Rigid insulated duct for ERV instead of flex

trepexApril 22, 2013

Hi folks,

Our current home (and new home under construction) has an Energy Recovery Ventilator that we run pretty much year-round. The house uses rigid-foam cladding, and it's very very tight, so we rely on the ERV for regular air changes.

The unit is installed in the basement and the HVAC installers use insulated 6" flex duct (see link) for the intake and exhaust runs that are about 25 ft.


In our current home we've had to get the intake duct replaced three times because of either slits and tears in the vapour barrier, or else poor sealing on one side or the other, allowing humid inside air to get into the insulation and condensate on the inner duct that is passing really cold winter air to the unit. Each time water has accumulated in the lowest point of the duct run.

After the first two times this happened, I stopped having the HVAC guys do the warranty repair and I terminated the ends of the ducting myself to make sure it was done properly. The installer was lazy and used screws to hold the duct to the exterior cap, and then just wrapped tons of "duck tape" around it, taping it on. I followed the proper installation procedures of sealing the duct and vapour barrier separately to the collar, etc. Problem is that when the installer shoved the duct up into the floor joist, it came in contact with a bunch of screws poking down through the subfloor from above. I tried to catch all the tears and seal them with tuck tape, but I must have missed some and after this winter, once again I'm getting a bit of water accumulating in the insulation.

I'd like to not deal with this in the new home and replace the flex duct with insulated rigid ducting myself. Does this exist? If so could someone recommend what I'd need? Could I maybe get away with a smaller diameter since rigid wouldn't restrict airflow as much as the flex does?

Thanks in advance!

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Metal duct with externally applied isulati.

Make sure the sheet metal is sealed at EVERY joint (every corner also) before the insulation goes on.

You likely will have to assemble, seal , then insulate the duct into one piece below its final position and then raise it up as required.

Luckily it is not heavy, just big and flimsy.

Since I usually had only a small work team, we would raise pieces of duct work like this using nylon webbing.

At least one strap for each section of metal ducting to avoid stressing the joints between sections any more than we had to.

Extra metal was often placed across the sealed joints from section to section to strengthen the large assembly.
Pop rivets to attach it all, and then mastic to seal the rivets.

We shortened the webbing by stapling it to the sides of adjacent joists over and over to raise the piece nearly level.

Then final strapping to hold it in place and removing the webbing used to raise it.

Attach to the fixed end (outside wall collar) and then fit to the ERV (since you can at least move it slightly during attaching and sealing).

It looks like on of the dumbest ways to install a run of duct, but it results in great sealing that survives installation with no leaks being introduced.

    Bookmark   April 23, 2013 at 12:29PM
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