Exterior wall with no insulation (& a missing window). How to do vapor?
January 24, 2017 7:48 PM   Subscribe

Older house in Portland OR- no insulation at all in two particular exterior walls (just siding, 2 by 4 joists, solid wood wall). Trying to think through vapor -- was planning on 2" polyiso, foil on one side. I should put the foil facing inside rather than out, right? And should the poly go between the joists (between siding and solid wall), or inside the wall with furring to attach sheetrock to? Or should I do the latter plus put unfaced pink fiberglass between joistsl?

My house in Portland OR (climate zone 4C, avg. winter temp 41 degrees F, summers very dry), built in 1900 but the problem is in a later addition. I knew there was a draft in the largish closet, so I removed the sheetrock (and a layer of soft wood fiber wallboard) and found this -- a window had been removed, replaced by nothing. Only a badly patched and leaking bit of exterior siding sits between the outside and the wallboard.

So there are 2 exterior walls. I have access to the interior of the shorter missing-window-side wall since I have to replace the very poorly patched siding -- but not to the longer exterior wall. I've read a lot online including this helpful AskMeFi post but am drowning in info at this point. I'm already caulking the hell out every conceivable gap, notably between the interior wall boards.

I kind of have to do some kind of insulation on the inside of the longer wall, unless I rip open the whole thing which is best avoided. So - foil facing in? (plus furring and sheetrock?)

On the missing window side, I can stuff some kind of insulation between the joists before I replace the siding, or not. Polyiso? Batting? Great Stuff? Is plywood fine to patch that window hole in the wall? (I'm replacing the entire course of siding on the outside. I'm guessing unfaced pink fiberglass between joists and polyiso on the inside of the wall is best, but if I was sure I wouldn't be posting this. Thanks!
posted by msalt to Home & Garden (8 answers total) 1 user marked this as a favorite
Hi, renovation pro here (unfortunately not in Portland).

Learn what the code says in your area. A common variation in the code is all about how to do the vapor barrier.

I'm pretty sure POrtland has both lots of online forums for homeowners doing DIY renovation, and some supportive organizations (maybe through the tool library?) who would be able to point you to the resources your need. In my town I can often just go ask the building department when I encounter a question I"m confused about.

There are also some excellent books on weatherizing and insulation (it sounds like you've read a bunch already), though I can't stress enough how advice specific to your climate is going to vary.
posted by girl Mark at 8:00 PM on January 24, 2017 [1 favorite]

Good suggestion. Portland's section on insulation seems to have been rescinded, but the state code requires vapor barrier on the inside: Section N.1104.9.1 (copying is blocked for some reason).

The amount of insulation is unclear -- searching through the code I found everything from R13 to R21. But nothing about the strategy of whether to put insulation between the joists or on the inside of the framing wall.
posted by msalt at 8:48 PM on January 24, 2017

Just to clarify -- I guess I was surprised to find no sheathing or insulation at all, just the layer of exterior siding on the outsides of the 2x4s in the exterior wall, and a solid wood slat wall on the interior of the 2x4s. is this typical?
posted by msalt at 8:59 PM on January 24, 2017

People build things badly all the time, especially if your house was at one point a rental with a crappy landlord, or maybe was once owned by a poor person, who might have hired underqualified cheap help or done work themselves that they weren't qualified to be attempting. In the past people didn't take insulation very seriously, either.
posted by girl Mark at 10:55 PM on January 24, 2017

It's either poly or foil but not both. You never want two vapour barriers in your wall.

msalt: "the layer of exterior siding on the outsides of the 2x4s in the exterior wall, and a solid wood slat wall on the interior of the 2x4s. is this typical?"

This was a not unknown building method at one time (pre WWII).
posted by Mitheral at 7:58 AM on January 25, 2017

Thanks. I'm puzzled though -- almost all of the polyiso I've seen at Lowe's and Home Depot has the foil on one or even both sides. What am I missing here? This is the stuff I'm talking about.

Reading more on all of this, I see that many people recommend keeping your sheets of rigid foam continuous -- either on the outside of sheathing or inside the wall -- as opposed to what some call "cut-and-cobble" insulation between the joists.

Some of that though is the bias of a professional construction person -- cut and cobble is far too time consuming for a pro, but might make sense in some for a cash-strapped home owner (such as me). Solid sheets seem more effective though due to heat bridging at joists, and just having a lot more edges to seal (which can unseal as wood expands and contracts). This implies that it's better to use longer lag screws to go through your sheetrock and foam into joists instead of using furring.
posted by msalt at 8:34 AM on January 25, 2017

There are three things you want to think about: your air barrier, your thermal insulation, and your vapour barrier. You need an air barrier to stop air from moving through the wall because that's a significant way heat is lost. This is a common problem in older buildings.

So, foil-faced polyiso with taped seams on the inside of the 2x4s plus fiberglass or other batts in the wall is a very good insulation system for your climate. Yes, it is far better to screw right through the insulation board for drywall unless you plan to use more than 2" of foam (in which case you should install furring strips on top of the insulation not between pieces).

Batt insulation is so cheap that it really doesn't make sense to not use the batts between the studs as well. The cheaper method would be batts plus a plastic sheet vapour and air barrier with taped seams, but that may not be enough insulation.

Cut and cobble is not a great idea if you can't spare the 1-2" on the inside of the wall. Spray foam is better to get full coverage and sealing, but the climate effects of most spray foam blowing agents are quite large.
posted by ssg at 2:41 PM on January 25, 2017 [2 favorites]

Thanks! That's the plan. Plus gallons of caulk at every possible air gap, on the outside siding and the inside wall.
posted by msalt at 3:40 PM on January 25, 2017

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