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Community and Q&A

Creating unconditioned attic (and ducting/furnace question)

jameshowison | Posted in Green Building Techniques on

I’ve been reading the excellent guidance on attics here:

I’m in Austin TX, so that’s zone 2. We almost never get snow, but we do have humid periods. The current attic is a very complex shape, with multiple gables, valleys etc. More, when the house was extended (at least twice) they left part of the old roof in place (asphalt shingles and all), so it’s hard to move around in most of the roof (and about 1/3 third of it has almost no crawl space under it, making sealing the underside of the roof almost impossible there). The attic currently has about 3 inches of a combination of old fiberglass, cellulose and rock wool. We have one room at the back stripped to the roof for renovation. There are two large solar powered attic fans. There are no soffit vents (venting is at the top of the gables, currently covered with a very fine mesh screen.) We’ve had lots of rodents in the roof in the past (droppings in the insulation we removed in the room we’ve stripped).

Our HVAC ducting runs around the place and is all flexible tubing with fiberglass wrap at about R2 lying on the attic floor. It’s failing in multiple places and is poorly sealed. Our Furnace and blower for the AC is a vertical Ruud Shiloutte II, about an 80% efficiency model. It’s not a sealed combustion unit. It’s in a closet on the ground, and the top of the closet is open to the attic. Our building inspection mentioned that it should have combustion air brought to it from outside.

So I’ve been considering sealing the sloping roof but it seems that I’d have to upgrade to a sealed combustion furnace, which given the ~1200 heating degree days and my budget doesn’t seem worth it. Especially as the 2″ spray foam on the underside would come to about $2000, with rigid foam under that to R-38 being another $2000. And that leaves out the section with no crawl space.

Short of replacing the entire roof which is beyond budget (but something we’d like to do long term, perhaps adding clerestory windows)

So I’m thinking that the best option is to leave this an unconditioned attic, based on blown-in insulation (maybe Green Fiber to about 12 inches, approx $600 all up). That means at least four things.

The first is new ducting. I’m thinking that fabbing them from duct board makes the most cost/effort sense, something like CeertainTeed Duct Board. But I’d like the ducts to benefit from the R-38. So I was thinking about how to do that and wondered if it made sense to build a box around the ducts (with plywood or OSB) such that the sides were covered by the blown-in insulation. On top I’d continue the sides up a foot so that the insulation could build up there. This then means that the ducts are in a semi-conditioned space. I think the major concern here is condensation on the ducts, which I hope would be ameliorated by the duct itself being about R6 (polyiso or fiberglass board).

We’d also like the dreaded recessed lights, very likely using LEDs. I think the move here is to build sealed boxes around them, out of rigid foam with spray foam sealing.

Third is sealing against rodents, for which we’re thinking metal mesh around the roof corners (where the walls meet the roof), sprayed with foam.

Fourth is dealing with combustion air supply for the furnace. Here I’m thinking that above the furnace closet we build a foam box rising to the roof, then a vent in the roof, so that the furnace has access to combustion air from outside, but air from the unconditioned part of the attic can’t find its way down into the house. Maybe the vent needs to be piped down to the combustion air intake somehow (is that even possible or do these just take air from the register for combustion)?

I’m not sure about how to go about air sealing the older parts of the house before blowing in the insulation. The older roofs are metal lath and some very hard plaster. Even assuming I seal all that well, I’m concerned that it doesn’t function as a vapor barrier between the conditioned house and the cellulose insulation.

I’m also wondering if, by the time I box the ducts and the can lights, it might not be easier to just build and seal a whole false floor for the attic, about 15 inches above the current rafters, sealing the sloping sides up to 15 inches with spray foam (hey, I can afford that much ;), with blown-in insulation on top, running the ducts and cans through that space. I don’t hear that approach discussed much, although it seems to make sense to me.

Any comments appreciated. (sorry for the length, but I wanted to convey the whole situation).

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  1. wjrobinson | | #1

    James, your furnace if in a small room just needs a register connecting to larger space. I think your plan is pretty good for a DIYer. Forget the last option. Keep all as simple as possible. I think your buried ductboard is fine.

  2. dickrussell | | #2

    Before you just bury the ducts in porous insulation, think about the new temperature profiles you'll get from hot humid attic air in summer in to the duct itself. The surface of any existing vapor barrier around the duct will become much cooler as a result of the added insulation outside it, to the point where condensation could occur where you don't want it. You ought to have the duct vapor barrier out a lot closer to the air in the attic space. That normally would be in the form of a new wrap of insulation, with integral vapor barrier. If you bury the existing duct in cellulose, then enclose all of that in something that is a vapor barrier, you'll be fine. I'm picturing the OSB box you envision building around the ducts, full of cellulose. If you first lay a sheet of polyethylene into the box, with the duct over it, then fill the box with cellulose, and finally bring the sides of the poly sheet over the top and tape the joint, this might do the trick. You will have to pay attention to air sealing at the ends of the duct run, of course.

  3. Expert Member
    ARMANDO COBO | | #3

    You have too many issues to give you simple answers; I highly recommend you hire a professional. In Austin you can call Doug Garrett, Building Performance & Consulting, 512-259-2324, or [email protected]. I know Doug to be one of the best in the country and he is from your neck of the woods (Leander)

  4. GBA Editor
    Martin Holladay | | #4

    To summarize, your attic has the following problems:

    1. Your attic has a very complex shape with multiple gables and valleys.

    2. When the house was extended (at least twice) they left part of the old roof in place (asphalt shingles and all), so it's hard to move around in most of the attic.

