bath vent duct in attic (cold climate): bury under loose-fill? or spray foam around?
Hello GBA readers,
Am in Vermont (CL zone 6A). On a two story house, I need to vent a upstairs bath+ laundry room. I learned from GBA that this vent outlet shall be in the Gable end (which is 25ft from the fan). I have a vented attic, and will achieve R-60 insulation with loose-fill cellulose (~17 inches of fill).
I am planning to use 6 inches metal duct, sealed with mastic to reduce friction loss, also hoping to last longer that a flex duct. If I run the duct along on-top-of lower cord of roof trusses, I have only 4 ~ 5 inches of loose fill left (~R13 -R15).
Considering the bath air is hot (75F) with near saturation (80% RH), with freezing temperature in winter months, condensation inside the duct will happen. I will maintain a slope towards the outlet to drain the condensation.
What are the best practices here ?
Does burring it under loose-fill cellulose is adequate? or shall I spray foam 2″ around the duct to increase R value and better seal?
Is the mastic sealant can be compromised in condensation?
Is an insulated Flex duct is a better alternative?
Thank you so much for your comments.
Rajib
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Replies
You can get insulation wraps and sleeves (“socks”) for rigid duct too. Burying the duct in the amount of cellulose you mention should be sufficient though. You do need to pitch the duct so that any condensate will drain out. I see no need for spray foam here.
Mastic won’t have a problem with condensation. Just make sure you put the mastic on thick enough to be reliable over time, many people don’t. The general rule is to apply mastic “nickel thick”, which is a little over 1/16” or so. If it looks like smeared paint and not a band with some thickness to it when you’re done, then you haven’t put on a thick enough layer.
Bill
Thank you Bill!
Appreciate the suggestion and tip on Mastic.
I'm in the same boat but my duct is closer to 30ft. It's going out through a soffit vent now and I gotta move it. Caused ice damage last winter in its current location.
I'd be tempted to get a spray foam kit for it. One that is class 1A. I've insulated attic ducts this way with great results. Then again if you bury it enough maybe foam wasn't needed.
Is 6in duct ok for a dryer vent? I always thought you wanted closer to 4in to have enough velocity to get the lint out.
Duct velocity can be calculated given the drier CFM. For typical 100 CFM flow, this will result in 1145 FPM velocity. For 6 inches, the it drops to 510 FPM.
Expert readers comment appreciated.
You are overthinking it. Just run a flex vent pipe to the attic and vent it as you see fit. Make sure your bathroom fan has a well fitting one way flap so you don’t suck cold heavy air from the attic when the fan is off
Code does NOT allow you to vent a bath fan into the attic space. Doing so is almost guaranteeing moisture problems in the attic. Flex duct is much worse than rigid duct too for air flow, which gets worse as the duct gets longer.
It's always best to try to keep the duct as short as possible regardless of the type of duct you use, and it's always best to try to use rigid duct whenever you can. You MUST vent to outdoors, and NOT under a soffit where the moist exhaust air can get sucked into the soffit vents where it goes into the attic. Gable end vents work, as do roof vents.
Bill
We decided not to vent through the gable (which would have been approximately 20-30’ duct run) but instead to go through soffit using this product https://na.panasonic.com/us/home-and-building-solutions/ventilation-indoor-air-quality/ventilation-accessories/ezsoffit-venttm-soffit-termination-system
which was mentioned here on GreenBuilderAdvisor as a good solution for soffit venting. Haven’t installed it yet as it for new construction, but hope this works. This would give us only 6’ of duct run to soffit. Our Attic insulation is also R-60 with 17” of cellulose so hoping it would cover most of the duct due to short run.
This places fan on the soffit cavity, instead of interior.
Regardless the moisture saturated air is dumped into the soffit. For a vented attic, I don't see how this solve the problem.
Could you please elaborate.
When we bought our house the 2 bathroom fans were ducted to the soffit, and one bad winter we ended up with a huge moisture problem in the vented attic.
I would avoid any bathroom vent into the soffits....
bongo30,
You can successfully vent through soffits, but the termination needs to be directional, and near the eaves. I have two of these. In cold weather when you can see the moist air clearly, it is vented well away from the house, not drawn back into the roof. https://www.lambro.net/product/4-white-plastic-eave-vent/
I have done several in Radon vents at high altitude Zone 6.
I come up above the insulation with a PVC Duct, and insulate around it with the an OSB box filled with loose fill. then put a two 1/4-inch per foot slope. It to the closest gable end. And insulate the down slope with an insulated wrap. Any condensation drains out of the pipe, so we don’t get freeze up. On the one building where we did get freeze up in the pipe, I added heat tape under the wrap. if the manometer shows a drop in differential we turn on the tap for a bit to clear the ice build up.
On a Bathroom I would slave the fan to the light switch, so the pipe gets some warm air during the period before teh shower starts, and have a delay to keep it running for a bit.
Neither is exactly green, but I’m dealing with historic buildings and SHPO is kind of picky about what I can do. The old things are so grossly inefficient that I don’t think much besides leaving the windows open would let any more air in.
1. Don't vent to your attic...but I'm guessing the OP knows this is very bad idea.
2. Like Bill says, there are a number of ways to deal with longer runs in cold attics. I prefer aluminium tape over mastic as post-shrinkage gaps will appear if not applied quite liberally. I've used insulation wrap, socks, and also spray foamed in place. The downside of foam is servicing after if required. On the last few projects we used cellolose over 2" of spray foam in the attic. Careful air sealing of the duct, then insul-wrap or sock (also taped), buried in the R-60 to R-80 of cellulose and you'll have zero issues.
3. On a long run, you really need to make sure the duct stays warm (so as close to attic ceiling as possible) otherwise warm/moist air will freeze inside, blocking the duct. I like the Broan Ecovent for an exhaust hood, but they only come in 4" flavour. At -30C, the insulated ball/hood works quite nicely to keep cold air out of the duct when idle.
4. We had water closet exhaust vents running over 40ft in a commercial project which had no issues after 4-5 years (sold the building). Air sealed, wrapped, then buried in R-80 of cellulose.
Thank you Dennis.
Great tips.
I run bath fans through the roof using insulated flex and a Broan exhaust vent. This method gives a double damper and have never had issues in a cold climate. Have seen many bath fans terminate in the attic space, nothing goods comes from this.
For a bath vent duct in a cold climate attic, it’s best to insulate the duct to prevent condensation from forming inside, which can cause moisture problems. You have a couple of options:
Burying Under Loose-Fill Insulation: Burying the duct under loose-fill insulation, like cellulose or fiberglass, can help insulate it and keep it warmer. Make sure the duct is as close to the ceiling as possible (not against the roof) and buried deep enough in the loose-fill to be well insulated. However, be aware that loose-fill alone might not be enough if your winters are particularly cold, as it can settle over time and provide less effective coverage.
Spray Foam Around the Duct: Spray foam insulation is another excellent choice as it creates a tight air seal around the duct, which can prevent cold air from reaching it and minimize condensation risks. Closed-cell spray foam is especially good in very cold climates since it has a higher R-value and provides more insulation per inch. Just make sure to allow for enough expansion room around the duct.
Best Option: Often, a combination of both methods works well. Apply a thin layer of spray foam around the duct to air-seal it and then bury it in loose-fill insulation to maximize insulation. This combined approach offers both an air seal and thermal protection, reducing condensation risk and keeping the duct warmer.
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