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Converting to a conditioned attic for HVAC efficiency

jpkad | Posted in General Questions on


We are remodeling our ranch in Massachusetts and will be removing our old HVAC system and replacing it with a Mitsubishi ducted mini split system located in the attic.

I have read the numerous articles on this website saying it’s a bad idea to locate an HVAC system in an unconditioned attic and fully understand why this is an inefficient option.

My wife and I would like to steer clear of spray foam due to chemical sensitivities in the family but would like to condition the attic as much as possible. The house has a hip roof, 2×8″ rafters, 24″ OC, with a ridge vent and vented soffits.

What are my best options to condition the space? Do all vents need to be sealed up in order to created a conditioned space? I saw in another thread someone had done the following (which sounds like a vented but conditioned attic):

Rafter vents
Rockwool between the rafters
2″ R-MAX polyiso thermasheath XP

The insulation contractor we have talked to says to either spray foam the entire roof, or just do dense blown in insulation on the floor of the attic. Neither of those options is really appealing to us, the first for the chemical sensitivity concerns, the second because the HVAC will be losing efficiency by fighting the temperature in the attic.

Would appreciate some guidance on how to achieve this without the use of spray foam.


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  1. matthew25 | | #1

    You are asking to both vent the attic, and also make it a conditioned space? It is usually either one or the other.

    Sealing off all vents and making it unvented but conditioned will help the HVAC efficiency issue but comes with added vapor risk. For example, you may need to control indoor humidity with a dehumidifier since you will not have exterior insulation to keep your sheathing above the dew point. Spray foam is the easiest but you can also staple up netting and blow in fiberglass or cellulose between the rafters as well. Batts between the rafters are another option.

    Keeping your attic vented and unconditioned is the safest option from a moisture standpoint. It is also the least expensive option. You will take up to a 30% energy penalty on your HVAC equipment in terms of operating costs. But if you work out the numbers it’s probably not that bad, and will save you money upfront by not converting your attic to a unvented space. Most ductwork has at least R-3 insulation but you may be able to find higher R-value duct insulation to help reduce heat loss.

    1. jpkad | | #2

      Thanks Matthew.

      As I was writing the post I was thinking to myself that everything I've read is either unvented and conditioned or vented and unconditioned and that I may be asking for something that isn't really done in practice. I had assumed that spray foam was needed to air seal in order to have a unvented attic where is where I think I was confused.

      To your point about cost, the dense blown in insulation is about half the cost of spray foam. My HVAC contractor has done R-8 insulation on work we've had done in the past, so I would ask for that again.

      1. matthew25 | | #3

        If you do a simple payback calculation of the energy penalty on your HVAC operating cost vs the cost difference between the two insulation methods, I bet you’ll find it hard to justify sealing up and conditioning the attic space. We all want the most efficient house but when you’re dealing with existing construction your options are more skewed.

  2. Expert Member
    BILL WICHERS | | #4

    Spray foam would normally be applied directly to the underside of the roof sheathing, and would seal off the ridge and soffit vents. This would give you a "hot" roof (trade term), and an unvented assembly. That will work just fine, is commonly done, but pretty much requires the use of spray foam to be safe from a moisture perspective.

    If you want to avoid spray foam, it's best to keep the assembly vented (the roof assembly, not the attic itself). The way I would do this is to tack (with a finish nailer) some 1x2 furring strips along the inside edge of each rafter tight to the sheathing, with furring parallel to the rafters. This will give you a sort of back stop 1.5" deep to create a vent channel, and you want that vent channel to run from the soffit to the ridge in EVERY rafter bay. Put some polyiso in the rafter bays against those furring strips to form the vent channel, and to gain you a freebie radiant barrier from the foil facing on the polyiso. I would use somewhere in the 1/2" to 1" range for polyiso here (R3 to R6).

    You should have about 7.25" depth with 2x8 rafters, so you've used (assuming 1/2" polyiso), 2" so far between the 1.5" vent channel and the 1/2" polyiso. You have 5.25" remaining, which is pretty close to a 2x6 and you can use batts for a 2x6 depth wall here. Mineral wool batts would get you about R23 in that space, as would high density fiberglass batts. Either should be able to fit, but fiberglass is probably a bit easier since it's squishier. You're now up to about R26. You ideally want at least R38 here, which isn't current code, but is a sort of practical minimum. If you put 2" polyiso under the rafters, that gives you another R13 for a total of about R39 for the assembly, which is pretty reasonable. If you need more, use thicker polyiso on the interior, or, better yet, two layers to build up to the total thickness you need. You may need a protective layer over that polyiso, which is usually 1/2" drywall but can also be plywood or 1/4" hardboard. Drywall is usually the cheapest option.

    You don't really have a double vapor barrier, since the upper 1/2" layer (which does NOT need to be air sealed, BTW) allows moisture to escape by sneaking out around the edges, migrating through the relatively thin area of wood in the edge of the rafter. The vent channel will safely carry this away. The inner layer (the 2" polyiso under the rafters) should be taped and air sealed, and will do a good job as a vapor barrier, limiting how much moisture can make it's way up into the assembly.

    This vent baffle / batt / polyiso assembly gives you a pretty robust assembly that is relatively easy to build and doesn't require any specialized contractors. You DO need to keep your soffit and ridge vents open with this assembly, since it's still a VENTED roof assembly -- only the attic space inside is conditioned space sealed off from the outdoors.


    1. user-7833485 | | #5

      The hip roof will make it virtually impossible to vent each rafter bay...

      1. jpkad | | #7

        Yes, looking at the roofline you are correct. I don’t think I could vent every rafter to the ridge vent due to the corners of the hips.

      2. Expert Member
        BILL WICHERS | | #8

        Ack! I missed that part! Yes, that does complicate things. You basically have two options then if you want to keep things vented:
        1- Add lots of roof vents, which aren't exactly aesthetically pleasing...
        2- Go the "mini attic" route along the ridge board at the hip, and then vent the "mini attic" somewhere near the peak. This way multiple rafter bays can "vent" into that mini attic space, and the mini attic acts as a sort of collection point to allow multiple rafter bays to share a common roof vent. You have to be careful with the net free area (NFA) ratings if you go this route -- ideally you want a bit more intake (soffit) vents compared to outlet (ridge/roof) vents.


  3. walta100 | | #6

    Let’s consider avoiding stupid and not putting the HVAC in the attic.
    Have you considers floor cassettes for your mini split.

    Or converting part of a closet into a space for the air handler and dropping the hallway ceiling for some ductwork.

  4. BirchwoodBill | | #9

    I am Zone 6A — I sealed one attic with 2 inches rigid foam insulation against the 2x4 truss rafters and then taped the seams. Another attic was sealed with 1/4 plywood that was covered with 2 inches closed cell foam. The rigid foam insulation roof performs better, I.e. warmer in the winter than the ccSPF. However, I monitor the RH of attic on a regular basis with Accurite sensors. No problems so far. We have a very simple gable roof that is vented.

    1. Expert Member
      DCcontrarian | | #10

      The moisture buildup you have to worry about is not in the attic, but between the insulation and the sheathing.

  5. walta100 | | #11

    “The moisture buildup you have to worry about is not in the attic, but between the insulation and the sheathing.”

    I agree if the attic is actually conditioned and more or less the same temp and humidity as the rest of the home.

    Inside the attic could be a problem should someone insulate the roof as described but fail to condition the new attic space by connecting it to the rest of the house and deciding not to remove the old insulation from the attic floor. IE hoping to find a free lunch. This encapsulated attic could often have equal dew point and temp allowing water to condense and become wet and moldy.


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