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I'm in the midst of a remodel in Austin, TX and I'm struggling to resolve the "right" way to go about insulation for exterior walls in this region. I'm generally familiar with the concepts of vapor barriers, vapor retarders, and managing airborne liquids in and around your living space, but all the sources of information I'm referencing have my head spinning at the moment.

My question(s) is/are this: in the southern US/Texas, where it's warm (read: hot as heck) for most of the year...

are vapor retarders required? are they prohibited? Are they allowed, but not good building science practice? Should they be faced towards the exterior (unconditioned space), flying in the face of what most sources suggest for insulation best practices?

Any thoughts would be appreciated!

One added detail, the makeup of my walls presently is (will be):

Nature >> Wood siding >> air gap/furring strips >> 2 layers felt paper >> OSB >> studs/R-15 fiber batts >> kraft paper face/tape on the seams >> gypsum >> netflix and cheetos

P.S. I know that spray foam insulation is probably best in a case like this, but unfortunately I didn't plan for that in the budget.

  • Why felt instead of modern building wrap? I'd think you'd want a breathable membrane. – isherwood Apr 5 '18 at 15:59
  • From my observations the way felt was used in the south was overlapping horizontal courses with sufficient looseness in the overlap to allow the sheathing to dry to the outside. So a double course of felt might have too much overlap to permit drying to the outside. – Jim Stewart Apr 5 '18 at 16:18
  • Isn't good primer (Kilz or better) on the living space side of the drywall going to prevent the movement of water vapor from the living space into the walls? So the problem is preventing water vapor from outside penetrating the wall and condensing in the wall during the cooling season. But the humidity in Austin in the hot months is not high. There is no much to worry about however you do it. – Jim Stewart Apr 5 '18 at 16:47
  • @isherwood - perhaps I'm wrong, but I thought felt WAS breathable? At the very least, I know it doesn't create an air barrier, but (as I understand it) as a vapor barrier, felt has the unique attribute of being somewhat absorbent, and becomes more vapor permeable when moist. Should liquid reach that layer of the construction, the felt can absorb it to an extent until drying conditions improve. My logic was coupling that with a rain screen "air gap" would mean improved performance overall. – JimboSlice Apr 5 '18 at 17:30
  • Since you're in Austin Google for Matt Risinger who is as builder in your region with an excellent blog on building science and also a YouTube channel. – DaveInCaz Apr 6 '18 at 0:17
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Just talked with the inspector and it seems using a insulation with kraft facing on the interior of the house is perfectly fine here (as of April 2018, at least). In his words, taping the seams is "overkill" but will certainly help to keep airflow to a minimum.

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Instead of "felt paper" hasn't most construction switched to Tyvek (or generic equivalent) on OSB? In Dallas newer medium high end construction is with the green Zip OSB sheathing instead of Tyvek.

I doubt that air gap vertical furring strips are really necessary in Austin. This is basically a "rain screen" treatment and Austin does not have that much rain. I have seen a very high end house in Dallas that was going to use siding of 3/4" thick clear cedar on furring strips, but in the end they switched to some even more expensive Brazilian wood siding probably on vertical furring.

There is a DuPont product called DrainWrap which is said to allow drainage of liquid water behind the siding, should any get there. In the event that was needed you would probably want to prime or stain all 6 surfaces of each piece of the siding.

http://www.dupont.com/products-and-services/construction-materials/building-envelope-systems/brands/water-barrier-systems/products/tyvek-drainwrap-moisture-barrier.html

  • All good info, but the question is about insulation and interior moisture management. – isherwood Apr 5 '18 at 15:59
  • I talked to a builder/architect on the site of a medium high end construction going up in our neighborhood in Dallas. He said the environmental conditions in our climate zone (presumably including Austin) are so forgiving that the designer has many ways to produce a well functioning design. – Jim Stewart Apr 5 '18 at 16:11
  • Thanks for input, @JimStewart. Housewrap does seem common these days, but I opted for the felt nonetheless. It helped that this 60-year-old house had paper that was still intact (albeit brittle) and still just as water-resistive as the new stuff. I agree the rain screen is a bit overkill, and complicated my trim a bit, but I liked the concept on paper and figured i'd give it a shot. I haven't hung the siding yet so I may change my mind yet... I know Austin's climate is very forgiving, but I'd like to ensure I don't accidentally violate the local code with my vapor-mitigation efforts. – JimboSlice Apr 5 '18 at 17:10
  • I had a section siding to replace in our 47-year-old tract house and I considered two rain screen options, but in the end just put on plywood sheathing, generic tyvek house wrap and nailed the cedar siding into the sheathing. I tried to over the studs. I did use special 304 stainless steel siding nails (Simpson Strong-Tie S6SND1 2" 6d and 2-1/2" 8d). The old siding was ruined because a previous owner had let an impulse sprinkler blast it for years. IIRC I did prime both sides and the cut ends of the siding and trim before attaching it. – Jim Stewart Apr 5 '18 at 20:17
  • I guess you are using fiberglass batts with paper on one side and wonder which side to put the paper on. I would put in on the living side, but it is not completely clear to me. – Jim Stewart Apr 5 '18 at 20:22

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