Expert: Ted Barnhill (Principal, Design45 LLC) - 4/22/2010

Question
I am going to install closed spray foam in new construction and cannot get an answer as to the application of the foam to faced batts. Most say to spray the roof deck but it seems that this approach puts the vapor barrier on the wrong side. I have some inaccesible vaults that I could put faced batts for r push them up 2 inches and spray the face thereby getting bulk of the r from the inexpensive fiberglass while getting the seal from the foam and having the vapor barrier where it is supposed to be. any advice would be appreciated. thanks
Todd

Answer
Todd,

If I understand your issue correctly, you would like to install both fiberglass batts and closed-cell spray foam in a vaulted ceiling assembly in new construction.  I gather that you are using the foam in order to have a great vapor barrier, but are hoping to save some money by using fiberglass for additional R-value.

A hybrid batt-foam approach is used by some installers, though they typically spray first, on to the sheathing.  This approach is criticized by some because, as you mention, this puts a vapor retarder in the middle/outside of the assembly, a potential problem in a cold climate.  Your suggestion to layer the hybrid the other way sounds good in theory, but I haven't seen it done.  This is likely because the foam offers advantages when sprayed to the sheathing that it would not if on fiberglass.

1. Foam is a great airseal, filling cracks in framing and sheathing, preventing air/wind from entering the wall or ceiling cavity.  If you put batts against the sheathing, you lose this benefit.

2. Foam as a seamless vapor retarder on the warm-in-winter side prevent moisture from diffusing into (and condensing inside) the assembly.  If your foam is to the inside of the wall/ceiling only, where there are penetrations from electrical boxes, exhaust fans, etc, you may allow paths for vapor to escape.  An all-foam system sprayed to the sheathing has less openings to deal with.

3. Foam contributes to the rigidity of the structure when applied to the sheathing.

At least in my areas, spray foam has come down in cost as it becomes more common.  I doubt that you will save enough by using a hybrid approach (rather than all foam) to make it worthwhile to give up any of the benefits of an all-foam system.  If you do decide on a hybrid approach, you should do a dewpoint calculation to determine whether the foam-to-sheathing aproach (with batts inside) poses a potential vapor condensation problem given your design temperatures and winter interior humidity levels.

Ted Barnhill
Design 45, LLC