[Strawbale] [SB-r-us] Re: Frost Heave

[Speireag Alden]

Sgrìobh Hank Carr:

>Derek Roff opined:
>
>  > There is moisture in the environment where most of us live,
>  > yes. That doesn't mean that there is always sufficient
>  > moisture in the right situation for frost heave to occur.
>
>This is absolutely true.  I didn't say "always".
>
>It really sucks when on those non-always occasions it does happen and it
>happens to you, though.
>
>>  Building codes are an interesting mix of prudent practice,
>>  insufficient attention to important details, extreme overkill,
>>  and voodoo. Understanding the building science and the
>>  specifics of a design and site help one make more refined
>>  judgments.
>
>Building codes are intended to protect families, friends, and future owners
>from people who think they know better and are based on the best
>understanding of the building science at the time that they are written or
>updated.

     Building codes are minimum standards.  They may not reflect best 
practice, and they necessarily lag behind ongoing best understanding 
of building science.

     This is not to say that building codes are bad.  But they're imperfect.

>The heat escaping from the building through the insulation under the slab
>keeps the substrate under the grade or rim beam or the thickened edge of the
>slab from freezing.  If the building is left unheated and the substrate
>under the field of the slab freezes frost heave can occur and can seriously
>damage the slab and the building.  (Note that I didn't say WILL or ALWAYS
>this time either.  I said CAN and meant exactly that.  Given the right
>conditions it CAN occur and if it happens to you it WILL really suck.)

     Yes.  You wrote:

>The substrate is never perfectly dry. The edge thickened frost
>protected slab foundation system (like the rubble trench and grade
>beam system) relies on heat loss through the slab to prevent freezing
>(see the DESIGN GUIDE FOR FROST-PROTECTED SHALLOW FOUNDATIONS, NAHB
>Research Center.)
>
>Since there is obviously moisture in the substrate (i) leaving the
>building unheated would allow freezing in the substrate (ii) which
>could lead to frost heave.

     You just overstated your case, that's all, and that's what I was 
reacting to, in an effort to clarify.

     Even though the substrate is not perfectly dry, because some 
small amount of water is always present, clearly there is a condition 
where moisture drops low enough that the ground won't heave.  Any 
foundation which creates those conditions down to the frost line 
won't permit a heave.  Rubble trenches don't have to rely on heat 
loss from anything, if they go down below the frost line.

     The same is true of frost-protected shallow foundations, but they 
achieve it by raising the frost line.  They can be designed such that 
they rely on heat loss from within the structure's envelope, or such 
that they don't.  I'm inclined to regard those that rely on heat as 
dubious design, because you never know that a building is going to be 
heated to a certain level forever.

     We're talking about huge masses of earth, so losing heat for a 
week or two isn't going to lead to heaving.  There's too much thermal 
momentum.  You'd have to have an unheated building for months, and a 
hard winter.

     I'd rather design a building such that an ignorant future owner 
could leave it to freeze in the future, and the structure would be 
unaffected.

-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)