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View Full Version : Duct Sealing and Insulating?



Matt Meiser
07-15-2015, 11:39 AM
On my todo list since we moved into this house 11 months ago was to work on sealing the very leaky ductwork in the basement. When the AC is on, the basement is COLD, main floor right where the thermostat is set, and the upstairs is warm, with my daughter's room getting pretty hot if she doesn't have her blinds closed as she gets the afternoon sun and our bedrooms have massive (~30sqft) windows.

I got a quote for Aeroseal but decided against doing it at $2000, plus I had a heck of a time getting the sales guy to actually show up so I don't really think I want to deal with them. I bought 2 products--a really good foil tape and the mastic you brush on. Last night I tackled the main trunk. I taped all the corner seams and joints. I also taped all of the seams on the plenum above the furnace. I used the mastic at every take-off from the main trunk though there was a couple spots I couldn't get to due to framing, ducts, plumbing, etc. Some joints had gaps around 1/4"x1/4" or larger at each of the 4 corners, plus leakage along the seams and there was massive leakage at most of the takeoffs. It made a HUGE difference in the amount of air movement you can feel in the basement when the fan is running. I wish I'd taken a thermometer down as it felt like the basement temp had risen to match the upstairs by the time I was done--before you could tell a very obvious difference as you walked down the stairs.

I haven't done any sealing on the round ducts that lead from the trunk to every register which general seem to be significantly better than the trunk. Not perfect but it would take a lot of work and a lot of materials to seal all those runs, where I can even reach to do anything with them. Also obviously I can't reach the parts that go into the walls and go up to the second floor (every second floor register is its own run from the basement trunk.) I'm wondering if this is worthwhile? Another thought I had would be to take out as much of the metal runs as I can and replace with the flexible insulated stuff. I'd leave the first elbow off the take-off and the last elbow before the duct goes up into the wall or to a register on the first floor.

Also, the return trunk is similar--any benefit to sealing that? The returns are all wall cavities and they just used a foil covered cardboard product made for that purpose to connect those to the trunk.

Lastly, the main trunk is just sheetmetal. Any payback to insulating it? That looks to be a relatively expensive project unless I can find the material a lot cheaper than Lowes.

Eduard Nemirovsky
07-15-2015, 7:13 PM
Matt, I can't comment on duct sealing and benefit of it. But I have same temperature difference in basement and second floor. As soon as I install EcoFoil radiant barrier in attic - I can sleep in our bedroom on second floor in summertime. Minimal difference in temperature. Very comfortable.

Ed.

Kent A Bathurst
07-15-2015, 8:36 PM
Seal everything that is easily accessible. Or, at least, not a huge PITA.

Clint Baxter
07-15-2015, 9:28 PM
... Another thought I had would be to take out as much of the metal runs as I can and replace with the flexible insulated stuff. I'd leave the first elbow off the take-off and the last elbow before the duct goes up into the wall or to a register on the first floor.

Also, the return trunk is similar--any benefit to sealing that? The returns are all wall cavities and they just used a foil covered cardboard product made for that purpose to connect those to the trunk.

Lastly, the main trunk is just sheetmetal. Any payback to insulating it? That looks to be a relatively expensive project unless I can find the material a lot cheaper than Lowes.[/QUOTE]

Don't replace your straight pipe with flex. Much more resistance to your air flow. If you wish to insulate the run, take the straight down, put you insulation on or over it and put it back up.

You won't gain much of anything by insulating your returns. Unless they are running through areas of excessive heat or cold such as an attic

insulating the main trunk goes on the same vein as the returns. Not enough gains to warrant the effort. If you get everything air sealed as much as possible, you should see great Improvement in your system

Clint

Matt Meiser
07-16-2015, 7:14 AM
Our first house was 2 story and we'd forgotten about the temperature differential as our second was a ranch and we lived there almost 11 years. When it gets warm again this weekend we'll see how much difference the sealing makes. We do have good insulation in the attic (inspector said he was pleased to see it is insulated to the current codes) but no radiant barrier--I'll look into that. I've also recently found running the fan continuously balances the whole house out nicely, but between that and the hail damage to our AC unit which struggled to keep up for about 3 weeks of the last bill cycle our electric bill shot up $50. I'd like to find a thermostat that can schedule the fan to run a percentage of time as I think that would do it.

