Insulating Rigid Ducts in attic space - best practices?

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Permalink Reply by Sean Lintow Sr on June 15, 2012 at 1:08pm

Bubble wrap is nothing but marketing BS, and the FG isn't even going to be close to R8 when you get it installed

First I would fix the duct leakage, air seal the attic area under & around the ducts & dense pack cellulose under the duct

Wrap duct with foam board on 3 sides - 2 layers offsetting your seams & corners (Liquid Nail actually holds pretty well & not much is needed, though that product you listed may work ok also) - finish off sealing all seams &edges with special UL listed HVAC tape

Finish off the attic air sealing & blowing in cellulose

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Permalink Reply by Craig Bird on June 15, 2012 at 2:21pm

Sean - I am with you on the duct/air seal - it is in all my scopes. While I like your idea in theory, the practicality is an issue for me. The labor involved would be 3 times as much as a fibrous insulation due to all the cutting and fitting you would need to do. Additionally, most codes do not allow for rigid foam board to be left exposed in the attic, so you would need a thermal barrier.

The compressed R-value on the 2.5in batt(R8.5) is 7.0, the 2.0in batt(R8.2) is 6.4 at 25% compression. It will have to do. It is going to get too hard to install anything thicker. There is a 3" Batt at R10.2 / R8.4 compressed. May try that if it is an easy wrap with lots of clearance.

If the clearance below the duct is too tight, I am thinking spray foam duct to the lid to create an air tight cavity below, bringing it into thermal envelope.

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Permalink Reply by Sean Lintow Sr on June 15, 2012 at 8:57pm

Having been in enough attics out there, I know it can be a nightmare but in reality fitting isn't to difficult  - you simply cut the panels to the average proper width or a fraction larger, adhere, and go (the cutting can be done in the carport or garage). Perfection isn't really required as a little foam can fix any little gaps

Check first with the AHJ, as many simply ignore that piece and/or look at the flame & smoke index rating which eliminates the thermal / ignition issues

Now as for the hallway duct runs where all you can get to is the top - yeah it can be a nightmare but you can easily use foam panels sealed properly above them to bring the ducts into the conditioned space & cover it with more cellulose - voila there is your thermal / ignition barrier 

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Permalink Reply by Patrick Michaelyan on June 18, 2012 at 10:02am

Craig, are the ducts a mix of round and flat (boxy) sheet metal?

For round ducts, consider slipping flex duct over them (which, if done correctly, can eliminate the need for sealing those ducts).

For flat ducts, I am with Sean. The flame spread ratings are sufficient for attic applications. Also, the install becomes quicker and better with each attempt.

Patrick

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Permalink Reply by Beau Bledsoe on June 21, 2012 at 8:33am

I'm not sure some of the replies below adhere to building science principles.  Duct should follow the same principles as the building envelope.  1) The pressure barrier should minimize air flow - seal the duct work at all joints and seams.  Mastic must be used.  Adhesive on tape will not last long term in the heat of an Arizona attic.  Flex duct will not seal rigid duct unless carefully sealed at each end.  2) the thermal barrier should be in contact with the pressure barrier to minimize loss / gain due to convection.  I'm not sure how to do that with a box fabricated with foam board.  The insulation on flex duct is minimal at best.  It would need duct wrap put over it to get anywhere close to R8.  
Best practice required by IECC is to seal with mastic a nickel thick and R-8 duct wrap with minimal compression.  Typically HVAC contractors staple an overlapped seam of duct wrap then seal any exposed fiberglass with foil duct tape.  This keeps moisture (humidity) from infiltration the fiberglass, then condensing on the cold duct.  This may be less of a problem in Arizona than here in Georgia.

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Permalink Reply by Ed Minch on June 22, 2012 at 4:36am

We did some studies with a major regional builder in the mid-Atlantic (hot humid).  The spec they finally wrote was: all ducts on the attic floor sitting on the bottom truss chord or the back of the drywall if possible, runs as straight as possible, but gentle curves when needed, sealed tight, then more insulation material blown over the top of the already-insulated duct to about 4-5 inches tapering gradually on the sides.  Computer modeling suggested that this accomplished most of what moving them inside would do - in fact that same builder is finally now moving the ducts inside for V-3.0 of EStar.  

They had an engineer on staff, so complaints from building inspectors were mostly easy to deal with.

For solving problems in this builder's older houses (and all other houses with attic ducts) our policy is cutting all straps holding ducts off the floor of the attic, disconnecting flex at the trunk and re-routing through the truss work to be straight from the boot to the trunk, cutting off excess length of the flex (up to 5-6 feet of duct which is 10-12 ft2 of surface area in a 7" flex duct, and lots of friction), sealing all duct seams, nestling the flex ducts into the existing attic insulation as much as possible, then blowing additional insulation to about an R-20 over the top gradually tapering to the normal attic thickness at the sides.  Some trunks are suspended a foot or so off the attic floor so we will wrap them with another layer of R-8 if blowing is not practical.  This has resulted in about 10-15% more flow out of the treated ducts and also about 4° better temperature on a design day, heating or cooling.  More air with better temperature.

Still leaves that damn air handle sitting there.

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