Add foam board to outside of existing walls.
driopel
| Posted in Energy, Heating & Insulation on
I would like to add rigid foam board insulation to the outside of my house. I am concerned about condensation inside the wall. My existing walls are 2x4s with blown in insulation, (no vapor barrier between drywall and studs) celotex sheathing and aluminum siding. My plan is to remove the aluminum, leave the celotex, shim out the studs with 2x2s, add 1 1/2″ foam board insulation between the 2x2s, cover this with osb then tyvek then vinyl siding. Also the bottom 3′ of the wall will be cultured stone over the tyvek.
I am concerned about condensation because I read that the perm rating on the cold side of the wall should be 5 times higher than the warm side of the wall. On the other hand, method #5 from the article “Six proven ways to build energy-smart walls” from Fine Homebuilding 208 Nov. 12, 2009 is pretty similar to what I want to do.
Any opinions or suggestions would be greatly appreciated.
Thanks
Don
Replies
Part of the answer depends on where ya are--where are ya?
Southeast Michigan. Brighton
Hi driopel
I think that the thickness you have to use to make it work properly depends on where you live (temp). I just talked to a contractor friend who did a reno just like you discribed with 2 inch foam. I know that here (coastal BC) they recomend 1-1/2" to 2". I have no idea what your climate is but I would imagin that it is colder there so thicker foam would be needed to avoid the condensation problems in the wall.
If you eliminate the 2x2s in your proposal you will would have far less thermal bridging which is one of the main advantages to adding foam on the outside. My friend hung the vinyl back on with three inch roofing nails right through the foam said it worked great.
There is some discussion about where to put the moisture barrier in the green building adviser web site.
I am interested in doing this here on our next new build,
Angus