Replies

1. Expert Member
   Akos | Mar 25, 2024 09:43am | #1

   Ideally in zone6 you would want a bit more exterior rigid insulation:

   https://buildingscience.com/documents/building-science-insights-newsletters/bsi-100-hybrid-assemblies

   Your plaster with many layers of oil paint is probably pretty close to a vapor barrier, so you can get away with less but going for the above amount (R7.5 for 2x4 wall) will avoid all condensation risks. If you go with polyiso or GPS that would be 1.5" or so, so not much extra space or cost.

   I don't see a way of doing this retrofit without stripping the siding, simply too many gaps otherwise. Normally with board sheathing I would recommend a peel and stick to seal it up but your dense packed walls are already pretty tight, so this is less of a concern. Add on there taped seams on the polyiso and and I think you should be good.

   Vinyl siding has no weight, I don't see the issue of installing it over slightly thicker foam.

1. Expert Member
   Michael Maines | Mar 26, 2024 10:28am | #6

   I've heard people claim that cellulose is an air barrier several times lately, which is concerning. It is absolutely not an air barrier. The best cellulose installer I know says he can get an old house down to around 3 ACH50 with air-sealing and installing cellulose, which isn't terrible but not great. I doubt that 9 out of 10 cellulose installers pack as tightly as he does. If you are stripping siding anyway, now is the perfect time to add a true air barrier to your assembly, significantly increasing energy performance and building durability.

1. 
   ScottRokisPhoto | Mar 26, 2024 12:07pm | #7

   Hi Michael,

   I'm not sure if something got crossed here. I wasn't saying anything about using cellulose as an air barrier. I am just stating what is on the house currently before I bought it two years ago. This is a 1905 build and in the process of discovering what is behind my siding I found that there is cellulose blown into the walls. Not all of them though. they've missed some of the cavities around the one window space I opened up. I'm only considering the cellulose from an insulation perspective and that at best, it would be R13 based on the 2x4 cellulose guidance.

   I'm new to this, but I'm assuming you are referring to an air barrier such Tyvek Commercial Wrap or another of the similar options on the market. I have planned for this layer between the vinyl and rigid foam.

   Does that make sense?

2. 
   ScottRokisPhoto | Mar 25, 2024 03:32pm | #2

   Thank you for the reply. I was initially dissuaded from poly iso due to the R value loss. But after more reading, I think I'll just go thicker than 1" and be ok. I looked at GPS just now (hadn't heard of that option yet), but it seems like poly iso will be fine.

   I've seen the houses on either side of me going both ways. One left the siding on and one removed it all. After watching their process, I was more drawn to stripping the siding and having a cleaner install against the sheathing. Glad to hear that is your recommendation as well.

   For the vinyl, do you think 2" of foam is too much space? I know the instructions are 1", and people generally stick closer to that 1". But just curious if there people are actually installing vinyl siding over 2" foam without issue.

   Thank you,
   Scott

1. 
   climbing_carpenter | Mar 25, 2024 09:07pm | #3

   Scott,

   When you're considering exterior insulation, fastening cladding should not be the primary concern- the primary concern should be getting the math right. And in climate zone 6, your wall will require 30% of its r-value be outbound of the sheathing. Of course, more is better. Possible that you have insulation outbound of the sheathing only and use the existing stud cavities as a service cavity.

   Strapping would alleviate any woes you experience fastening the cladding- would also provide a nail base and an air gap for a different form of siding, such as LP Smartside. You may wish to reconsider vinyl.

   Mento 1000 is superior to any Tyvek. If you seek something more readily available, look into Blueskin VP100.

   Strip the existing cladding, no-brainer!

3. 
   ScottRokisPhoto | Mar 25, 2024 10:01pm | #4

   Thanks for the follow-up.

   I've evaluated lots of siding options...considered the pros/cons to all. For this particular home, vinyl is the best choice. Although, for a future home, I'll probably choose something else. I was trying to avoid the furring strips to keep the surface behind the vinyl smooth and flat. I know I could fill in the cavity created by furring with foam, but that all seemed messy. So, I went on a path to maximize the outbound insulation. 1.5" is the happy medium of most factors, but if I could stretch it to 2", then it gets better for most factors, with the only downside being vinyl attachment ... that's why I was concerned with fastening cladding.

   I could play it safe and stay with 1.5", but I was trying to go for more is better.

   Thanks for the rec on Mento 1000/Blueskin. I'll take a peak at those and see if that works for this setup.

   Thank you!

1. 
   climbing_carpenter | Mar 26, 2024 02:28am | #5

   LP is similar in cost to a high grade vinyl!

   Alas, nailing vinyl through thick foam will be tricky. Not reinventing the wheel though, you'll have to use long roofing nails which can have a tendency to bend. 3" should be widely available and substantial enough with 2" foam as you'll only be driving the nails into the sheathing.

   Whatever layer you define as your air barrier -the WRB or the foam- be certain you are meticulous with detailing as you have board sheathing. Give good effort at the sill and roof connections too.

1. 
   ScottRokisPhoto | Mar 26, 2024 01:34pm | #8

   Thank you! Your points are noted and very helpful. I appreciate the time.