    3. The attic is very poorly insulated, with only 3 inches of a combination of old fiberglass, cellulose and rock wool.

    4. There are two large solar powered attic fans, which are almost undoubtedly depressurizing the cramped attic enough to increase the rate of air leakage through your leaky ceiling.

    5. You attic is infested with rodents and rodent droppings.

    6. Your ductwork is located in your unconditioned attic. The ducts are in poor shape and the seams are not sealed.

    7. You have an atmospherically vented furnace that is located in a room that is connected with your unconditioned attic.

    8. Your leaky ceiling is punctuated with recessed can lights.

    9. Your budget is tight, and you can't afford to install spray foam to create an unvented conditioned attic. Nor can you afford to replace your roofing.

    Wow! Did I miss anything? As Armando said -- that's a lot of problems. Your attic could be used as the poster child for a bad attic.

    I can't solve all your problems, but I'm happy to provide you information on burying attic ducts in insulation:

    1. Here's an article I wrote for the February 20005 issue of Energy Design Update.

    2. Here's a report by researchers from Steven Winters Associates on a study of buried ducts.

    Good luck.

  5. user-659915 | | #5

    James, if I read your post correctly it sounds like you are planning to make matters worse by adding can lights to the list of problems you have to deal with. In a word - don't. In addition to the air leakage issue they are intrinsically inefficient light sources and even if you mitigate the situation by using (expensive) LED luminaires you will still be paying an unnecessary energy penalty, as well as stealing money from your limited renovation budget that could be better spent elsewhere.

  6. wjrobinson | | #6

    Gents. Read James' post.
    He is going to seal for squirrels etc. check
    He is in a cooling climate, the recessed lights are no biggy and he will overinsulate them well. check
    He is going to add insulation like blowing in cellulose or Spyder since there are lots of obstacles. check
    He has a tight budget. check we all do. Cellulose and DIYing takes care of budget.

    James, go to it. I think you will be doing quite a bit to improve your situation.

    The furnace can take air from anywhere convenient. IF it is inside isolate it to inside air. IF it is outside the envelope then isolate it to the outside.

    Yes, some contractors taking a look would help with ideas. You can still do the work and save over 50% in costs.

    Get err done.

    Edit add, Recessed lights in a cooling climate with insulation above them (non-cathedral ceiling) are not the problem they are in a heating climate and stuffed into a cathedral ceiling. Use your head folks. Cold air in the home drops (does not want to go through the light). Warm air above the light rises and does not want to drop into the home. If my physics is off here let me know otherwise cooling climates with modern sealed insulated recessed cans and CFLs or LEDs are absolutely OK.

  7. homedesign | | #7

    James H.,
    I think Armando, Dick, James Morgan and Martin are giving good advice

    Can lights are NOT "absolutely OK" in a cooling climate
    When a home is air conditioned ... Cold dense air can spill out thru openings in the lower part of the house.... reduce the pressure in the house and allow/induce attic air to spill into the living area.

    An atmospherically vented furnace can not simply "take air from anywhere convenient"
    The venting is a serious matter and should be carefully considered by a professional.

  8. wjrobinson | | #8

    Brooks, the posts asks for help keeping his cost down. The recessed cans covered with boxes and much more insulation is fantastic. Your post is not. My furnace thoughts since it is staying is the way they are installed, you parsed what I said. I said it has to have access to the homes air or the outside, one or the other is how it is done with existing air leaky hones. I also said advice is a great option via getting some contractor quotes. As usual brooks you are falsely stirring the pot with me. Have a nice day my man.

    This home can be improved without involving an architect but if you really want to help, you could stop by the home and volunteer.

  9. jameshowison | | #9

    Thanks to all for your input. Martin, you listed the issues very well. I was chuckling by the end :) The links to the building science articles are also very much appreciated.

    I'm pretty convinced now that I'm on the right track. Rather than bury the ducts I'm leaning towards building a box around them and vapor-barrier sealing that (perhaps build the box out of taped foam), which should take care of the warm, humid air penetrating to the cool ducts problem. That also should make service on them a lot easier (much easier to clean off the top, then lift it off). The roof structure doesn't involve complex trusses inside, thankfully, so essentially I'd be building something that in the end would look like the plenum trusses shown on p 19 of this Steven Winters Associates report:

    Didn't realize that recessed can lights were such a touch-point, but they are an aesthetic requirement here and as long as they are low-heat (ie LCD) and well sealed and insulated above I think they'll be fine.

    FWIW, no rodents currently in the attic, but given that they were there before and we're using loose-fill insulation again I think we'll mesh and foam seal the likely entry points.

    I'll be heading up to the attic shortly to disconnect the solar power fans (this is a no brainer while we have part of the ceiling completely open!).

    The furnace remains a bit of a mystery to me. I'm considering sealing across the top of the furnace cupboard (and around the furnace cupboard), but using a three inch pipe to connect that into the attic, well above the loose fill. Then there should be only one source of combustion air, from the vented attic.

    Thanks for the contacts, at first I my response was "Hey, I want to figure this out myself" but as I reflected on it more, I decided to contact Doug to see what his consulting rate might be for a site visit.

  10. wjrobinson | | #10

    James, you are preparing well. I am sure you will complete your project well.

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