Roofing underlayment for vented cathedral ceiling
After finally deciding on my insulation thanks to this previous thread: Insulation Delirium (thanks again everyone). I am now talking with my roofing contractor and I have a follow up question, though it seems a new thread is most appropriate.
I have a contractor who I am discussing the bid in regards to the changes I made to the roof as per the previous thread. In my discussion they mentioned installing a ‘synthetic underlayment’ when pressed for details I was told it is “Weather Teck Select Underlayment”. The question I have is if this sort of underlayment will cause a problem by not allowing the roof decking to dry out. It doesn’t seem I can find too much in terms of permeability, etc.
Do you think I am being overly worried or should I contact the manufacturer to find out this information? I’m just concerned that allowing the contractor to use this underlayment will undo all the work going into the venting. What do you think?
Andrew
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Replies
Andrew,
Most types of synthetic roofing underlayment are vapor-impermeable. If you are planning to have a vent channel below your roof sheathing, the permeance of the roofing underlayment is irrelevant (for most types of roofing). However, if you are planning to have a vent channel above your roof sheathing, then you definitely want to install a roofing underlayment with variable permeance, so that the roofing underlayment allows some drying to the exterior.
So, if you plan to install a vent channel above your roof sheathing, you definitely want #15 asphalt felt -- which is a "smart" vapor retarder with variable permeance -- not synthetic roofing underlayment.
For more information on this topic, see my Fine Homebuilding article: Synthetic Roofing Underlayments.
Perfect I will stick with this and tell the contractor to switch it.
Thanks again.
Lastly, if not #15 felt is something like GAF Deck-Armor or other permeables mentioned in your underlayment article acceptable in this scenario?
Andrew, It depends on what the rest of your roof is build up (asphalt shingles? metal?). And as Martin states if you plan to vent under above the sheathing. If you are venting above the sheathing, you are better off with a vapor open underlayment - the SOLITEX MENTO membrane we supply at 475 is lighter than #15 felt, is wider so applies faster and very waterproof - it can be a temporary roof for 3 months and will function as a subroof at a minimum until your next roofing job (see photo). More info about roofs vented above the sheathing (or membrane) in the blogposthow to properly vent a high performance roof
Andrew,
Yes, if you want to use a synthetic roofing underlayment, there are a few products that are vapor-permeable that would work in your application. Note that they are all significantly more expensive than good old asphalt felt.
The product you mention (GAF Deck-Armor) has a permeance of 16 perms. Floris works for a building material supplier in New York City, and his company sells a European roofing underlayment called Solitex Mento. It is also significantly more expensive than asphalt felt.