GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Detailing Exterior Insulation on Basement Walls

Cortland15B | Posted in General Questions on

Hello, I want to insulate the rest of my basement above grade. The previous owner dug out the foundation, applied water proof membrane, insulated with 1.5” or 2” of foam, added drainage pipes and backfilled with rock. They didn’t go above grade though so I want to do it from there to the bottom of the siding. House is in zone 7 so very cold. How thick if foam should I go? Code says R-20 but that’s 4” of foam.

The problem is I haven’t been able to find a good way of flashing this junction where the foam meets the siding. The foam is going to stick out past the siding as well. I don’t want to remove the siding as it will probably break its thin redwood siding that is old and dry.

The foundation above grade is skim coated and painted, do I need to put down a waterproof membrane first before the foam?

What do you do at windows? How do you frame them out?

What are my options for protecting the foam that is not a stucco like coating? I fear anything applied like that will eventually start cracking off. There is a foam board that comes pre-coated with small rocks or stucco that would work but it’s only 1-2” thick. I also saw this plastic roll that you fasten to it but it’s pricey. Can you use that vinyl stone siding stuff?

Thanks!

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.

Replies

  1. Expert Member
    PETER Engle | | #1

    Here are two articles from this site:

    https://www.greenbuildingadvisor.com/article/how-to-finish-exterior-foundation-insulation
    https://www.greenbuildingadvisor.com/article/faq-insulating-foundation-walls

    In your very cold climate, R-20 would be best but anything is better than nothing. Remember that you can install half of your insulation outside and half inside. That would help keep the outside insulation from sticking out too far past the siding.

    1. Cortland15B | | #2

      I don’t have GBA prime that’s why I’m here. I can’t the information behind a paywall.

      1. Expert Member

Log in or create an account to post an answer.

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |