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What to do with tongue & groove sheathing, cavity insulation, etc?

TomChaplin | Posted in General Questions on


Many thanks for your excellent website!!! It’s been a big help!

I own a house built in 1953 about 10 miles north of Jay Peak in southern Quebec, elevation 715 feet, zone 6 / 7.
I started renovating our master bath which is now stripped down to the 16 o.c. studs. We are planning to install a larger window in the bath. Next spring & summer we are planning to re-side with Maibec cedar siding and replace all of our windows and doors. In the process of working out details for the bathroom, I was slow to realize I also needed to work out my approach to insulating, siding, etc. for next year’s project because of the bathroom window. I am not a professional contractor, but plan to do the work with a carpenter friend.

House is 1 story, roughly 55 ft wide by 40 feet deep with 2×4 exterior stud wall sheathed on both sides with 3/4 inch thick (6 ½ inch wide) tongue and groove pine. We have cement shingles on the sheathing. Wall insulation is minimal, a few inches of old mineral wool. There is no poly in the walls or ceiling. The exterior wall is about 8 1/2 feet from the foundation sill to the underside of the roof.

Foundation wall has a rubberized asphalt coating covered with 2 inches of XPS rigid foam (4×8) panels covered in stucco. We are surrounded by woods and have had some problems with carpenter ants but no sign of termites as yet.

Attic was reinsulated, rewired, screened at the eaves, with baffles for venting, all tongue & groove seams and cracks on attic floor sealed with great stuff and insulated with cellulose to 16 inches minimum depth a couple years ago.

House has no ventilation system, is heated with electric baseboard until real cold arrives and then primarily w/ wood w/ electric back-up in colder months. All interior walls with a few exceptions have t&g pine on both sides beneath the drywall which complicates the installation of air exchanger ductwork, etc.

The bathroom window did not have a header. Having worked on the house for some years, I feel there will be more windows (perhaps all) without headers (we have about 20 windows and 2 or 3 doors). It seems likely we will have to open up around the windows inside and out to deal with the headers.

Questions :

1) Is the exterior tongue & groove such a liability as an air barrier that I should remove it & resheathe?
(I figure I have roughly 1000 feet to caulk or tape on the back side of the house alone).

2) My budget is limited to about $50,000 for the insulation, siding, windows, doors & labor. I want to add exterior insulation to reduce the amount of thermal bridging, but is this an opportunity I shouldn’t pass up to also re-insulate the cavities from outside before I continue with the re-siding work? I am not a big fan of spray foam because of off-gassing and difficulty of getting to wiring afterward. Should I reconsider?

3) What about the exposed bathroom ceiling T&G? Should I caulk, foam or tape it?

I am thinking that once the t&g question is resolved and I have finished with my sheathing / air barrier that I want to put felt paper as the next layer because it has proven durability & is forgiving, seals better against nail and staple holes and lasts longer than plastic wrap.
I read that rigid foam over felt doesn’t give a good 2ndary drainage plain so am willing to pay for crinkled plastic housewrap to serve that function but it seems plastic wraps don’t really work as air barriers and perhaps not as vapor barriers if they are so susceptible to damage by nails and staples.

I am reluctant to use EPS given our ant activity. Foiled polyiso is too much of a vapor barrier for my liking.
So I am thinking of Owens Corning Foamular Codebord 4′ x 9′, 2 inches thick. I have had some difficulty determining the actual vapor permeance but it looks like around .85 perms or 45 ng.
4) If I paint on my vapor retarder on the drywall does that look like a wall that would be functional
in terms of being able to dry itself out, resist air movement and keep the cavity warm enough??

5) Will 2 inches of XPS be enough on the exterior??

6) The bathroom window is a double-hung aluminum clad wood window made by Lepage with low e glass, double glazed. It does not have nailing flanges. I think I read somewhere that in outie windows I should use masonry clips. It seems that nailing flanges on windows are a less common in Canada or is that only for wood windows?

7) Do you know of a resource that describes how to seal non-flanged windows?

8)Should I be making my own flanges?

9) The window 36×48 is quite heavy. With even larger heavier windows such as picture windows should the window buck or box be blocked underneath to strengthen the construction?? Is that standard procedure?

Thanks for any help you can give me!


Tom Chaplin

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  1. GBA Editor
    Martin Holladay | | #1

    You have lots of questions.

    First of all, tongue-and-groove boards are not an air barrier, and it doesn't make sense to caulk the cracks. Tongue-and-groove boards make perfectly acceptable sheathing.

    You have some very basic questions. I suggest you start by reading these articles:

    How to Install Rigid Foam Sheathing

    Questions and Answers About Air Barriers

    Using Rigid Foam As a Water-Resistive Barrier

    All About Water-Resistive Barriers

  2. user-869687 | | #2

    In your climate you could really use R-40 walls. It would take 6 inches of polyiso to get there, and that is doable but maybe not within the budget. Another way to get there is with Larsen trusses, or else a freestanding curtain wall of 2x4 framing. That would be a new wall surrounding the house with enough cavity space to get your desired R-value in cellulose. This could be more economical than polyiso and more resistant to ants. All windows and doors would be reinstalled in the new outer wall. You'd have to look closely at the foundation and roof details to make this work, and it would dramatically change the look of the house. Something to think about anyway.

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