To all,

We live near Chicago and have a split level house built in 1959. I would like to use rigid foam to convert our two vented attics to unvented so that we can use them for storage. The rafters are 2×6, so there is 5.5″ of depth under the roof deck to put layers of rigid foam into as a “stack of pancakes” between the rafters. I would also like to attach 4’x8′ sheets of rigid foam (at least 1/2 inch thick) to the lower edge of the rafters to prevent thermal bridging.

I can get clearance value bundles of scrap pieces of polyisocyanurate for $50 per 4’x8’x4′ stack of all different thicknesses and various facers. The total area under the roof deck that needs insulated is approximately 1,850 square foot, so I estimate that for between $350 and $500 I can buy enough polyiso to insulate between the rafters. I plan to cut the scrap pieces and squeeze-fit them between the rafters to a depth of 5.5 inches. (I will put a layer of all one thickness in, then stagger the seams of the next layer, until the cavity between the rafters is full.) I plan to then buy better quality 1/2 inch thick 4’x8′ sheets to attach to the lower edge of the rafters.

The pitch of the various parts of our roof varies from around a 2.8 in 12 pitch to a 4.3 in 12 pitch. The greatest height from floor to ceiling of the attic is around 44.5 inches, so no one can stand up in it. We have to move around on our knees. The storage is very valuable to me, so I don’t mind moving around on my knees. I want to make our roof area very well insulated and energy efficient, but I’d like to keep the thickness of the insulation attached to the bottom edge of the rafters to a minimum.

We need to re-roof our home soon. I plan to insulate everywhere vents are not, then have the roofing crew eliminate the vents, then finish the insulation project.

THE BIGGEST SET OF QUESTIONS: I understand that to air seal this perfectly is absolutely critical. Can you help me to understand exactly how to air seal this, and whether the air sealing should be at the lower edge of the whole assembly or exactly where? If I very thoroughly seal the seams of the 4’x8′ sheets of the lowest layer with foil insulation tape, will this suffice? (Or maybe caulk at the seams and then cover with foil insulation tape?) If some moisture problem starts to occur now or far into the future (maybe because warm moist indoor air somehow leaks through the insulation layer, hits cold roof sheathing, and water condenses on the sheathing), will we be able to tell that the moisture problem is occurring? I will probably be doing this project solo, and it may take me months. If there is a months-long period in the winter when I have started to insulate this way but have not finished yet, could this cause damage to our house?

Here are some other less important questions that maybe someone else can help me with if you don’t have time to:
*Will the various facings (foil, gray latex, black felt) cause any problem if they are sandwiched between layers of polyiso between the rafters?
*Our city requires R38 for this type of “cathedral ceiling”. If I install a total of 6 inches of polyisocyanurate rigid foam as I have described, would it meet our city’s requirement? All city requirements aside, would a 6 inch depth be entirely sufficient to prevent ice damming, cold drafts in winter and stifling hot upstairs bedrooms in the summer? (We have suffered from all these problems for many years.)
*Our city requires that we drywall or attach a foil rated fire barrier to the underside of the insulation. If the “factory seconds polyiso rigid foam boards” I plan to purchase have a foil facer, does this foil facer provide the needed foil rated fire barrier?

Thank you so much.

Rebekah