Expert: Bruce E. Johnson - 7/26/2010

Question
In my (somewhat strange) setup we are installing 2 layers of 3" polyiso boards in an attic (on top of attic floor, glued to plywood floor). Polyiso boards are foil faced (Thermax).

Attic will have venting (continuous ridge and continuous soffit vents). Do I have a risk with this setup to trap moisture ? Would I be better off puncturing little (1/4") holes in the foil to eliminate vapor barrier this way ?

Answer
Hi Alex, you won't be trapping moisture under the polyiso so why poke holes in it?  Most new construction today would put the polyiso in between the rafters with no ventilation.  I do all of my commercial roofs with polyiso on top and no ventilation.  The thinking is that once the space gets conditioned then it remains so.  We stretch out the insulation envelope so that even the attic is conditioned space.  But to answer your question I don't think you are creating a moisture trap that will require additional efforts such as poking holes in your vapor barrier.  I hope this information helps, please feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com