Foam boards on outside walls?
Long time reader first time poster. The two outside walls in my living room are currently down to the studs. The combination of the window leaking (no flashing) and leaks behind the brick veneer on the front of the house and a giant chipmunk nest rotted most of the siding and the studs. I had a framing crew replace the walls and T-111 (T1-11?) siding.
I am planning on using R-13 unfaced fiberglass batts for insulation and could use some thoughts on placing 1/2 inch polyisocyanurate foam sheets over the studs and then the drywall over the foam sheeting. The foam sheets are rated at R-3.3 and are foil faced on one side. The foil would be the vapor barrier with the seams taped with the proper tape. I am in northern Indiana so the vapor barrier has to be on the inside. Would this be a waste of time?? Will the extra R-3.3 make a difference?
Most of what I’ve read says that using foam sheets on the outside will minimize the thermal bridging from the studs. I can’t figure out if the foam sheets on the inside would have the same effect since it would be on the warm side.
I would have to use jamb extensions on the window and possible trim the carpet a little
One wall is 16′ long and will have a new Andersen 400 series window in it. It faces west. The other wall faces north with no openings. The walls are 2×4 16″ on center, 8’tall. This is a 35 year old ranch on a slab.
I called a insulation company about spraying foam, but they have a $400 minimun charge. 7 foam sheets would cost about $60.
Thanks for reading this and for any comments.
Dave
Replies
Take a look at this site: http://www.umass.edu/bmatwt/publications/articles/insulating_on_the_outside.html
>>I am planning on using R-13 unfaced fiberglass batts for insulation and could use some thoughts on placing 1/2 inch polyisocyanurate foam sheets over the studs and then the drywall over the foam sheeting. The foam sheets are rated at R-3.3 and are foil faced on one side. The foil would be the vapor barrier with the seams taped with the proper tape. I am in northern Indiana so the vapor barrier has to be on the inside. Would this be a waste of time?? Will the extra R-3.3 make a difference
1/2" blue foam board will serve the same funtion as the foil faced board (Tuff-R ? ). The blue board has a polyethylene layer on it. Taped with agood guality builders tape, it will serve as a vp in your wall assembly.
Read the instructions on the foil faced sheathing board. IIRC it is intended for exterior applications.
Why not dense pack cellulose for the walls? You get more bang for you insulation $$.
dave
Thanks for the advice. I will check into the dense pack cellulose.
Dave
We're nearly finished on a whole house with wall systems similar to what you proposed, although our framing was 2x6 and the thermax panels (inside the frame) were 2". I see nothing wrong with your approach, but why use only 1/2" iso? You already see the issues with building out the jambs and carpet, so why not use at least 1" and really improve the R-value of the wall? The drywall/interior paint will slow down most water vapor, but you still want to tape all the joints, as well as the edges where it meets the floor, etc. and you will have a very snug wall.