Let me start by saying that I am a builder with over 15 years experience. I am thinking of doing something to my house that gives me concern. I have run it by a number of my associates who haven’t been very helpful, so I thought I would run it by the sages at FHB online.
I bought a house in central Ontario, that has exterior walls constructed as 1/2″ drywall, poorly installed 2mil poly, 2×4 framing with R-12 fg-batt insulation (also poorly installed), 1/2″ tentest sheathing without windbracing, and aluminum siding.
I am systematically replacing the drywall which allows me to repair/replace the insulation and vapour barrier and I would like to replace the sheathing with osb/plywood to provide structural integrity, and add a rigid foam to the exterior and reside.
Does anyone see a problem with putting foam up against sheathing like this? I am concerned about having a vapour barrier on both sides of the wall and having water condensing on the sheathing. I am thinking of installing the house wrap between the foam and sheathing instead of over the foam but I’m sure this wont provide me much protection.
On the other hand, maybe I’m worrying too much and I should just be careful to tighten up any sources of leaks on both sides. What do you think?
Replies
Wog, this (exterior rigid foam insulation) is becoming standard building practice in Ontario, because 1" allows builders to meet current code insulation requirements while keeping a 2x4 wall. I'm in the outer-outer-GTA.
I too was concerned about the double vapour barrier, so I emailed one of the companies making the insulation, Atlas Roofing products.
Their response, essentially, is that if you keep exterior moisture from penetrating the exterior shell, and you have a proper interior vapour barrier, then inside the wall will never be wet enough to cause concern.
Seems almost reasonable, but perhaps a little...hopeful...to me. We'll see if in 40 years, houses built in this time period start rotting out.
Regards,
Tim Ruttan
P.S. Here's the exact email I received:
The existence of the 2 vapor retarders is not a problem when the exterior sheathing and the interior vapor retarder are installed per their respective code requirements and code designated exterior flashing procedures are followed.
By properly excluding the intrusion of moisture from the outside of the structure, the moisture commonly present in building materials does not promote the growth of organisms or promote wood deterioration. Properly installed interior vapor retarders will prevent additional moisture from entering the wall system from the interior.
The intent is to prevent moisture entry from sources outside the wall structure - from either side. The equilibrium of moisture content in the new materials will be reached soon after the building is finished and will be at levels which will not be harmful to the wall system components.
Thanks
In your climate, it's safest to keep the VB on the interior wall, although exterior VBs are becoming more cmmon as construction practices and materials improve. There are ways to get around this regarding the location of the dew point within the insulation on the exterior facade of the wall...but fuggetabout that for now.
To me, the hardest part of the process will be adding an integral vapor barrier on the interior face ot the exterior walls where the exterior wals meet interior partition walls. There are ways to do it, but it depends on the details.
Were I you, since you're gutting the interior, let me offer this option:
1) Gut to the studs.
2) Install 4' by 8' sheets of foil-faced RFBI over the studs. The sheets of RFBI (I prefer polyisocyanate @ R7.2-ish per inch) can break over the stud bays, they do NOT have to break over a stud. When installing the sheets, gap them by about +/-3/8ths of an inch.
3) Use a sharpie marker to make a tick mark in the face of the RFBI showing the centerline of the studs you just covered up.
4) Use canned foam to foam the 3/8ths inch gaps you left earlier. Use AL tape to tape over the foamed gaps.
5) Run horizontal furring, 16" oc, up the exterior walls. Use screws to screw through the furring, through the foam, and into the studs. I cut furring strips to the desired width (1-1/2" to 2-1/2" for drywall edges, wider fr around windows/doors as a trim backer) from sheets of 3/4 CDX. Also, the furring strips don't have to break over studs, but each strip should span at least two studs.
6) Cut round access holes through the RFBI to access the stud bays, and blow dense pack cellulose insulation into the bays.
7) Go back and use canned foam to foam the access holes, or cut circles from scrap RFBI, and foam them into the holes. Cover with Al tape.
8) Sheetrock over the furriing strips as usual, using 1-1/4" screws.
You'll have to add jamb extensions to your windows, don't forget the furring strips around the perimeter of the windows/doors so you'll have something to fasten your window/door trim to. You'll also have to build out your electrical boxes.
Sounds like a lot of work, and in some ways it is...but you'll have an amazingly tight house which is something you'll never get with standard FG batt construction. Air infiltration is a thermal killer.
Attend to moisture generators in the house. Baths, kitchen, etc.
Use canned foam to foam any penetrations between the living space and the attic (if attic space not conditioned), as well as the basement (if not conditioned) and the living space.
All that said...I'm not familiar wth the exterior "tentest sheathing", but I'm guessing it's not structural? If not that opens a proverbial can of worms regarding how much interior demo can take place at one time without adversely affecting the structural integrity of the house. That might make pther options more viable.
If you;re looking at stripping the exterior to the studs, then you might not have to worry about the interior VB. there is a possibility that you can leave the interior drywall and vb in place, and gut from the exterior of the house.
Remove siding and insulation, add sheating/RFBI, then pump cells inot the stud bays from the exterior. then add new siding.
Cells handles vapor that might get into the stud bays better than FG, and with exterior insulation that's thick enough, the dew poitn should not occur within the thickness of the cells (inside the stud bay itself).
Edit: korrecktid speling
Edited 6/13/2004 11:45 am ET by Mongo
Thanks
Check out the Par-PAC system. http://WWW.parpac.com. I did this on my 1800 sq. foot addition. It went up pretty easy, seals tight as a drum. You can use the blower they have at Home Depot or Lowe's. I got the plastic from National Fiber, 1800-282-7711.
Thanks
Wog
When I built my house 20 years ago, no one did the exterior insulation.
I framed with 2x6 studs and sheathed with OSB. Then installed the standard tar paper. Next came the 1 in insulation, with the siding on top.
20 years later, still no problems.
Now, every builder is doing it around here.
Jeff