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Gaby:
I saw your other post and I’ll try to help.
It was very important that you stated your locality. Quebec is very much in what is catagorized as a “Heating Climate.” Therefore, you have it about right in your wall’s cross sectional make-up, infact, you might have a little redundancy built-in. Here goes:
What you are trying to do in a wall is stop air leaks and moisture diffusion.
The Tyvek is a moisture barrier. If you use it, and I think you should if you have it at your disposal, then it is a little redundant because of the FURRING AND EXTERIOR INSULATING SHEATHING. But, this redundancy is good being that you live in such an extreme climate, just remember to seal it well at the top and bottom plate locations.
Extruded Polystyrene is good because it provides and external air barrier AS LONG AS YOU SEAL ALL SEAMS. One inch of extruded poly has a low perm rating of about 1.2. Your going w/ 1-1/2″ so you’re better off. This thickness of insulation keeps the temperature of the wall cavity high enough to prevent warm house air from condensing should it get into the wall cavity. This E.P. also adds a redundant “capilary break” to the wall, thus any or all wicking action (of moisture) is eliminated – and stopping water from entering the wall cavity is the ultimate purpose. I should say here that the Tyvek is also eliminating any ‘wicking’ so if you had to choose one or the other (no redundancy) then keep the extruded polystyrene because of its added insulative qualities (over Tyvek).
So, the rigid foam, seams sealed, is fine w/ the furring and shingles; this with the Tyvek is even better.
You mentioned the air/vapor barrier of 6mil poly on the inside, this is essential because it is stopping the moisture from entering the wall at the other side (interior). Just remember to do ceiling too. It all must be continuous. Seams and joints kept to a minimum and any seams sealed w/ special seaming tape made for that job. All penetrations of the barrier, plumbing, electrical boxes, and what have you, should be sealed.
And remember, there are vapor-retarding paints that can provide vapor barriers as well; this is especially helpfull if you have a conc. walled basement “out of the ground.”
This is really a clear cut case since you live in a clear “heating climate.”
If construction isn’t so far along you may wish to inspect and/or seal the rim joists at the deck to wall locations where the sheating might meet. An acoustical caulk is recommended — then the e.p./Tyvek.
OK!
Replies
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Gaby:
I saw your other post and I'll try to help.
It was very important that you stated your locality. Quebec is very much in what is catagorized as a "Heating Climate." Therefore, you have it about right in your wall's cross sectional make-up, infact, you might have a little redundancy built-in. Here goes:
What you are trying to do in a wall is stop air leaks and moisture diffusion.
The Tyvek is a moisture barrier. If you use it, and I think you should if you have it at your disposal, then it is a little redundant because of the FURRING AND EXTERIOR INSULATING SHEATHING. But, this redundancy is good being that you live in such an extreme climate, just remember to seal it well at the top and bottom plate locations.
Extruded Polystyrene is good because it provides and external air barrier AS LONG AS YOU SEAL ALL SEAMS. One inch of extruded poly has a low perm rating of about 1.2. Your going w/ 1-1/2" so you're better off. This thickness of insulation keeps the temperature of the wall cavity high enough to prevent warm house air from condensing should it get into the wall cavity. This E.P. also adds a redundant "capilary break" to the wall, thus any or all wicking action (of moisture) is eliminated - and stopping water from entering the wall cavity is the ultimate purpose. I should say here that the Tyvek is also eliminating any 'wicking' so if you had to choose one or the other (no redundancy) then keep the extruded polystyrene because of its added insulative qualities (over Tyvek).
So, the rigid foam, seams sealed, is fine w/ the furring and shingles; this with the Tyvek is even better.
You mentioned the air/vapor barrier of 6mil poly on the inside, this is essential because it is stopping the moisture from entering the wall at the other side (interior). Just remember to do ceiling too. It all must be continuous. Seams and joints kept to a minimum and any seams sealed w/ special seaming tape made for that job. All penetrations of the barrier, plumbing, electrical boxes, and what have you, should be sealed.
And remember, there are vapor-retarding paints that can provide vapor barriers as well; this is especially helpfull if you have a conc. walled basement "out of the ground."
This is really a clear cut case since you live in a clear "heating climate."
If construction isn't so far along you may wish to inspect and/or seal the rim joists at the deck to wall locations where the sheating might meet. An acoustical caulk is recommended -- then the e.p./Tyvek.
OK!
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This is related to "Insulated sheathing causes moisture trap"
You guys have me all confused and worried. I'm renovating a 1942 home.I spoke to people around here(Quebec,Canada)about increasing the R value in my exterior walls(2x4).Everyone says use 1.5" rigid Styrofoam and Tyvek. From inside it's like this...Drywall,6 mil poly,batting,10"x7/8" plank sheathing(existing),1.5" styro,Tyvek,1x3 ferring,shakes.Now is this bad or O.K?Should I pull off the Tyvek(shakes are not on yet).Why do Tyvek instructions say to put over sheathing or over insulation?What about air infiltration if I don't use Tyvek? Help!!
Gaby
*Gaby:Here's my *opinion*.There is some great talent on this board but also some of those same talents tend to preach gloom & doom.I doubt that you are really going to get your 1942 house tight enough to have to worry about much about trapping moisture in the walls.The Tyvek is designed to breath. Both the Tyvek and the Styrofoam is going to be well perforated by the firing strips(strapping) and siding nails. The styro will leak at the seams - unless you seal them really well. The poly won't be air tight - there will be leaks at the room corners, at electrical penetrations, around the windows, possibly at the top and bottom plates(below the baseboards), etc., etc. Probably there will be air infiltration through the top & bottom plates at the site of wires and pipes. The fiberglass batt insulation (I am assuming) will allow some air circulation as well. Your "sheathing" is better ventilated than any used today. Get the picture?I think your proposed wall configuration is fine, possibly even great as it has the airgap between the siding and the Tyvek for a drying effect. On the other hand, I live in a much warmer (and probably damper) climate than you so take what you read here with a grain of salt!
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Thank you very much guys!! I can finally get some sleep!
Gaby