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Community and Q&A

Best Practices for Bringing an Attic Into Conditioned Space

jonathanb | Posted in Energy Efficiency and Durability on

As we look to upgrade our HVAC, we want to air seal the attic in California (Zone 3C). The question: should we do that by encapsulating the attic with spray foam under the roof deck, or should we air seal the top of the ceiling and add thicker insulation (fiberglass or blown-in) on top?

Our vented attic is only 19″ high at the peak. It’s got 6″ of fiberglass insulation, and has never been air sealed. It’s acquired some recessed lights over the years.

Before we start this project, the house will be all electric, so no gas furnace or water heater ducting through the roof.

If we air seal at the roof deck, the whole space becomes conditioned, and we could maybe run ducts through the attic. If not, we will go with mini splits, either high wall or in-ceiling, buried under some insulation.

I’m not sure if it’s feasible in such a small space to encapsulate the attic, or if it’s feasible to air seal and replace the insulation. What would you do?

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  1. JC72 | | #1

    IMO Air-seal the ceiling then add insulation on top in the attic. I don't see another option other than a re-roof with rigid foam.

    Spray foam wouldn't be my first choice.

  2. Expert Member
    BILL WICHERS | | #2

    Is your HVAC equipment located in the attic? If it is, then I'd encapsulate the attic. If it's not (let's hope it's not :-), then I'd air seal the attic floor, including boxing over the can lights, then install code depth blown cellulose.

    I'd only use spray foam if you're encapsulating the attic and you have to do it as an unvented roof.


  3. jonathanb | | #3

    Thanks, John @JC72 and Bill @Zephyr7!

    Spray foam is also really expensive, I've discovered... We won't be using it.

    Good question about the HVAC. We're considering running HVAC ducts in the attic. No equipment up there though. That will be in a utility closet accessible from the living space. If we do put ducts up there, the only way we could get enough insulation is by encapsulating the attic, sealing off the vents, air sealing, and filling up literally all of the open volume with insulation. The whole attic is only 19" high at the peak, sloping to zero at the edges.

    If it's just a duct in the attic, and there's nothing else we would ever need access to, it seems like it ought to be ok to completely fill it in with blow-in insulation.

    Anyone else ever do that? Any issues?

    1. Expert Member
      BILL WICHERS | | #4

      You could insulate over ductwork, but they are prone to leak.

      You should avoid running any sort of mechanicals (ducts, plumbing) in the attic. The only plumbing that should go up there are vent runs, and I'd try to limit wiring too, even though wiring in the attic isn't generally a problem from an energy efficiency standpoint. You should really avoid running any part of your HVAC system up there at all.


      1. jonathanb | | #5

        Thanks, Bill! In that case, we could avoid the ducts by putting mini split ceiling cassettes in the upstairs rooms. We'd still run the lines to them through the attic, just so we don' t have them running along the outside of the house under the eaves. With mini splits, we could keep the attic ventilated, and still fit enough blow-in insulation to keep it efficient.

  4. exeric | | #6

    I think standard procedure in the non-humid temperate climate you live in is to bury the ducts under cellulose insulation. It's important to weigh and balance the cost and efficiency of using different routes to get to an endpoint. Sounds like your attic is non-useable space with it's small volume. People run ducts all the time in attics in California and then bury them. It's a non issue as long as you don't use that space for anything. That includes storage. You do not want people trundling up there stepping on buried ducts.

    The main thing is to air seal the attic floor before doing any work up there. When you put in the ducts you absolutely must make sure that it's also airtight. You can have that done if you have a contractor you trust. If it's done once and individuals never go up there then it will stay tight. That's the key, don't define the attic as something that is accessible and useable except for emergencies after that work is done..

    1. jonathanb | | #7

      Thanks, Eric @exeric! Great information.

      We definitely won't have anyone up there, and we don't store anything up there. It's just enough space to run R8 flex duct and bury it.

  5. climbing_carpenter | | #8

    It's going to be very difficult for you to airseal the attic floor.

    Because your roof tapers to zero at the eaves, you'll continue to experience a good deal of heat loss through the top plates of your walls and the eaves of your roof assembly.

    Seems to me exterior insulation is your best option- in conjunction with robust cavity insulation (would effectively fill much of your attic). That would, however, dictate you replace the roof. If not possible, consider spray foam under the roof deck and down over the top plates of the walls. You'll have to box-in any venting on the exterior as the spray foam will fill your soffits. Fill the remaining attic cavity with cellulose.

    There are plenty of posts, articles, and tables on the site that will guide you to the appropriate amount of foam. Do not follow advice of an installer as many operate on rules-of-thumb.

    Run mechanicals elsewhere. Consider wall mount heads instead of ceiling cassettes and find a different HVAC contractor who will be more tactful about concealing linesets.

    1. jonathanb | | #10

      Thanks, Chris!

  6. exeric | | #9

    After thinking about it some more it seems like it may not make much difference to air seal at the attic floor or at the underside of the roof deck. They will both be hard to access at the perimeter considering the low height of the attic. I would say just use closed cell foam in a 1" thick layer for whichever one you choose. It will be easier to assure a good sealing job on the perimeter using cc foam where access is tight at the perimeter.

    It seems like overkill to replace the whole roof because of the shallow perimeter insulation when living in such a moderate climate. One can be practical and get 90% of ones weatherizing fullfilled at a much more moderate cost. I understand because I live in the same climate with the same limitations on my home. The cost structure is very much different between building from new with current best practices and retrofitting an older home. It is a completely different balancing act.

    1. jonathanb | | #11

      Thanks, Eric @exeric. That's a good argument for using foam around the perimeter.

      I'm realizing that energy improvements don't necessarily pay for themselves, particularly in a mild climate. California still sells natural gas for super cheap compared to electric. We put in a heat pump for our first floor, and our heating bills went up a lot compared to the old 80% gas furnace. Our solar panels now cover that usage, but of course extra solar panels aren't free.

      We're going to go all-electric, air seal, insulate, upgrade glazing, etc. But it's not for the money, or even for comfort.

  7. jonathanb | | #12

    I wanted to give an update: we ended up installing ducts and then burying them in blow-in insulation. It works pretty well!

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