Author Topic: Yet another discussion on wall insulation ...  (Read 3088 times)

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Offline JRR

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Yet another discussion on wall insulation ...
« on: March 14, 2011, 01:14:29 PM »
I'm lucky (or unlucky...?) to live in an area with little building code enforcement.  My teeth may be getting too long for many more construction beginnings.  The following is a insulation scheme, of many, that I have pondered.  No foam.  Principally for cold climates.  Might also work in warm climates.  Might work nowhere.  Your comments appreciated.  (Be kind to the old man.)

Layout of exterior wall section:

1. Interior air ... warm and moist in winter.
2. Interior wall surface material layer ... plaster preferred.  Includes electrical service runs, and some PEX water runs.  Service runs covered with appearance trim as necessary.
3. Vapor membrane.  An unbroken covering of non-permeable plastic, such as Visqueen ... applied with mastic onto ...
4. Plywood sheathing.  That's right... this scheme has the structural sheathing "inside" the studs
5. Studs and cellulose insulation.  I would prefer staggered/offset 2x4's.  Cellulose to be wet blown or wet stacked... then skreeded to be flush with outside edge of studs, leaving a flat surface.
6. One or two layers of felt.
7. 1/2" thick strips of PT wood attached atop studs ... to provide ample drain plane.
8. Shiplap wood or stucco....attached to studs.
9. Exterior air.....

I can think of no reason the sheathing could not be located inside the studs ... there would have to be engineering solutions for multi-floor buildings, perhaps let-in angled bracing.......this leaves the balance of the wall for easy breathing to the outside.
.
« Last Edit: March 14, 2011, 01:29:00 PM by JRR »

Offline MountainDon

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Re: Yet another discussion on wall insulation ...
« Reply #1 on: March 14, 2011, 01:32:01 PM »
A/C in summer?
Just because something has been done and has not failed, doesn’t mean it is good design.

Offline JRR

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Re: Yet another discussion on wall insulation ...
« Reply #2 on: March 14, 2011, 03:26:39 PM »
Why not?  What happens?

Offline MountainDon

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Re: Yet another discussion on wall insulation ...
« Reply #3 on: March 14, 2011, 03:34:15 PM »
A/C means the interior wall surface is the cold wall. That means moisture from outside humid air migrates through the wall and condenses inside. Happens all through the south in places with vinyl wallpaper. Makes for mold.
Just because something has been done and has not failed, doesn’t mean it is good design.

Offline JRR

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Re: Yet another discussion on wall insulation ...
« Reply #4 on: March 15, 2011, 04:57:52 AM »
A/C means the interior wall surface is the cold wall. That means moisture from outside humid air migrates through the wall and condenses inside. Happens all through the south in places with vinyl wallpaper. Makes for mold.

I know.  But would a free-breathing (to the exterior) wall mitigate the situation ... so that mold growth is minimized/prevented?  Is there no real "solution" other than exterior foam?

Offline Alan Gage

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Re: Yet another discussion on wall insulation ...
« Reply #5 on: March 15, 2011, 05:14:11 AM »
There's another camp that says don't install any vapor barrier or exterior foam. Then the wall can breath to the inside or outside depending on the season. They say as long as it can breath condensation on the inner surface of the sheathing isn't a problem. They also recommend plywood (or horizontal wood siding) instead of OSB for helping it "breath".

So the wall assembly would be: Painted sheetrock (air tight approach)>studs with cavity insulation>sheathing>house wrap>siding.

Both camps say that a well detailed air barrier is a necessity as air leaks will introduce way more water than what can drift through the sheetrock.

The more I read the more I think neither of them are right or wrong. I don't doubt either would work well if done right and either would fail if done wrong.

Don't take anything I say as gospel though. No experience here yet, just lots of research so far.

Alan

Offline MountainDon

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Re: Yet another discussion on wall insulation ...
« Reply #6 on: March 15, 2011, 05:30:54 AM »
Keep in mind that there are most likely two completely different solutions here; one for a cold northern climate and the other for the humid south.



JRR, if the wall breathes why would the moisture move outwards if the interior was air conditioned. The A/C will lower bothe temperature and the relative humidity inside the house. Heat moves towards cold. Moisture level will try to reach equilibrium.  ???
Just because something has been done and has not failed, doesn’t mean it is good design.

Offline JRR

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Re: Yet another discussion on wall insulation ...
« Reply #7 on: March 15, 2011, 10:38:34 AM »
Keep in mind that there are most likely two completely different solutions here; one for a cold northern climate and the other for the humid south.

I'm not sure, but I think there is one "one size fits all" solution.  A solution that suits hot wet southern and cold northern climes.  Unfortunately, it requires foam.  A simple "formula".....

Whatever the required R number is ... provide 60%, or more, of it in well-installed exterior closed cell foam.  Protect the foam from the elements.  Works on any wall design.  Period.

May be expensive.  May not be "green".  That much foam insulation dictates that the "condensation zone/wet-bulb temperature" will occur somewhere in the foam itself.  No wetting ... no mold.

(Is this not correct?)

Offline MountainDon

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Re: Yet another discussion on wall insulation ...
« Reply #8 on: March 15, 2011, 12:08:34 PM »
Right, cover the exterior in enough layers of staggered seam foam and you can leave the stud cavities empty. I didn't think either of you were likely going to end up with that though.
Just because something has been done and has not failed, doesn’t mean it is good design.

 

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