The problem sealing the runs is they did a pretty good job of tucking them up into the joist bays so they are pretty much against the floor above and at best have a couple inches each side, or worse pushed against the floor and one side. I need to see if I can get some narrower tape which would probably make life easier there, especially on the elbows.

I didn't think about the resistance of the the flex--good point.

Julie Moriarty
07-16-2015, 9:16 AM
On commercial jobs, the tinners seal every joint with duct seal and use insulated ductwork for all main lines and larger branch lines. The insulation is on the inside of the duct.
http://www.certainteed.com/images/products/highRes/ptoughguardmd.jpg

They also use sweeping bends or 90s, with turning vanes installed, for all but the last branch. This reduces turbulence.
http://www.symscape.com/files/pictures/windtunnel/turning-vanes.png

I've never been on a residential job where I've seen even duct seal used on the joints. let alone insulated ductwork or sweeping bends.

The main line in our house sits directly above the furnace. The original installers T-tapped it, creating turbulence. When I had our furnace replaced, I told the installer I wanted him to add vanes or sheetmetal that would create sweeps at that transition point. It made a difference but didn't solve all the problems. For the weakest flow branch, I added a Soler booster fan.
http://s1.img-b.com/build.com/imagebase/resized/x800/solerandpalauimages/td100x.jpg
That thing moves some serious air! It has two speeds and is set up for installing a hard-wired remote controller.

Besides the lack of joint sealing in my house, there are too many bends in some of the runs. The company that did the original install didn't have very well-trained workers.

Matt Meiser
07-16-2015, 9:39 AM
We have talked about adding a booster to the run to my daughter's room which is also the furthest. We have the T-situation at the top of our furnace as well. I'm not sure I've seen one that didn't around here.

We had duct board like your first picture in our last house.

Julie Moriarty
07-16-2015, 11:29 AM
If you want to tackle moving your furnace so you can access the horizontal main duct, you can install pre-made turning vanes. But the booster will probably be your best bet. Before I bought the Soler unit I had tried the HD "booster" and it was a joke. Soler makes a lot of different types of boosters. The quality is first class. I think the one I installed is 300 CFM. http://www.solerpalau-usa.com/ilccf.html

Mike Lassiter
07-16-2015, 12:27 PM
We have talked about adding a booster to the run to my daughter's room which is also the furthest. We have the T-situation at the top of our furnace as well. I'm not sure I've seen one that didn't around here.

We had duct board like your first picture in our last house.

you may find after the sealing you have done that was allowing leakage that more air is now going there and is making enough difference you no longer need that. I fixed extremely poor ductwork in a 93 Clayton single wide mobile home. I bought a ductblaster testing and actually tested before and after. Furnance blower rated at 1200 cfm, if memory serves 458 cfm was being blow in the belly and lost. That creates negative pressure in the living envelope which then sucks hot humid air inside from ever crack possible. So not only does the conditioned air you pay for NOTgo to where you want it, but a double negative is now to be pulling in outside hot humid air that mixes with the inside air and makes the living space much more uncomfortable. I was very unhappy with the things that I found and had to fix. Even where the furnance sat over the ductwork - out of sight in the belly under the floor where open gaps I could stick my hand through. I had to work on the front side of the main ductwork at the furnance by cutting the back side open and patching and sealing from the inside, then close the hole I made to work through and seal it.
I hope you had better people doing your ductwork. Loosing over 1\3 of the air to leakage caused $300 electric bills in a 16x76 mobile home and a central unit that ran continuiously until 10:00 PM at night and still had 90 degree inside temperature. Daughter was much happier and cooler after sealing every register and the furnance up. I don't recall the after test duct leakage after sealing but it was below 100 cfm.

Matt Meiser
07-22-2015, 2:36 PM
Well the sealing I'd done so far definitely made a difference. Late evening the upstairs was about 5 degrees warmer than downstairs according to an inexpensive thermometer. Didn't really feel like much to me. Daughter's room, when I can get her to close the blinds and not pile crud on the register (she's 13...) feels the same as the other rooms. Basement is still cooler, and I would expect it to be, but not freezing cold like before.

IF I could find a wifi thermostat that allowed me to schedule the fan to run from say 5PM until 10PM I'd add that and I think we'd never notice either a temperature differential or a cost on our bill. I MIGHT be able to do that with an Insteon thermostat but haven't been able to figure it out for sure. Thinking about just going over to Walmart and buying one-think they stock them.

