Hello all - So I am having a lot of frustration trying to determine WHERE to locate my house wrap (It's going to be Tyvek Drainwrap or Greenguard Raindrop). I will have 2x6 studs with mineral wool insulation between them, then 5/8" OSB sheathing, and then either wrap then 2" foam + 3/4" Firing strips OR 2" foam, and then wrap + 3/4" Firing strips. I am worried about 2" of XPS or EPS foam being against my water sensitive sheathing while I feel the house wrap would protect the sheathing from moisture better than on the outside OVER the foam. Please give your opinion - and if any of you have done this...what did you do?!

If you have a sufficient thickness of foam to raise the temperature of the sheathing enough to prevent condensation, you should not have moisture at the sheathing, unless you have poorly done flashing. (The amount of foam necessary relates to your climate.) If so, install the wrap no the outside of the foam where it will help keep the foam dry. Some practitioners install it next to the sheathing - so take your pick.

Posted By CTSNic on 26 Jun 2015 12:10 AM
Hello all - So I am having a lot of frustration trying to determine WHERE to locate my house wrap (It's going to be Tyvek Drainwrap or Greenguard Raindrop). I will have 2x6 studs with mineral wool insulation between them, then 5/8" OSB sheathing, and then either wrap then 2" foam + 3/4" Firing strips OR 2" foam, and then wrap + 3/4" Firing strips. I am worried about 2" of XPS or EPS foam being against my water sensitive sheathing while I feel the house wrap would protect the sheathing from moisture better than on the outside OVER the foam. Please give your opinion - and if any of you have done this...what did you do?!

If you seal the house at seam house wrap act as an air barrier. House wrap should be installed over the sheathing and behind the sides. Now, many people avoid installing it over siding because of fear of its material. A good quality House wrap can be installed on any kind of material. You must get it installed from General contractor. I have get it installed from http://www.stonebridgecontracting.com/. There are many more places from where you can get it installed. An insured and certified contractor is ideal for this task.

If you have a sufficient thickness of foam to raise the temperature of the sheathing enough to prevent condensation,

Almost invariably such a design prevents condensation most of the time, not all. So call it "reduces condensation". And then realize that even sustained 70% humidity can cause a problem. So while foam helps, IMO it's still important to get the air barriers and vapor retarders correct (ie, breathable in both directions, more so to the cold side). Cellulose is also a plus (vs less absorbent fiberglass).

For air sealing, I'd tape the OSB (taped rigid barriers outperform films) and at least consider putting it on the inside and then also taping the foam (on the outside).

I don't understand Drainwrap and a vent gap, but the OP is probably already finished.

Yes, this is a very old post and the OP is likely on to other things... For future reference, the answer on how best to accomplish a building assembly largely depends on the climate that the building is located. In other words, you need to know the relative humidity and the temperature exposure of both the exterior and interior sides of the building assembly before you can actually determine what is the best insulation and vapor barrier strategy. As others have said, the goal is to minimize the likelihood of having moisture enter and condensate within the building assembly. Joe Lstiburek has written extensively on this subject:

Moisture Control for New Residential Buildings

We have software that is based the ASHRAE dew-point or Glaser methodology for analyzing and evaluating moisture accumulation and drying within building assemblies:

Borst Building Assembly Moisture Analysis Software