Lots of good reading at buildingscience.com

Yeah, I think you have to spend some time on this...

Three issues:

1. walls have to dry from the inside or the outside. You don't want a vapor barrier on both sides of the wall or it will rot.

2. If you have foam on exterior that's not permeable, you must be careful about having enough insulation that the water vapor doesn't condense on the wood sheeting - or again it will rot.

3. Tyvek is water vapor permeable, but R-Max isn't (it's foil faced). Not sure what it is that you are trying to do there.

I believe you really need to re-think your approach or you will have a big ugly problem with mold and rot.

Jake,

Currently as the house stands I have a double vapor barrier with the foil polyiso and the poly sheeting on the inside. I am trying to take the correct approach here while I replace the siding, and address anything in the inside at this time. For what it’s worth I have noticed a number of homes in my area which where built around the same time and probably by the same contractors with the same wall detail have been getting new siding and they would remove the old siding, apply house wrap new siding and call it a day. Obviously these contractors that are doing the siding replacement are looking to just get in and out, not saying it’s right or wrong.

I understand what you are saying...but:

Here is the abstract from Building Science: At issue is the dew point of the back of the sheeting. That has to be designed based on your average outdoor winter temps.

Here is some of the debate about the issue on from Green Building Adviser

If you use the foil face of the foam as the drainage plane, you have to be clear about detailing the joints and such.

Again, I think you need to do some significant homework to understand where the vapor barrier is, what the dewpoint on the back of the sheeting is, where the drainage plane is, etc. You will also have to think through how the windows will attach, etc. What was done originally obviously wasn't the right thing... from the T1-11 to the double vapor barrier (and possibly the 3/4 poly-iso foam board on the exterior also).

Why are you thinking of going with Roxoul?

I choose the Roxoul based on the higher r value/in, hydrophobic properties, fire resistance, workability, and I got it for next to nothing.

I skimmed through the article from Building Science, I want to look through it more thoroughly, based on my calculations of average winter temperature, indoor temperature and RH, and desired wall r-value I come with being just about spot on of r15 batts with r4 foam (3/4” XPS), but don’t quote me on that as I need to look at it more thoroughly. If I recall reading an article in that XPS beats PolyIso in cold temperatures, I don’t remember the specifics but the r-value would increase during those months.

Can you enlighten me on the drainage plane? And how/where the windows will attach? I figure they would be tied into the tyvek.

I opened a wall up to see what I am dealing with, now this was a north facing wall where the T1-11 was in decent shape, aside from the bottoms for being weathered and rodent/insect infiltration. The wall cavities look clean and dry, the rim joist/back of T1-11 in that area had some signs of moisture around the infiltrated areas. The polyiso did seem to shrink a bit, all seams have atleast a 1/2”+ gap or more, all studs were exposed. If there were any condensation on the interior side of the polyiso from the winter it would have been very minimal as I did not notice much/if any signs of dried moisture, but was probably more so from the poor fitment. The existing poly sheeting, back of drywall, fiberglass all looks in good shape, no signs of moisture in the few cavities I had opened. Proper air seating should tighten it all up nicely. So probably going to just use the current fiberglass, scarifie the little r-value for extra warmth on the sheathing side, is that is more advisable then put the new 1/2” plywood and 1” foam overtop.

Remove all existing material, inspect for any signs of moisture. Leave the poly sheeting(it’s lasted 30+years without problems), air seal penetrations in drywall and framing with low expansion window and door foam.

Reuse the existing fiber glass (rather then replace with R-15 rockwool)

1/2” plywood caulk and tape seams

1” Dow Tuff-r tape seams

Tyvek house wrap

Windows tied into the house wrap