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How about sealing and insulating both the roof deck and the attic floor?

scramgravy | Posted in GBA Pro Help on

I just read Joe Lstiburek’s excellent Crash Course in Roof Venting. It caused me to have an unorthodox idea about which I seek your opinion: sealing both the attic floor and the roof deck. Please read on before you dismiss me as nuts.

My wife and I live in a 100 year old four-square in Joliet, IL (Zone 5). It has a hip roof with about eight feet of main ridge, eleven feet on each of three dormer ridges, and three feet of eave overhangs all around. I had dense-pack cellulose blown into the balloon-frame stud cavities, and I removed the old attic floor and the old and sparse mineral wool, sealed all openings and cracks that I could find, reinstalled the mineral wool and supplemented it with cellulose to the tops of the joists, and replaced the old flooring with ¾” plywood. The attic floor joists are 2x8s, so the insulation, although greatly improved, is still not adequate for the climate zone. Gas usage is down from over 12 BTUs/HDD/sq. ft. when we bought the place in 1995 to just under eight now.

Heat is via hot water radiators, but the CAC air handler and ductwork is in the unconditioned attic. Rafters are 2×6 except in the three dormers, where they are 2×4. We put a new roof on in 1996 and ventilated the attic with about 35 feet of ridge vents, twelve one sq. ft. holes in the eaves, and a powered roof vent fan.

So. What harm might there be in increasing the total amount of insulation above the conditioned living area, with an intervening air cavity (i.e. the attic), by sealing the roof deck and insulating its underside, thereby moderating the attic temperature summers and winters and reducing the temperature differential between the conditioned living space and the attic and reducing the load on the AC air handler? We could have at least 14 inches of insulation (six inches or more between and beneath the rafters and eight between the joists) and dead air in the attic.

Okay, am I nuts?


Bob Nachtrieb

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  1. stuccofirst | | #1

    The problem with insulating both surfaces is that you trap moisture. With no ventilation, your warm air convecting up into the attic will condense most likely in the attic floor. Either insulate the air handler and ducts, or insulate the roof underside and remove the floor insulation.

  2. wjrobinson | | #2

    Bob, your plan may work. Keep your eye on moisture and air leak paths along with all the other details, venting or not venting and more. And post back as you progress.

  3. Expert Member
    ARMANDO COBO | | #3

    I hope when you installed new insulation on your roof decking, you installed wind baffles, so your soffit vents are not clogged up with insulation. Installing powered vents to attics is a really bad idea, unless you have your ceiling completely air tight and used an ADA installation approach on your ceiling sheetrock. You could gain much by blowing more insulation on your attic ceiling; sealing and insulating you air handler and ducts, and keeping the ventilated attic. You should installed 1sf of vent per 150 sf of ceiling area; and as Joe said in his article, install 60% on the soffit and 40% on the ridge to create a slight positive pressure in the attic.
    To do a conditioned (unventilated) attic in CZ5, you should install at least 2”-3” rigid foam on top of your roof decking to avoid thermal bridging and condensation, and then open cell foam under the roof decking, and allow your roof assembly to dry to the inside if it gets wet.

  4. user-869687 | | #4


    You are correct that the insulation at the attic floor is inadequate, and your suggested upgrade can work. Before adding insulation at the roof you should consider creating a vent space to reduce risk of moisture issues. One method would be to install 1x1 wood strips alongside the rafters under the roof deck, then cut rigid insulation to fit between the rafters against those wood strips. This way air can still move below the roof deck to carry out moisture. If headroom in the attic is not a concern, it would be easier to just install rigid insulation (polyiso) below the rafters.

  5. GBA Editor
    Martin Holladay | | #5

    Your proposal might work, but I don't know if it is the best approach. It might make more sense to install more insulation on your attic floor and improve the duct sealing and duct insulation, as Armando suggested.

    Whatever you do, unplug that powered attic ventilator, which is wasting energy and providing no benefit.

    Shane, I don't know why you suggest that Bob's attic will end up "trapping moisture." You say that Bob has warm air convecting up into his attic. I hope you are wrong. Bob told us that he air-sealed the ceiling plane. In any case, even if the air in the attic contains moisture, it shouldn't condense anywhere if Bob insulates his sloping roof assembly.

  6. scramgravy | | #6

    Thank you all for the excellent insights! Shane, I, too, am worried about any moisture in the dead air. Yes, I did seal all the apertures I could find in the attic floor, but this house is 100 years old and who knows if I did a perfect job. AJ, I agree that any leaks I missed could be found with a thermographic imager and then fixed. Armando, the ventilation that was put into place in 1996 is pretty good. The rafter chases are clear, but then this is a hip roof, so they end well before they get to the ridge. Tom, this is one of the reasons why I'm thinking of reversing the ventilation and sealing the attic. Martin, I agree that more insulation at the floor level is easier and cheaper, but, against Joe Lstiburek's advice we have a lot of stuff in the attic for which we have no better place, which is why I have new flooring up there and am limited to the depth of the joists for insulation and why the idea of two separate layers of insulation – on the attic floor and under the roof deck – occurred to me in the first place. And, yes, I did disconnect the powered vent fan two minutes after watching Joe's video version of the written article. The bottom line: this house is 100 years old, and I would like to avoid doing anything that would lessen its chances of lasting another 100 years. I welcome and hereby solicit any more thoughts and suggestions on the matter.

    Regards to all!

  7. user-659915 | | #7

    Robert, with a hipped roof I suspect your floored storage area is considerably less than the entire attic footprint. If this is the case you might consider blowing significant levels of additional insulation in the un-floored area, with a dam of OSB or plywood at the inner edge, to bring the *average* R-value of the attic up to a reasonable level. Go crazy and add a couple of feet of cellulose.

    By the way, the flooring will (marginally) improve the performance of the insulation below it by eliminating the potential for wind-wash. Oh, and do I need to mention to be sure that the attic access stair is not the weak point in your ceiling air-seal?

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