Bob Turkovich
07-22-2015, 3:18 PM
Well the sealing I'd done so far definitely made a difference. Late evening the upstairs was about 5 degrees warmer than downstairs according to an inexpensive thermometer. Didn't really feel like much to me. Daughter's room, when I can get her to close the blinds and not pile crud on the register (she's 13...) feels the same as the other rooms. Basement is still cooler, and I would expect it to be, but not freezing cold like before.

IF I could find a wifi thermostat that allowed me to schedule the fan to run from say 5PM until 10PM I'd add that and I think we'd never notice either a temperature differential or a cost on our bill. I MIGHT be able to do that with an Insteon thermostat but haven't been able to figure it out for sure. Thinking about just going over to Walmart and buying one-think they stock them.


Given the recent warm weather in SE Michigan, I was wondering if you saw a more balanced temperature grade this past weekend. Glad to see your efforts have paid off.

I have a ranch with a finished basement. When the A/C is running it's 75 in the main floor and 66 in the basement (where my workroom is:(). All the ducts to the basement are closed. (I have a similar difference in the winter with ducts open - 72 vs. 65). The furnished part of the basement has a suspended ceiling but access to the main duct is tight (but doable). I'll have to look into duplicating what you did.

I have tried, however, running the fan (manually - with A/C off) to try to get a better balance and have not seen any marked change in temperature differential between the two floors.

Matt Meiser
07-22-2015, 3:27 PM
I think the balance is more about where the returns are drawing air. There's just as much leakage in the returns in the basement as there was the supply, if not more, so it draws a good amount of air from the basement as well as the upper floors.

Brian Elfert
07-22-2015, 4:07 PM
A good residential HVAC contractor will seal the duct work when installing it. Unfortunately, a lot of houses are built as cheaply as possible so the builder takes the low bid on the HVAC. I looked at some new houses last summer on the lower end of the price range and none of them had the ducts sealed.

I had a house built in 2001 and the duct work is all sealed. I bought a foreclosure last summer and had all new duct work installed. It was all sealed without asking. I have very little temperature difference between upstairs and downstairs in the summer. This past winter the basement was a little cool, but that is because all the heat is going up the stairway. I have plans to add a door this winter.

Matt Meiser
07-25-2015, 3:05 PM
I found out the Honeywell RTH9580WF thermostat sold by Lowes, Home Depot, Best Buy, and others allows scheduling the fan to Auto, On, or Circulate for each period in the schedule which should work perfectly for us.

Also found that its $229 at Lowes but only $199 at Best Buy. Printed that out, took it to Lowes and got the 10% price guarantee so it was only $179. I'm also eligible for a $50 rebate from my power company. Listed the old Wifi thermostat on Craigslist for half of a new one so I might be able to get another $50-75 back there too.

Hardest part of the install was matching the background color of the display to our wall color :rolleyes:

Matt Meiser
07-25-2015, 3:24 PM
I had a house built in 2001 and the duct work is all sealed. I bought a foreclosure last summer and had all new duct work installed. It was all sealed without asking. I have very little temperature difference between upstairs and downstairs in the summer. This past winter the basement was a little cool, but that is because all the heat is going up the stairway. I have plans to add a door this winter.

At our old house we had an insulation contractor evaluate our house. His first three recommendations in order of priority:
- Add insulation in the attic (built to 1991 code) to bring it to current code.
- Insulate the rim joist
- Add a basement door

He didn't even offer the services to do the last one but said those were the 3 that would be worth doing. Anything after that the cost-benefit was not worth it.

Our basement had an open staircase so it could be finished in the future. We put a french door at the bottom of the stairs, with a small landing area so it was still inviting when you looked down but kept warm air from rising out of the basement drawing in outside air behind it.

Lee Schierer
07-25-2015, 10:27 PM
Another thought I had would be to take out as much of the metal runs as I can and replace with the flexible insulated stuff. I'd leave the first elbow off the take-off and the last elbow before the duct goes up into the wall or to a register on the first floor.

Also, the return trunk is similar--any benefit to sealing that? The returns are all wall cavities and they just used a foil covered cardboard product made for that purpose to connect those to the trunk.

