Properly Sealing HVAC Duct Work

craymer45 · 11-06-2008, 08:45 AM

I am currently in the process of sealing the duct work in my house. I have a standard HVAC forced air system. I am using expandable foam for gaps and HVAC mastic tape for duct joints. I have received conflicting advise on the following issues, so I thought I would ask the experts. Here are my questions:

1. Do I need to seal all joints and gaps on the duct work for cold air returns? Or is this just a waste of time and money?

2. When I inspected the inside of the duct work for cold air returns, I noticed there are gaps allowing air to flow into the floor joists instead of the walls. How concerned do I need to be about these gaps? (See attached pics).

3. Finally, Do I need to seal gaps between the end plates and joists of the cold air returns in my basement?

Marvin Gardens · 11-06-2008, 09:47 AM

It looks like your basement is a conditioned space. Therefore no more insulation is needed.

Mastic is the correct way to go on all the feeds to the upstairs.

No need to seal the return joints. A waste of time and money. Leaks in the return mean nothing unless they are coming in from outside the conditioned space. In that case something needs to be done.

It looks like a new house and as such I have to assume that the HVAC people did it right.

For the opening on your second picture it would appear that there is a wall stack coming down and it is open at the end of the return. Either this is by design or by mistake. I would do nothing about this since if it is by design then you would cut down on the return to your furnace which is a bad thing. If it is my mistake there is little problem with it if you have flow in the basement.

That being said it is a rather large opening and I would contact the HAVC people and ask them if that was designed to be that way. Just for piece of mind.

sweaty · 11-06-2008, 03:36 PM

I would seal the returns from the top floor, since that level seems to be the hottest in the summer and the coldest in the winter. I would think that, the more circulation there, the better.

**beenthere** · 11-06-2008, 06:06 PM

Seal those return leaks.
They are a HAZZARD.

You have a natural draft gas water heater. Those unsealed returns can draw combusted gas from the dilution hood, and spread it into the rest of your house.

Winchester · 11-06-2008, 09:05 PM

I would seal all the returns. My energy auditor had this as #1 or #2 on a prioritized list that was conducted earlier this spring. Situations are different for everybody and we're not aware of the layout of your house. But as for me, I have a 2 story and the suggestion made sense to me. Hard for a system to work correctly, pulling air from those rooms upstairs if there are several leaks in the basement. You're going to lose that air current upstairs when it's primarily taking it from the basement. IMO, it's the path of least resistance working here.

Once again, I'd go for the sealing all you can. You can use cheap caulk to seal this joints just as well. A very inexpensive fix that I feel is "bang for your buck."

To each their own though.

hvac122 · 11-06-2008, 09:47 PM

As said "seal the returns". If you don't it will cause a negitive pressure in the basement and cause your water heater to backdraft.

Also in the second picture there is supposed to be a header blocking off that joist space. Looks like installers missed it. I would use mastic to seal ducts and seal panning and all holes in the return air even along the joist space.