DIY Home Improvement Forum banner

Styrofoam Insulation Question

1406 Views 7 Replies 5 Participants Last post by  Gary in WA
Is it a good pratice to install Styrofoam Insulation sheets over the wood sheathing (to form a thermal break) and then put fiberglass insulation on the inside between the 2 x 6 studs?
1 - 8 of 8 Posts
No because you would be installing a vaper barrier on the wrong side of the wall.
The vaper barrier needs to be on the inside.
Yes, it's the best way, short version:

Long version;

Depends on the sheathing (OSB or PLY) and the siding type. Most important= where are you located?

PS. Welcome to the forums!
Pleas go back and add your location to your profile for ansewers not questions.
Just go to the top left and tap on your name where it says welcome to make the change.
in a cold location its perfectly fine depending on the type of foam your using as open cell foam has to have house wrap installed over it. .closed cell foam can just be put on hte wall by itself but be sure to use shiplap foam and tape the vertical seams horizontal seams are left open so it can still breathe
  • Like
Reactions: 1
As long as the poster knows Blue Styrofoam (EXPS) by Dow Chemical from bead board (EPS) The White Cheap Stuff.

Styrofoam is Blue, and is a Registered Trademark of Dow Chemical Co. It is a closed cell extruded polystyrene insulation. It is so resistant to water that it is used in inverted configuration in low slope roof systems where it is placed on top of the roof membrane. (Protected Membrane Roof System).

And Yes, we need to know where you are to know where the Vapor Barrier/ Retarder should go, but Generally Speaking you want the dew point to fall in about the middle of the insulation package.
Thanks for your reply regarding the styrofoam insulation over the OBS, I am having a new home built in Nova Scotia (framing in-process) and was wondering if it may prevent the vapor from excaping.
Exterior foamboard is fine anywhere, warm or cold location. It should be thick enough to keep the backside of the sheathing warm so moisture doesn't condense there or on the cavity insulation (dew point= first condensing surface);

The open-cell foam (EPS- more than one type) is fine for side-wall applications (hard to get the R-value/window detail with thickness for your location), even below grade;

The best place for the WRB (housewrap) is between the foam/structural sheathing, whether cc or oc;

Describe the wall make-up.

See less See more
1 - 8 of 8 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.