I need some advice on an attic retro fit. The attic is rough cut 2x8 pine (2"x7.75"), 24"oc, with a plaster lathe ceiling underneath. Right now it has about 8 to 9 inches of blown-in cellulose. I'd like to bring up. Code is 38 in my part of MD. I'm thinking using 4x8 sheets of ridged insulation board covered with plywood over the cellulose filled studs for the storage area and 12 to 14" of extra cellulose over the rest. Two questions; Is this a good idea? Which insulation would be best for the storage area, polyisocyanurate or Extruded Polystyrene? Thanks,

Is there an air channel in the rafter bays? Or are the bays filled with cellulose to the sheathing?

Only the blown cellulose is in place now. An air channel could be added before the sheathing is laid down.

EPS is typically best from a R value per $ basis, polyiso is best in R value per inch.

I wasn't sure which would support the most weight. No big deal because it's just basic, bulky, light weight materials that will be stored up there.

Posted By Bob I on 07 Jan 2015 05:12 PM
Is there an air channel in the rafter bays? Or are the bays filled with cellulose to the sheathing?

Should I worry about an air channel under the sheathing?

EPS will better avoid the vapor barrier being on the wrong side as faced polyiso often does and will be strong enough with plywood over it. Using multiple layers helps avoid gaps.

If you are adding insulation above the rafters, an air space above the cellulose is not necessary and can lead to air infiltration into that cavity, short circuiting the foam insulation. The main goal with exterior foam is to install enough to prevent condensation between the layers, and to install it so that it is airtight. A good, high quality peel and stick tape* is the best way to create an airtight seal. *Siga from High Performance Building Products (Main Green Building Supply) in Maine, or a tape from 475 High Performance Building Supply in Brooklyn are the best sources.

So for the areas covered with foam, seal the wood/foam junction, seal the foam seams and seal any holes or joints in the rafters.