GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter 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

Exterior siding details

JP Patrick | Posted in GBA Pro Help on

My house is a 1500 square ft bungalow located on Lake of the Woods in Ontario near the Minnesota border (Climate Zone 7) . Before I purchased the house the previous owners replaced all the windows with triple pane pane windows but had the installers build them out 1.5″ with intentions of adding exterior foam when the siding was to be replaced. I have since gutted the entire main floor and rewired, plumbed and insulated with 3.5″ of closed cell spray foam in the walls. I added a couple more windows and a set of patio doors and stuck with the 1.5″ build out.

My question is what is the best way to detail the exterior before adding siding? If all the windows are built out 1.5″ how do I prep the exterior before adding Marvin Apex Fiberglass siding without having the windows stick out. All of my openings have been detailed correctly with blueskin prior to the windows being installed.

I don’t really want to add exterior rigid foam as I believe this will cause moisture problems with my sheathing as It will not breath to the outside. I was thinking of installing good housewrap (What is the best housewrap for my application?) and adding 1.5″ furring strips before siding. Will a 1.5″ gap cause problems with my siding warping, or attract insect/rodent/bats?

The previous siding was aluminum over #15 felt paper with no moisture damage to the sheathing in 30 years. I would like to keep it this way.

Any suggestions and details are welcome. What would you guys do if you were faced with this?

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.


  1. Patrick Walshe | | #1

    I used rigid roxul comfortboard IS sheets instead of foam - you would need to calculate the minimum foam thickness for there which I think would be more than 1.5 ". Its not an issue with roxul because the walls can breath through it. I used Typar house wrap and 1x4 strapping with the metal bug and rodent screening on the bottom (goes over roxul and 1x4 boards). The gap will help your wall and siding last longer. I have details on our blog at

  2. Patrick Walshe | | #2

    I should say the walls can dry to the exterior

  3. GBA Editor
    Martin Holladay | | #3

    Once you have installed closed-cell spray foam between the studs, it's usually not a good idea to install additional rigid foam on the exterior side of your wall sheathing. It's best to avoid encapsulating your OSB or plywood wall sheathing in this kind of foam sandwich.

    You've boxed yourself into a corner by choosing closed-cell foam. Since your wall assembly can't dry to the interior, you'll need to take Patrick's suggestion of using mineral wool insulation on the exterior, or just install a fat rainscreen gap as you suggested.

  4. Lucas Durand - 7A | | #4

    Given where you live, I would take the opportunity to add more insulation to your walls by following Patrick's suggestion.

  5. JP Patrick | | #5

    Thanks guys. I appreciate your advice.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |