Metal shed roof / Ice & Water Shield / Insulation over and under
I am doing a remodel in the Seattle area – zone 4C. This question is related to roof build up and insulation.
– 2:12 shed roof, high side faces south
– Joists are 20″ oc and 12″ deep
– Decking is 5/8 plywood
– Roofing material will be metal standing seam
During construction we applied some ice and water shield over the entire the decking – mainly to temporarily keep the decking dry and prevent internal water intrusion until we were ready to install the roof.
Our original plan was to lay the metal over the decking without insulation; however we altered our plans and our now going to do 2″ build up of insulation over the decking. We wanted to get a thermal break for the decking and wanted to get more R overall.
Under the decking we will build a 2″ vent space in the joists with Majvest (stapled and caulked) the entire bay right up to the ridge and low edge. The ridge and low edge have bird blocks vents to the soffit.
Under the vent space we will use 10″ of Roxul or a fiberglass batt.
1) Should we remove the ice and water shield before laying down the insulation? I read a few posts about breath-ability issues with the decking. My take is that since have a 2″ vent space running end to end it should breathe well from below. Plus, the added top insulation helps minimize any top condensation.
2) What insulation is recommended for over the decking? If we left the ice and water shield down I was thinking that permeability of top insulation becomes less of an issue. We are using cork of the majority of external walls, but I’d like to save some money on this roof section if possible.
3) What insulation should we use for the joist bays? Roxul does not come in a 10″tx20″w batt. Its 23″w and 7 1/4″t. I’d have to cut all of the bats to get them to fit and then layer another 3″ to get maximum insulation pack.
4) Is building the venting path in the joist bay a solid approach? If not what else do you recommend?
5) Is it common to put a layer of ice and water shield over the top side insulation?
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