Metal ducts have less resistance to air flow than the flexible ducts, which have a convoluted interior even when stretched tight.

If the top of the wall cavity above th3 air inlet is not sealed, you could be drawing warm air from the attic. Open the cold air registers and seal the stud cavity above the register. You can also get leaks from wires or plumbing runs that pass through stud cavities used for air returns, but unless you open every wall you won't be able to seal them all. Any difference between air delivered to the rooms of the house and the air going back via the air returns sort of evens out providing there are enough air returns built into the system. The square feet of air returns must be equal or slightly greater than the square feet of air delivery ducts. If the air returns are less you are starving the fan so it can't deliver its full capacity.

Andrew DiLorenzo
07-26-2015, 4:39 AM
On my todo list since we moved into this house 11 months ago was to work on sealing the very leaky ductwork in the basement. When the AC is on, the basement is COLD, main floor right where the thermostat is set, and the upstairs is warm, with my daughter's room getting pretty hot if she doesn't have her blinds closed as she gets the afternoon sun and our bedrooms have massive (~30sqft) windows.

I got a quote for Aeroseal but decided against doing it at $2000, plus I had a heck of a time getting the sales guy to actually show up so I don't really think I want to deal with them. I bought 2 products--a really good foil tape and the mastic you brush on. Last night I tackled the main trunk. I taped all the corner seams and joints. I also taped all of the seams on the plenum above the furnace. I used the mastic at every take-off from the main trunk though there was a couple spots I couldn't get to due to framing, ducts, plumbing, etc. Some joints had gaps around 1/4"x1/4" or larger at each of the 4 corners, plus leakage along the seams and there was massive leakage at most of the takeoffs. It made a HUGE difference in the amount of air movement you can feel in the basement when the fan is running. I wish I'd taken a thermometer down as it felt like the basement temp had risen to match the upstairs by the time I was done--before you could tell a very obvious difference as you walked down the stairs.

I haven't done any sealing on the round ducts that lead from the trunk to every register which general seem to be significantly better than the trunk. Not perfect but it would take a lot of work and a lot of materials to seal all those runs, where I can even reach to do anything with them. Also obviously I can't reach the parts that go into the walls and go up to the second floor (every second floor register is its own run from the basement trunk.) I'm wondering if this is worthwhile? Another thought I had would be to take out as much of the metal runs as I can and replace with the flexible insulated stuff. I'd leave the first elbow off the take-off and the last elbow before the duct goes up into the wall or to a register on the first floor.

Also, the return trunk is similar--any benefit to sealing that? The returns are all wall cavities and they just used a foil covered cardboard product made for that purpose to connect those to the trunk.

Lastly, the main trunk is just sheetmetal. Any payback to insulating it? That looks to be a relatively expensive project unless I can find the material a lot cheaper than Lowes.

Good job on the sealing Matt. I just wanted to comment about a similar sealing job on a house I had up north that had a basement. The wood framing sat on top of a masonry wall and the joint between them leaded a lot of air. I had no idea how much it leaked until I sealed the junction between them with expanding foam, and the basement warmed up. It was amazing, and the best bang for the buck on energy savings that I have ever done!

Brian Elfert
07-26-2015, 3:52 PM
The current recommendation for insulating rim joists is generally spray foam. My previous house built in 2001 had spray foam on the rim joists. For an existing house that you can't afford spray foam the recommendation seems to be be foam board with canned spray foam around the edges.

Matt Meiser
07-26-2015, 9:39 PM
Fiberglass is still pretty common from what I've seen (looked at a couple new build houses last year.) When I paid to have it done at the old house he quoted both ways and suggested that the basement door would make a bigger difference than foam vs fiberglass. But we usually don't get the same cold you do.

Brian Elfert
07-26-2015, 10:57 PM
The rim joist insulation keeps the heat from leaving the house and saves energy. A basement door doesn't really save energy unless you are putting additional heat into the basement to make up for the heat going upstairs. If a basement is rarely used you probably want the heat to go upstairs, but in that case just close the register vents in the basement.

Matt Meiser
07-26-2015, 11:16 PM
His reasoning was about stopping air flow. As heat rises it pulls air up from the basement. That air is cooler (a benefit in the summer) which gets replaced by sucking in air from outside (rarely a benefit). Reputable guy in business for a long time so I tend to believe him.