I am going to put a new roof on my house. I have a cathedral ceiling with 2×10 rough cut rafters and 1″ boards for sheathing. The rafters are insulated with 9 1/2″ fiberglass insulation leaving a half inch air space with soffit and ridge vent. There is a plastic vapor barrier on the heated side. I have decided to cover the 1″ sheating boards with a smoother surface; planning on using OSB board. Since I am re-sheating, I have decided to also increase the insulation. I am going to put rough cut 2X4s on top of the existing roof deck and put the new sheating on top of that. Here is my question:
I am deciding what to use for insulation… two thoughts:
1. Install 2″ of rigid insulation in the new cavity leaving a 2″ air space that will be vented with soffit and ridge. Do I need to worry about trapping moisture? The section view would be (from the inside out) plastic sheating/9 1/2 rolled fiberglass / 1/2″ air space / 1″ sheating board / 2″ rigid insulation / 2″ air space / new sheating.
2. Use 3 1/2″ rolled fiberglass leaving a half inch air space instead of the rigid insulation.
Any suggestions?
Replies
Is there venting from soffit to ridge vent now? When you add your new insulation you need to cut off the old air flow from the old system. If not when the heat radiates through your current fiberglass and gets to the 1/2" free /vented area it will flow through the vent thereby making any insulation above that point useless.
Now if you can get this done then I would install 1 1/2 "insulation board to existing roof deck and shingle over that with 2 1/2 " long nails. Your roofing supply yard will point you in the right direction as far as the type of board / nails you would need. This would be the most cost effective way to do it. Now if you want that last 1/2" of foam board and are willing to pay a lot more. Then I would call roofing co. and ask them for insulation board with osb already glued to it and screw down into existing rafters through the 1' existing sheathing. Then roof over. You don't need a sleeper system or venting in this system as long as you seal all penetrations perfectly it should be fine.
Piffin will be along soon enough to chime in with some idea's himself I am sure.
The no vent idea in this system may draw some critics but if done properly its the most cost efficent way to "get er done".
Edited 3/5/2007 1:19 pm ET by AllTrade
Edited 3/5/2007 1:20 pm ET by AllTrade
Edited 3/5/2007 1:25 pm ET by AllTrade
bump-da-bump -bump-BUUUMMMP!
every court needs a jester
I've got a headache! Quit pounding on the floor up there!
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
Better this way - place the foam directly solidly on the old sheathing, then apply the osb over it and use screws through all.
If you want to add vented cold roof, then still apply solid foam over old and use 1x4 furring to screw through on rafters.
Then nail the new osb on over the furring. Detail at edge to allow venting but add screen to keep bugs out.
Welcome to the
Taunton University of Knowledge FHB Campus at Breaktime.
where ...
Excellence is its own reward!