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OK to put rigid foam between rafters to convert a vented attic to unvented, for storage?

Rebekah Haworth | Posted in GBA Pro Help on

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.


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  1. Expert Member
    Dana Dorsett | | #1

    If you are re-roofing, put the foam above the roof deck sufficient for dew point control on R21 or R23 batts or ~R20 blown cellulose between the rafters, and don't sweat the air-sealing perfection. That would have a LOT more resilience than stacked polyiso (which at 5.5" would be well under 1 perm even if you peeled the facers), since it could dry toward the interior. With foil facers it's a moisture trap- the roof deck can't dry toward the interior, nor can it dry to the exterior, which is a fairly risky. All roofs develop minor leaks some where, some time, most of which have no consequences if there is a drying path.

    Foil facers on generic polyiso do not have fire ratings. Dow Thermax facers do. You would have to install half-inch gypsum or similar to meet code if it's living space- you can't be looking at the unrated foil.

    The amount of above deck foam needed for dew point control varies with climate. Where is "*Our city..." ?

    To hit the same thermal performance as R38 between 2x12 rafters with R20-R23 would take about 2-2.5" of polyiso above the roof deck, and that would also be sufficient dew point control for US climate zone 4 or lower, but for zone 5 you'd need at least 3", and in zone 6 it would take 4"..

  2. GBA Editor
    Martin Holladay | | #2

    The approach you are talking about is called the "cut-and-cobble" approach. I don't recommend the use of the cut-and-cobble method for unvented roof assemblies; there have been failures (roof rot) when this has been attempted. For more information on this discredited method, and more information on the failures, see Cut-and-Cobble Insulation.

    As Dana suggested, it would be far better to install the rigid foam in a continuous layer or layers above the roof sheathing when you re-roof.

    If you insist on following the cut-and-cobble approach, I recommend: (a) that you create vent channels that connect your soffit vents with your ridge vents before you install the cut-and-cobble foam (for more information, see Site-Built Ventilation Baffles for Roofs); and (b) you include a layer of drywall on the interior side of the rigid foam (the drywall is required for fire safety).

    Building codes specifically forbid any exposed foam insulation if the attic is accessible for storage.

  3. Rebekah Haworth | | #3

    Dana and Martin,
    Thank you so much for your quick responses! I will spend time now studying up on how to install the foam above the roof sheathing.

  4. GBA Editor
    Martin Holladay | | #4

    Here is a link to an article that you probably want to read: How to Install Rigid Foam On Top of Roof Sheathing.

  5. Rebekah Haworth | | #5


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