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  1. #1

    Which spray foam for A/C ducts in attic

    Hi all. New here.

    We just finished installing an A/C air-handler with rigid round metal ducts in our attic. The installation was done by a HVAC pro. (We are located in Southern Ontario, Canada).

    Our plan is to insulate the ducts with spray foam. The question is which type/brand to use?

    Each dealer/installer we talked to talked up their product and diss the competitor's.

    We are most confused about VOC and other nasties released during application and the time required to vacate the house.

    We are also confused about fire rating and requirement for a fire barrier.


    Any suggestions, opinions, comments etc most welcomed!



  2. #2
    spray foam?

    wrap that **** with real insulation....don't take the lazy way out.

  3. #3
    Join Date
    Jul 2007
    Phoenix, AZ
    I would also suggest wrapping it with insulation. Exposed outside duct work we have foamed, but never duct work in an attic. The duct work we had foamed was done by a roofing contractor.

  4. #4
    Join Date
    Jul 2004
    The only times we've ever foamed is in coastal flooding areas where the duct work can actually meet up with the ocean! Foaming helps prevent rusting and if it does rust, the foam maintains the shape! However, there are different flammability requirements for foams. In order to foam in the attic, you may need to provide a fire barrier if you use the wrong stuff. So be sure to check the requirements and specifications so you don't run into a problem. Personally, I'd seal the ducts with mastic, let it dry, then wrap with the highest R-value I can find. If that's not enough, double wrap it!
    If YOU want change, YOU have to first change.

    If you are waiting for the 'other guy' to change first, just remember, you're the 'other guy's' other guy. To continue to expect real change when you keep acting the same way as always, is folly. Won't happen. Real change will only happen when a majority of the people change the way they vote!

  5. #5
    Join Date
    Jun 2006
    YCC, why are you insulating the ductwork? We did as well, but I am curious as to your reasoning. Is it to keep the ducts colder during the cooling season, or for insulation in the winter season (or both)?

    We installed an a/c system in our attic last summer. We have a 3,200 sq. ft. bungalow (I think Americans cal them "Ranch Style") that has radiant heat so we had to install new duct work in the attic. The contractor used a joint sealing compound for all joints in the duct work and then used a quality tape to cover each seam. On top of this, a double layer of fiberglass insulation with aluminum backing was wrapped around all of the duct work and taped with aluminum tape.

    Have you considered what to do about the inevitable moisture build up in the vents during the winter months? Our contractor ( and several others) advised that during the winter, moisture from the house (breathing, cooking, shower, etc.) would enter the vents and condense and freeze. In the spring the frozen condensate would melt and run out of the vents, or worse pool in the vents and grow mold.

    The solution was to either prevent the moisture from entering the duct work or prevent condensation from forming. The former involved fitting vents covers that would seal 100% (I couldn't find any) or install heaters in the ducts to circulate warm air through the duct work.

    We chose to install duct heating equipment and since our air movers have variable speed ecm motors, we have them set to turn very slowly (just enough to circulate air). Our control system measures the temperature of the air in the room and at the exit of the ducts and only adds enough heat to maintain the same temperature (i.e. we don't use the duct heaters to add heat to the house).

    This system added cost to the install, but after one winter we have had no problem with condensation in the ducts. Ideally we would have preferred a simple system to seal the air registers for two reasons: Firstly, it would have cost less and secondly, we wouldn't be adding any heat to the attic.

    This second point is something to consider. Even though the duct work is double insulated, there must be some heat loss to the attic while the heaters are running and warm air is moving through them and this heat ends up in the attic. If you live in an area where ice damming is a problem (or your house is susceptible to it) , any heat loss into the attic will aggravate the problem. We added additional roof venting when we replaced the roof last summer to take this into account.

    In the end, I think you are on the right path insulating your attic ducts, especially if you live in Ontario), but I wouldn't take any shortcuts. You'll pay for it later with moisture buildup in the ducts or heat build up in the attic.

    By the way, are their any building codes in Ontario that specify how the ducts are to be insulated (if at all?)

    At times, a forced air HVAC system seems like a simple answer, but I have to say we love our hydronic heating system.


  6. #6
    Join Date
    Feb 2003
    one can just foam under the roof
    one can also seal the whole attic

    one must consider how many hours the outside Realitive Humidity is above 60% -- to avoid drawing in moist air [such is the case here!!! ]
    read at

    seal all joints & seams of the ducts & boxes & unit with HVAC mastic. one does not have to overwrap this with tape.
    seal all 'boots' & elec boxes to the ceiling | floor | wall -- use spackling -- or caulking or mastic.

    what is your present attic insulation?
    how thick?
    are the ducts above it?
    harvest rainwater,make SHADE,R75/50/30= roof/wall/floor, use HVAC mastic,caulk all wall seams!

  7. #7
    Join Date
    Jun 2006

    Your advice may be correct for the warm climate you live in, but I doubt insulating above the ducts would address the problems of duct condensation in YYC's location, where winter temperatures can approac -20F on a regular basis. Duct condensation is a real issue.


  8. #8
    Hi all, again. Wow, lots of replies. Thanks.

