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I'm not quite sure that this question has been asked, but I might also be misreading some earlier questions. Regardless, I am installing fiberglass insulation batts to go in my (sadly) 2x4 ceiling joist bays. The underside (ceiling side) will be faced, and once all the batts are in, I plan to cover some portions with OSB to make attic access/storage easier.

My attic has two vents at the peaks of the gables, and two Whirlybird vents, so it's definitely an unconditioned space. I have no reason to make it a conditioned space, but should I also install insulation in the rafter bays (like rigid foam cut to fit the bays) or under the rafters (like panels of rigid foam sitting on the rafters, leaving an air channel behind them)?

My guess is that this isn't useful, but I'm simply unsure.

EDIT: I should add that there are no soffit vents. The only attic vents are the gable peak vents and the Whirlybirds, which makes for a very hot attic in the summer.

EDIT: I'm in Redwood City, CA. Winter temperatures get close to freezing at night, though rarely to freezing or below, and easily up to and over 100 °F during the day in the summer.

  • 100 Deg F in summer is cool, 140 is hot FL. You have gable vents. leaving an air channel behind rafters won't accomplish anything,your vents intake below that into the attic you don't have soffits requiring a channel either. While it might reduce overall attic temperatures - they spray foam insulate these now, I don't like that method for a number of reasons roof and shingles are hotter (reduced life), a leak can be easily missed for a long period of time and mold that can't be dealt with by DIY can grow in that same time period. If you have no intent to live in that attic save your money. – Ken Jan 18 '18 at 5:18
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You should not. Because the attic is vented, the upper insulation does almost nothing. In the summer at the attic will get much hotter than it does now, increasing cooling costs.

Also, you would be creating what's known as a hot roof. This can be damaging to the roof sheathing and shingles.

Most truss bottom cords are 2x4. This is normal, and the insulation usually reaches much higher than the tops of the ceiling joists. The usual routine is to raise the floor with additional framing to allow for more insulation.

  • they spray foam these units all the time - I agree with you - the roof/shingles get hotter reducing the lifetime of both. Not too mention what happens with a leak - you don't know until your whole roof is full of mold! – Ken Jan 18 '18 at 5:19
  • @isherwood Thanks for the advice about the rafters. I'd suspected I shouldn't insulate them, but had no concrete reason either way (except saving money). I understand that the attic floor is often raised, but since I have plumbing and electrical running in the attic, I can't close off access too much. It's already only 4' tall to the peak. – Hari Ganti Jan 18 '18 at 20:17

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