Replies

1. GBA Editor
   Martin Holladay | Sep 10, 2014 08:58am | #1

   Terry,
   Q. "Is it overkill to ventilate this in this manner?"

   A. No, this is an excellent roof design that will last a very long time. The ventilation will reduce the chance of ice dams.

   Of course, there is an added expense to the ventilation channels. But if your budget can handle the expense, it's a great way to go.

   Q. "Is it still necessary to tape each layer of foam sheathing if I do have this ventilation in place?"

   A. Not really, because your Ice & Water Shield is your primary air barrier. Nevertheless, the tape won't hurt, and it will (slightly) improve the thermal performance of the rigid foam.

2. 
   primitivelamps | Sep 10, 2014 04:08pm | #2

   Thanks Martin

3. 
   Sheila8201 | Sep 12, 2014 03:36pm | #3

   I too am in Z5 & considering cc foam. I've reviewed option of rigid foam over T.O. deck myself. The link below suggests 40-50% of the insulation r-value needs to be on top, to control condensation @ deck. My calculations indicated 3" of board on top would be req'd. to meet the present 49R energy code. If I were to use a hybrid system of 4" cc foam w/ 3.5" HD batt in the existing 7 3/4" cavities.

   The GC dosen't prefer insulation @ the exterior, perhaps its a more difficult application. I'm also concerned with increasing the depth of the fascia @ the soffit, mostly aesthetics reasons.

   Hope this link is helpful.
   http://www.buildingscience.com/documents/reports/rr-0108-unvented-roof-systems/

4. GBA Editor
   Martin Holladay | Sep 12, 2014 03:56pm | #4

   Sheila,
   For information on code requirements for insulated roof assemblies with rigid foam above the roof sheathing and air-permeable insulation below the roof sheathing, see this article: How to Build an Insulated Cathedral Ceiling.