    Our house is a 50's era double brick side split. There are no insulation on the outside walls. The outside walls and ceiling has tar paper as a "vapor barrier". The inside walls and ceiling are 1/2" drywall skimmed with 1/4" plaster. Air leakage is minimal for a house this age. (Every opening have been caulked, sealed, etc) The attic floor is insulated with fiberglass batts covered with blown-in fiberglass for about 8 inches (the height of the joists). Roof has 9 "top" vents and some soffit vent. There' probably not enough
    soffit vents because the spaces between the rafters at the eves are about 80% obstructed with "excess" brick and mortar.

    The house is about 1600 sq ft with 4 bedrooms. Bathrooms, kitchen and dryer vented to outside. Boiler hydronic heating, gas cooktop and gas water heater.

    Previous A/C system in attic was put in in the 80s. Combination of metal and plastic flex ducts. Factory pre wrapped. All ducts joined with metal "off the shelf" joints. Return air was the only custom made metal. Joints and return air was screwed, cloth taped, no mastic and wrapped in foiled backed insulation. Air delivery into rooms was thru metal diffusers on ceilings. Diffuser had a butterfly damper that leaves about 1/8" gap all around perimeter when closed. Damper and return air closed in winter.

    The old A/C was removed due to massive condensation problems. Literally gallons of water collected at the low points of the ductwork. Even though the A/C was not used at all this summer with the dampers and return air was closed all summer, the ducts were all dripping wet on the inside when they were removed 2 weeks ago.

    The new installation is all rigid round metal ducts. The trunk and return were custom. The joints were screwed and taped. No mastic. Ducts are all hung and suspended off the floor of the attic. All air diffusers were replaced with new plastic ones which closes completely. (100% plastic to plastic contact. Way better than before, but I don't think it is a 100% vapor seal.)

    This new system was installed with spray-foam in mind. But the spray foam contractor didn't tell us about the off gassing that required us to be out of the house for 48hrs. (My fault for not researching this deeper. I will post a rant later.)

    So, now I revisiting the issue of insulating this new A/C system.

    We originally chose not to wrap thinking that spray-foam is superior to wrapping. We thought that spray-foam, assuming a proper installation in a cramped attic, will seal all joints, cover all the surfaces and better insulate.

    But dangerous off-gassing during installation is now weighing heavily on where we go from here.

    Is spray-foam really superior to wrapping? Is the "superiority" worth the health risks? Are there other spray-foams out there that don't off-gas during installation? (Most spray-foams claim to be inert after curing. What they avoid saying is that the propellant and the curing process off-gases some real nasty gases.)

    Again, any suggestions, opinions, comments etc most welcomed!

    Thanks again, especially, kayjh.


  9. #9
    Join Date
    Jun 2006

    I'm not sure how much off gassing from spray foam will make it's way into the house from a well ventilated attic space as I haven't done any research in that subject area.

    I remember the UFFI (Urea Formaldyhyde Foam Insulation) debaucle of the 1980's. That product was injected into the walls of homes from the outside, where there was presumably an inside wall vapor barrier. Health Canada determined the material was a health risk even though there is a vapor barrier and helped consumers remove it.

    In your case, I'm not too sure why you want to use foam insulation if you are at all concerned with off gassing. In my experience, poor environmental regulation in Canada means that manufaturers can provide product that may have know health effects and not label accordingly. Doing your own research (as you are trying to do) is the best way to satisfy yourself as to health risks.

    I would wrap the pipes with insulation (fiberglass) unless you can satisfy yourself that there are no health risks associated with the foam. Prior to installing our new A/C system, we had vermiculite insulation (tested positive for asbestos) removed from our attic (very expensive) and had the space certified by an environmental company (Pinchin Environmental) as a "clean space". We then installed, sealed, insulated and outer sealed the ductwork just in case there were any rogue fibres left in the attic space (we didn't want those in the house).

    For new insulation, I contacted an environmental company to get an opinion on the safest insulation to use. They recommended celulose insulation as being the safest. I might suggest you contact an environmental company to answer your questions as these forums have alot of "experts". We used a fellow by the name of David Rousseau in Vancouver. He has written books on safe materials for use in home construction (see the reference below) - check the library or try to contact him as we did. We paid a $50.00 consulting fee:

    Rousseau, David & Victoria Schomer, Global Choices Resource Guide: A Guide to Env. Sens. and Low-Toxic Alt. Prods. & Educational Information, Vancouver, BC, Canada

    You could also try this link for a list of environmental resources:

    You should keep in mind that mold can be a serious health hazzard. Just because you are using plastic pipes doesn't mean you won't get moisture buildup in the ductwork - just that it won't rust it. I'd try to find some way to seal the ductwork during the winter months or circulate warm air so you can enjoy good air quailty in your home.

    Finally, you may want to contact Can Am Building Envelope Specialists. They are in Mississauga. We used them to air seal our attic (from the house fixtures, wall top plates, etc.) and insulate with cellulose. I'm pretty sure they used a safe foam product for the air sealing.

    Let us know how you make out.


  10. #10
    Join Date
    Aug 2007
    foam insulation is best r value possible, never sprayed duct in an attic but we had some on a rooftop sealed with it and it worked great. Water and air tight, have em do it on a Friday and go hang at a poolside room at Holiday Inn for the weekend
    You can't fix stupid

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