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Exterior insulation in MN attic

I have two walls of a bedroom that are adjacent to my garage attic. I am hoping to beef up the insulation on the outside of the wall in the attic. I am worried about a few things - cost, installation, and moisture issues.

I have a 2x6wall , FG batts, with vapor barrier on the inside - so wouldn't be in for something that is too vapor resistive on the outside (i think?). Currently the wall on the exterior is sheathed with vapor permeable fiber sheathing (Brace-rite,28perms). I was under the impression that xps or polyiso wouldn't be advisable...

what about at only 1" thickness on the xps? seems to me that through reading about basement insulation - at 1.5"-2" is when the perms gets to near 1..?

Otherwise would EPS be do able in the 1-2" range (3 perms at 1")? EPS has a nice cost/R

Thinking rigid board would be easiest to install. Can't find a easy source for mineral wool board... and strapping/attaching fiberglass batts (cheap) doesn't seem like it would be very effective.

Asked by Nick T - 6A (MN)
Posted Sun, 12/29/2013 - 11:24


3 Answers

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Your question boils down to this: "If my walls have an interior polyethylene vapor barrier, is it dangerous to add exterior rigid foam?"

I have answered the question several times, and I'm happy to share my standard answer.

Many energy experts have worried whether it's a good idea to install exterior foam on a house with interior polyethylene. Although it would be better if the poly wasn't there, the fact is that tens of thousands of Canadian homes with interior poly have been retrofitted with exterior rigid foam, and there haven't been any reports of widespread problems. According to building scientist John Straube, all indications show that these retrofits are "not so risky as most people think. These homes will probably be fine."

That said, the installation of exterior foam is not advised on any home that has suffered wet-wall problems. If you plan to install exterior foam during a siding replacement job, keep an eye out for any signs of moisture problems before you install the rigid foam.

It's also important to keep your interior relative humidity within reasonable levels during the winter. Never use a humidifier.

To summarize, here are three caveats:

1. Be sure that your foam is thick enough to keep the wall sheathing or stud bays above the dew point in winter. Read more on this topic here: Calculating the Minimum Thickness of Rigid Foam Sheathing.

2. Before installing any rigid foam, inspect the wall carefully for any signs of moisture problems. Don't proceed if you see mold or rot.

3. Keep your interior humidity under control during the winter; if the interior humidity gets too high (above 40%), operate your ventilation fan more frequently.

Answered by Martin Holladay, GBA Advisor
Posted Sun, 12/29/2013 - 17:08

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So with indoor humidity in the 29-36% all winter at 67° - and an external sheathing perm of currently 28. My walls are shown by this BSC paper could be frosty/damp for most of the winter... but dry very well or diffuse the moisture during times of no frost/condensing. (pg 17)

... so by adding say 1" of cheap EPS - that ups the R value by more than 25% of the assembly - but only raises the outwards perm of wall to around 4-5 (5perm EPS@1" & 28perm sheathing)? So less periods of condensation for part of the winter, but still more drying potential outwards then a code built wall with OSB/plywood (perm .8-1)? (although i suppose un-taped sheathing would leak enough to dry faster then perm1 would imply?)

So insulation up a bit, moisture risk changed little(+/-), and the 'risk' lower than most homes built to code (circa 2009-2013) ?

The best case and higher cost option being adding 2.5" of XPS , greatly increasing total wall Rvalue(~double) and eliminating the need(&ability) to dry.

Answered by Nick T - 6A (MN)
Posted Mon, 12/30/2013 - 14:53

Helpful? 0

When it comes to designing a high-performance wall, I think it makes more sense to choose exterior foam sheathing that is thick enough to keep the interior of the wall warm and dry, rather than to choose thin rigid foam insulation that results in moisture accumulation, but allows the moisture to dry (slowly) through the foam.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 12/30/2013 - 15:05

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