House wrap on interior of sheathing
TL/DR: Will Tyvek on the interior of sheathing prevent wind washing? Will it cause any problems?
Hi all,
I’m retrofitting the walls of my 1940s home from the interior. They are true 2×4 walls with plank sheathing and cedar shingles on the exterior.
I’ll be furring out the 2x4s to 5.5″ and putting rock wool in the cavity, topping that with a Membrain air barrier and vapour retarder, and then horizontal strapping to create a service chase.
However, with my walls open I’ve definitely noticed that a ton of wind comes through the cladding and sheathing (I live on the top of a hill). I’m concerned about wind washing with the rock wool, so I’m thinking about first putting house wrap (Tyvek) against the interior side of the sheathing (with proper sealing). Will this have the benefit of preventing wind washing, as I hope? Will it cause any issues? Any specific detailing I should consider? I’m in climate zone 6.
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What kind of sheathing do you have? If it's plywood simply caulking it should make it windproof. If it's boards Tyvek should work, but it's expensive. For about a quarter the cost you could line the stud bays with rosin paper which should work just as well.
Hi DC,
Thanks for the comments. The sheathing is boards of various widths. My guess is 3/4" thickness but I have no confirmation yet. Over time the boards have shrunk and there are cracks inbetween them.
I’m actually in the same boat, Ian, and everything I’ve read suggests that would be fine as long as the the tyvek was taped in each stud bay. You mentioned that it’d be properly detail, and I assume that’s what you meant.
One thing I wonder about relative to the assembly is the need for some sort of air gap to allow for the sheathing to dry. Something like a 3/8” gap provided by something like Slicker Max (or wrinkled Tyvek in lieu of both) between the sheathing and the WRB makes sense to me In that it’d allow any water that is wind driven into the sheathing to dry more quickly. That might be overkill though, but I’m sure others can chime in to correct me if it’s just an added expense.
Risinger did an episode where he did a vented dimple mat just to the interior of board sheathing.
https://www.youtube.com/watch?v=jhoQ_f9ZnzQ
Thanks for the link, Wooba; I’d seen that but couldn’t find the link.
My uncertainty over the necessity of the dimple mat is relative to the difference in climate zone. Austin, where Matt Risinger remodeled that house, is CZ2. I’m in CZ5, and the OP is in CZ6.
In a cooling climate I understand the rationale for it. But is it still necessary in a warming climate?
You are not quite there but it sounds like you don't have a WRB on the outside of your sheathing. I would read through this:
https://www.greenbuildingadvisor.com/article/insulating-walls-in-an-old-house-with-no-sheathing
In your case, the housewrap in the cavity is definitely a good idea. Along with air sealing a bit your leaky sheathing, it will prevent liquid water from getting into your insulation. Crinkly housewrap is a also a good idea.
When insulating old houses, make sure to fix any window and door flashing details. These tend to be non-existent in old houses and any bulk water leaks could result in a moldy insulation mess.
Real 2x4 walls tend to be hard to insulate with mineral wool as the batts are too wide to fit well (they crinkle up). You would have to trim them all to fit. Instead of trying to trim 1/2" off, I find it easier to buy batts for 24" oc framing, turn them sideways and cut them to width. This also works much better with the uneven stud spacing.
A quicker approach, and much better for air sealing, is to get the walls dense packed with either cellulose and fiberglass. The dense packing does wonders in tightening up these types of old buildings. You would still need the housewrap in the cavity.
P.S. Dimple mat in heating climate is a very bad idea on the outside of the wall. Since it is a vapor barrier and you are installing it on the cold side, it creates a condensing surface that will trap moisture.
Thanks for suggestion with the horizontal insulation! I was dreading the 1/2" cuts.
I would consider dense pack but I'm DIYing it one room at a time so it won't really make sense to call someone in for each room. It can't be done from the outside because the existing walls are actually insulated with 1/2" of old school rock wool, plus there is random blocking and other obstructions throughout the walls.
If you are DIYing it, a quick way to pick up some extra R value is to use foam for your furring strips:
https://www.finehomebuilding.com/2014/11/26/breaking-the-thermal-bridge
If available, you can also use R6 ZipR ripped to 2" wide strips. This would bump up your assembly from about R18 to R21. Not much but something.
Yeah I was planning on doing the furring strips as a thermal break, but was going to use 1.5" rigid foam to get my stud bays out to the 5.5" to use 2x6 batts. This R-sheathing looks interesting, and would certainly simplify fasteners having the OSB surface. However, it doesn't look like it's available in Canada yet?
If liquid water is reaching your sheathing you have a problem that needs to be addressed. Otherwise your sheathing and framing will rot out, your fasteners will rust and fall apart. Those parts aren't meant to get wet. Tyvek inside the sheathing won't help.
What you're looking for is a wind barrier. Tyvek works for that, it's just expensive for that purpose. Older houses used rosin paper, cardboard or even newspaper for the wind barrier.