing rigid (eco friendly, of 
>course) insulation on the interior side of basement walls and 
>plastering on them with earth?

     The main con is substantial:  the existing concrete foundation is 
a very effective vapor barrier.  If you insulate on the inside, it's 
pretty much guaranteed that you can't create a better vapor barrier. 
So, some air will circulate in back of the insulation, where things 
are cold.  The vapor in the air within the house, where things are 
warm, will condense on this excellent, cold vapor barrier.

     You might get away with it for a long time.  But it's entirely 
possible that you won't, and that you'll get mold, or rot of whatever 
you used to frame out from the walls.

     Better, by far, to dig a perimeter trench and insulate on the outside.

     On the other hand, there are times when that is impossible.  In 
those cases, I would insulate in such a way that (a) interior air 
can't get into the insulation space and (b) if it does get in, it 
can't get back out.

     However, easier said than done.

>As I understand it below grade walls serve many functions i.e. 
>providing a foundation which mitigates frost heave issues for the 
>rest of the building, it  also (amoung other things)  provides 
>thermal mass and acts as a heat sink for ambient ground heat.

     If you insulate on the inside, then you put this thermal mass 
advantage on the outside of the insulation envelope.

     If you insulate on the outside, on the other hand, you've got 
that thermal mass back inside the envelope, where it belongs (at 
least in any continuously-occupied building).

>New building practice is to insulate to the outside of the 
>foundation, whether it is following the foundation wall or panning 
>out from the perimeter as for shallow insulated slabs. What are the 
>potential problems of insulating to the interior, if any??

     See above.

-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)