A local business has the main duct plenums for return and supply located outside going to an RTU furnace/AC on the ground. Two trunks around 10ft. They are insulated on the inside but does not look to be the best done job. And, NO it is not possible to move the systems inside at the moment. They are planning an addition in a yr or two and would like a temp solution. It is in Iowa, in a heating and cooling climate. 

How do I insulate the system better. I would like to spray foam the outside of the duct work with a closed cell product. 

Does anyone know of a good working product that is UV rated? Or, is it best to just paint over top?

Tags: HVAC, insulation, outdoor

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Spray closed cell & then paint over / check with manufacturer for recommend products

Thats what I figured. Just wanted to see if there was a product that would do both in one application. 

I have done this a lot.   I use sheet foam and wrap with PTO - PVC Just like a roof

Hello, Another approach would be to wrap the duct in fiberglass and then use a sheet metal (aluminum) cover, caulked at the seams to prevent any sort of moisture from getting in. This will be much easier to modify later on. 

Yours,  Larry

Due to just needing a temporary fix, I'd seal the ducts, wrap them in vinyl coated R-19 and build a weather resistant cover over everything. This would be the easiest to de-construct in the next couple of years and also give your clients enhanced HVAC performance. 


You have to think about the skills of worker.  cost of the Job.  There is maybe 8000 ways to seal and add "R" to duct work. 

One system we specify was a contractor suggestion. The "old way" was board insulation and so-forth, pretty common but with a covering of roof membrane to match the roof material. That worked great except it was not a sanctioned application for warranty of the roof membrane and the labor aspects crossed lines of filed sub-bid rules with our public bid laws. Whatever.... 

The substitution we now accept is the same base layer: Seal the duct work to Seal Class A/Leakage Class 1 if you can test that low. ("Seal the crap out of it"- if I could put that in my specs, I would.) Two layers, staggered and taped but with the vapor barrier ONLY on the outside layer. Not two layers each with FSK for example. 

Minimum application would be a base layer of 1.5" which infills between the Ductmate/TDF flanges and subsequent "build layers" over that for full R value.  Makes a smooth surface.  An inverted 1" angle iron and score on the top sheet gives pitch if you like.  Corners are finished with drywall type corner beads and finishing cement. 

Now the finish:

The exterior part is a product of Venture Systems, Venture Clad, which is an aluminum sheet with peel and stick adhesive.  It has, usually, a leather-grain finish, is highly UV resistant though reflective. It is applied using good principles of  lapping. Start low and overlap as you go up over the top.


Insulating any kind of ducting in your home is important to prevent heat loss, which can result in higher than normal heating bills. Ducting that runs outside your home requires even more insulation because it is exposed to the outside elements, such as cold weather and rain. You can insulate an outdoor duct with the proper supplies and some moderate handyman skills.

Step 1
Remove old or damaged ducting insulation with heavy duty cutting shears. Many outside ducts have the wrong kind of insulation on them and are probably not weather resistant.

Step 2
Scrape off any remaining insulation pieces, glue or tape residue with the putty knife.

Step 3
Clean the duct area with the degreser and clean rags. The new ducting insulation won't stick well unless the surface is clean.

Step 4
Install new, foil-style duct insulation, which comes in rolls that are different lengths and widths and that has a foil backing on both sides with air pockets in-between. The R-value on foil-backed duct insulation is usually 65 or higher, making it a good choice for outside ducting.

Step 5
Start at one end, and wrap the foil-backed duct insulation around the ducting. Use overlapping wraps as you go from one end of the duct to the next.

Use metal tape to seal any seams. It has a sticky back and is specially designed for duct sealing.

It tears off of the roll just like duct tape does.

Step 6
Cut any corner seams with the heavy duty cutting shears. Cut them at a 45-degree angle.

Use the metal tape to close the cut seams. Overlap the coner seams a bit to make certain no leaks occur.

Step 7
Install heavy plastic sheeting over any ducts exposed more directly to rain or snow. Staple the heavy plastic sheeting to the walls and the bottom of the roof overhang.

If you are looking for just a couple year fix, take a look at the attached pictures.  I think spray foam and then wrapping with Radiant Barrier Foil For Outdoor Use will work great.  These are pictures a customer sent us.  They were using portable AC Trucks to cool the building while it was being renovated/replaced but could not get the air cold enough due to the length and sun.  They wrapped the ducts and instantly the air temperature dropped over 20-30 degrees.  I would not recommend this as a "forever" solution, but would work great combined with foam for a couple years.  Hope this helps!


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