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I have what appears to be R-30 unfaced fiberglass insulation batts between my ceiling joists. The installation looks like I would expect, except that the batts appear to have been pulled away from the ceiling drywall such that there is an air space between the drywall and the insulation. Some batts are bowed up and others have their edges exposed to the attic (not fully in contact with the joist as a result of being pulled up). Because the house has non-IC can lights, the insulation isn't continuous, meaning that the otherwise insulated air pocket is effectively vented to the attic every so often.

I would expect that insulation in direct contact with the drywall would perform best. It's not clear to me if this was done by the builder to make the insulation appear to be thicker than it actually is, if it has crept over time due to thermal expansion or other movement, or if this is a superior way to insulate an attic.

If it matters, I live in CA and the house is mid-1980s vintage.

Example of installation This is typical of what I'm seeing. The batt on the left has its sides touching the ceiling joist, but will sit 2-3 inches lower if I push it down against the ceiling drywall. The batt on the right does not have its sides consistently touching the joist.

ubakus.de output Here's a quick takeout from ubakus, though I couldn't find a way for it to show as a vertical space rather than a wall. The air gap between insulation and my roof sheathing ranges from 2-16 feet; I used 4 feet in this model.

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    Adding a picture sure would help us give you a better answer. – JACK Feb 4 at 18:34
  • In order to avoid the huge amount of condensate, the air space no. 3 must be ventilated, f.e. with slots or holes covered with fine mesh against insects and rodents. – xeeka Mar 8 at 3:26
  • Just try ventilated instead of stationary air. – xeeka Mar 8 at 3:29
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Sounds like a sloppy install to me, and as a result the insulation that's not in continuous contact with the ceiling panels is doing nothing. If air can circulate around the insulation, virtually no heat is retained or blocked (in winter and summer, respectively). If this was some sort of intentional air gap there would also be an air barrier over the insulation (out of necessity).

I would do my best to lay and fit the batts that are there, then I'd consider overlaying blown cellulose (an easy DIY job) to fill voids and bring your R-value up to modern standards.

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It doesn’t matter if there’s a 2” air gap or 2’ air gap between the ceiling insulation and the ceiling finish, the air gap is considered part of the living space AND will be conditioned to the room temperature. (You are heating/air conditioning an extra 2” of ceiling space.)

The graph is confusing because it’s rotated 90 degrees to how the space is constructed. (It looks like a wall space instead of a ceiling/attic/roof space.)

You can see from the direction of the black line why the attic space needs to be vented.

A bigger issue is the method of construction for your recessed can lighting. I’d build a box (usually made out of gypsum board) around the lights to keep the insulation away from the light fixture (so it doesn’t catch fire) and then install batt insulation around and over the box to compete the insulation envelope.

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  • My concern is that as-installed, it seems like unconditioned attic air can get under the batts (between ceiling drywall and batts), which seems like it would largely negate the benefit of having insulation at all. – HikeOnPast Feb 5 at 17:33
  • As for the can lights, it turns out that they are IC-rated fixtures but installed in a non-IC manner. My plan is simply to insulate up to the cans as part of fixing the other issue. – HikeOnPast Feb 5 at 17:34
  • @HikeOnPast Why would it be like “no insulation at all”? The air below the insulation is “conditioned air” and the air above the insulation is “unconditioned air”. It doesn’t matter that the air is above or below the ceiling drywall. – Lee Sam Feb 5 at 18:00
  • @HikeOnPast Remember, you’re trying to create a complete “envelope” around your conditioned space. Any holes in the envelope (like recesses lights that are NOT insulated) will leak conditioned air (heated or air conditioned air). The goal is to not have any leaks in the envelope. – Lee Sam Feb 5 at 18:04
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    It would be like no insulation at all because unconditioned air can reach the drywall, which is effectively conditioned air per your description. Seems like the ceiling is effectively partially insulated (the percentage of batts that aren't vented to the unconditioned space) – HikeOnPast Feb 5 at 18:09
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An air layer is sometimes part of insulation constructions in order to get rid of moisture, depending on the location of the due point. A reliable way to get all necessary information is to input each layer into the "Ubakus"-Site, which is free for private use: Ubakus.de

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  • Of course, an air layer that's exposed to the outdoors (or, in this case, the unconditioned attic space) almost entirely negates the effectiveness of the insulation. – isherwood Feb 4 at 20:34
  • @isherwood An application (wall) would be f.e. - from inside to outside - wall plaster, bricks, fiber mat in wooden frame, air layer, rain curtain (fiber cement plates). The insulation effect is not negated if the air layer comes after the insulation layer like in this simple example. To better understand the layer sequence of the thread opener, a picture (or screenshot of the Ubakus output) would help. – xeeka Feb 4 at 21:08
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    Right, and the insulation in that example is tight to the wall surface against which it rests (toward the conditioned space). That's my point. In the case of the attic, the same air gap would hypothetically be above the insulation. – isherwood Feb 4 at 22:12

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