Re-siding with vinyl: To rigid foam or not to rigid foam?
I apologize for the length of this post in advance, but many posts I have read have SOME of my circumstance, and not all. I have read a few articles regarding this topic on this site and others, and while I have learned a lot, I don’t know that I have a definitive decision on how to proceed.
I am in upstate NY, Zone 5-6, 800 sq/ft end unit townhome, single floor. 2X6 construction consists of original (1988) cedar siding, rarely re-stained when we purchased, 1/2″ OSB sheathing, no housewrap, fiberglass batts, polyethelene sheeting, and then drywall.
The northern wall of the home is my back yard and has a roof overhang of 18″. There are several trees, so NO sunlight hits the back of the house. I have two windows back there that I replaced last year.
My wife and I found some surface mold on the inside of the OSB sheathing, and it looks like the rim joist on the back of the house has some moisture impact as well. The only mold on the inside of the sheathing is directly above where the heating duct comes through the floor. The rim joist is impacted the entire length of the back of the house.
We had a mold inspector come in and have received quotes from remediation companies and we figured the mold came from a combination problem:
1) No sun on the back of the house to dry
2) Siding not having been re-stained recently/cracks
3) gutter guards left in causing rain to overshoot gutters and splash back on house
4) patio paver patio that grades against the house due to frost heaves
5) dehumidifier in basement running, but pump failed. RH in basement 70% at time of mold inspection (now 30-35%)
6) No drip cap installed above newly installed windows (I thought the overhang was enough as the windows were new construction, I flashed them in good, and the tops are 12″ from the soffit.)
7) P/O had no insulation along rim joist in basement (typical temp around 55 degrees)
8) poured concrete foundation wall has no insulation with 18″ exposure outside? (Not sure if this is a factor but someone suggested it)
Basically, we purchased new vinyl siding to be installed, with drip caps above the windows and housewrap. We are going to have a remediation company remove the mold and encapsulate spay. We replaced the dehu and check the RH daily. I pulled up the entire patio, laid new soil, stone, and sand; and graded away from the house at the appropriate rate.
We are considering quotes to spray foam with HeatLok soy based closed cell foam along all rim joist spaces after the remediation is complete and the siding is installed. We are also considering having 3/4″ XPS blue foam board installed under the new vinyl siding and over the housewrap. The application does not allow for foam thicker than 3/4″, and we do not have the budget to completely rework all the openings to accommodate thicker rigid foam.
Basically, I’m looking for opinions on whether I am covering all the bases, and perspective on the rigid foam board. My siding installation is scheduled for Dec. 3rd and I need to let the contractor know right away if we are doing the foam.
From what I have read on here, I shouldn’t because it isn’t thick enough to provide an R-Value high enough to keep the sheathing warm and with the polyethelene on the inside, it will create a vapor lock in the walls. With the problem wall being a northern exposure with no sun, that could continue my problems.
Thanks for the help on this and all the time invested in previous posts. As a newer homeowner, it makes a big difference you all rarely get to see the impact of.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
It seems like you know the answer. In your area, 3/4 in foam is not enough, not matter which type you choose. And as you noted, exterior foam and interior poly don't belong together. I know you've been reading a lot about this, but I want to make sure that you saw this article, which clearly answers your question:
https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing
There have been some people in similar situations who have used rigid mineral wool panels instead of rigid foam on the outside. The rigid mineral wool panels breath and allow the sheathing to dry while still adding some continuos insulation to your house along with the energy saving benefits that brings.
I’m pretty sure there is a GBA article about this, and I remember the crew had issues with birds nesting in the insulation before the siding went up. Hopefully Brian shares Martin’s magic article finding mojo and can provide you with link to that article.
Bill
Here's Martin's "all-about" on mineral as exterior insulation. I'm not sure about the nesting birds. Perhaps that was in a Q&A.
https://www.greenbuildingadvisor.com/article/installing-mineral-wool-insulation-over-exterior-wall-sheathing
Thanks for posting that link Brian. The bits about bird nesting might have been in a Q and A. I remember talk about the birds making nice round holes and the need to stuff leftover mineral wool into the holes as the siding went up. The birds were a surprise to the crew, and it stuck in my head.
Bill
IMO, both external EPS (unfaced) and mineral wool are permeable enough that any amount is more beneficial than none. Both are more than 1 perm, which doesn't trigger the requirement for the full amount of recommended exterior foam and a low perm interior.
Take the opportunity to air seal (tape) the sheathing.