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Have exterior wall open-questions about improving insulation

9 years ago

I have a 1980s era house, so not the greatest in terms of energy efficiency but not as bad as some. I have some walls exterior walls open for a bath remodel and would like to improve the insulation.

They are 2x4 walls with brick veneer over 3/4" RMax Thermasheild (foil on both sides). The bays are filled with R-11 FG batts and there was a plastic moisture barrier over those, then the drywall.

We plan to upgrade the batts to either rockwool or R-13 FG but would also like to use some foam over the studs to help with the thermal bridging. I've seen where 1/4" XPS fan fold can be used for that and they have some perforated foam like that at Lowes, so it's easy to get.

Would that be a good plan to make some improvement?

What moisture barrier should we used and where? We are in the Dallas area, so much more air conditioning than cooling.

Thanks for any advice!

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Comments (5)

energy_rater_la
9 years ago
if the 3/4" foam board is in good shape just leave it there.
if I'm understanding your description correctly the foam
board is on the exterior of the wall under the brick.
with the foam board in this location of the wall it
is stopping thermal bridging from ambient into
conditioned space.

this foam board has an insulation value depending
upon type. that adds to the value inside the cavity.

you don't need the vapor barrier to the interior.

http://www.buildingscience.com/documents/insights/bsi-001-the-perfect-wall/?searchterm=perfect%20wall

upgrading the insulation in the walls, would be easy
enough to do with it all open. you can special order R-15
batts.
while things are open, air seal. invest in a case
of caulk & seal things up as the project progresses.

best of luck.
lucas_tx_gw
Original Author
9 years ago
Yes sorry if I was confusing. The 3/4 foam is under the brick and is essentially the outside of the house, so we definitely plan to leave it. Just didn't know if we a layer of foam over the studs on the inside would help.

Thanks!
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worthy
9 years ago
A vapour retarder/barrier on the inside wall combined with the 3/4" foam board on the exterior will create a double vapour barrier, making it impossible for trapped vapour to move anywhere. So, no not a good idea.

As noted in this Building Science Corp. paper:

"Vapor barriers installed on the interior of assemblies prevent
assemblies from drying inward. This can be a problem in any
air-conditioned enclosure. This can be a problem in any below grade
space. This can be a problem when there is also a vapor barrier on the
exterior. This can be a problem where brick is installed over building
paper and vapor permeable sheathing."
lucas_tx_gw
Original Author
9 years ago
I can see the logic in that. Do residential building codes specifically address this question? Will I have any trouble with an inspection if there is no vapor barrier on the inside?

What a about the shower itself? Won't the use of Kerdi against that outside wall cause essentially the same condition? How do people deal with that?

Thanks for all the help!
worthy
9 years ago
"Do residential building codes specifically address this question?"

Yes.

"Will I have any trouble with an inspection if there is no vapor barrier on the inside?"

Depends on which Code is used and your climate zone. If the tx in your handle means you're in Texas, very unlikely to be a problem. Even in cold climates, International Residential Code 2006, for instance, provides for exceptions.

Here is the status of Code adoptions in the US.

"Won't the use of Kerdi against that outside wall cause essentially the same condition?"

Just one portion of one room would be no cause for concern.

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