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Is it worth applying housewrap from the interior, when retrofitting with insulation?

  • Yes, apply housewrap to the sheathing from the inside (wrapping the studs)

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  • No, not worth the effort.

    Votes: 1 50.0%
  • Other (please see comment)

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Discussion Starter · #1 ·
This is a low-budget rehab on a small, inexpensive, mid-century 'modern' house; near-flat roof with soffits, in the Pacific Northwest. The house will be torn down whenever it sells (probably within 10 years). For now, because it has zero wall insulation, I'd like to add some insulation in the walls to save energy costs in winter. The house is currently gutted inside; all drywall (walls and ceilings) has been removed. So I'd like to add the insulation from the interior side before I put drywall back up.

Question: should I apply housewrap, i.e. on the inside of the sheathing and wrapped around the studs?


The walls currently consist of (in order from interior to exterior):
2x4 studs
ship-lap 1"x8" boards for sheathing (horizontal)
thin black paper (air and/or vapor barrier?, original 1958)
cedar board-and-batten siding (vertical)

The walls are not tightly air-sealed. I.e. neither the ship-lap sheathing nor the cedar siding has been caulked, I don't know if the black-paper was taped, but I doubt it. The black-paper is the main air-barrier. In a few places the black-paper is visible through knots/imperfections in the ship-lap sheathing. (I used Great Stuff foam where the bottom of the wall meets the sill-plate. I haven't yet caulked the top-plate nor the soffits.)

I will install Roxul 3.5" (R15) between the studs. (I already got it, on sale).

Is it worth it to apply housewrap (like Tyvek) from the inside, wrapped around studs and on the interior of the ship-lap sheathing? (Not optimal, I know, but $ is scarce, its winter, and the house is occupied.) It's a small one-story house with large windows, and I won't touch the cedar-paneled living/dining area until spring, so this isn't as bad as it sounds: only 100 linear feet to insulate for now (8' high).

Or is the original black paper enough of an air barrier? I don't want to cause rot/mold problems, e.g. if putting in Tyvek trapped moisture near the sheathing. I'm not sure if there are any advantages to adding Tyvek, at this point; I guess the goal would be to reduce air flow and heat-loss. What's the collected wisdom on this? Pros and cons?

If I do install housewrap, should I tape the top and bottom edges of it to the walls (wall-plates)? If so, with what kind of tape? Or is stapling enough?

I'm guessing retrofitting with insulation from the interior has been discussed before but I searched and couldn't find relevant threads.

Many thanks for any knowledgeable insights!
 

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Discussion Starter · #3 ·
I'll look into MemBrain, thanks! I'd been thinking of putting housewrap between the sheathing and the insulation. In the CertainTeed link it looks like Membrane goes over the insulation, between it and the drywall -- although I suppose it could instead be put directly against the sheathing.

I could do rigid foam, "cut-and-cobble" with spray-foam to seal it in place. Martin Holladay has a good article with some pros and cons and tips: http://www.greenbuildingadvisor.com/blogs/dept/musings/cut-and-cobble-insulation Apparently it is fairly time-consuming. For my application, I'm not sure it's worth the cost. I was able to buy bags of R15 Roxul ComfortBatts for $35/bag (12 pieces at 48", enough for 6 stud openings of 8'). Versus e.g. 2" RMax polyiso double-foil goes for roughly $30/sheet (4x8), enough for only 3 bays (with a 2.2" strip left over), so almost double the cost, plus some cans of Great Stuff.

Still, something to think about and I appreciate your response! Thanks again.

If you are going to do something like that, use something called Membrain. http://www.certainteed.com/products/insulation/mold-prevention/317391 I would also look at a combination of rigid foam inside the wall to slow the moisture drive from the exterior and get a great air seal.
 

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IMHO, leave out the HW as you already have tar paper for a better job of water resisting as it wicks- HW doesn't. Far as air sealing- it should be your drywall (or Memb., poly)- in contact with the thermal barrier (on interior) as per minimum code. I doubt you will get any incidental water from exterior through the 3/4" thick vertical cedar boards and 3/4" battens over them- (both in grain orientation to drain water down)-- AND 3/4" thick board sheathing under it all.

Neighboring Canada requires the stud cavities vented to the exterior in their colder climate. With Roxul CB R-15 at 2# cubic ft. density, most minimal infiltration will be slowed/stopped anyway, not requiring HW application as an air barrier (in the wrong place). Rock wool is twice the density of R-13 FG; http://www.diychatroom.com/f98/biggest-loser-fiberglass-insulation-90438/ Dense pack (3-4# density) cellulose is almost airtight...
Poly (4mil min.) sheeting, Membrain, and paper faced (asphalt coated) FG batts (require a perfect application) are fine, if they are interior of the insulation. Don't use Mem. and faced insulation for problems. No double-foil next to the sheathing boards.... Gary
 
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Discussion Starter · #6 ·
Thank you for the input!

I had dinner with a half-dozen builders/contractors/DIYers, and their emphatic and universal advice for here in the Pacific Northwest is: "Do not add a layer of housewrap on the interior, between sheathing and insulation. Don't add any barrier." Dampness and mold is the big problem here, not extreme cold (nor extreme heat, there's no A/C in the house). The original 1950s tar-paper should be fine (and advantageous per Gary in WA's comment, above), combined with a tight seal on the drywall (around outlets, bottom- and top-plates, etc.). So not only is it not worth the time and money to wrap the inside with Tyvek, but it could contribute to mold problems.

Thanks again!
 

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