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Greetings:
I recently installed a bathroom exhaust fan. I used flexible 4″ aluminum duct to the “exhaust damper” on the roof. The duct runs in my “attic” above the bathroom ceiling (two story home). This is non-usable space and it is well ventilated with ridge and soffit ventilation so it does get cold in the winter. Can I just wrap a piece of leftover R-13 insulation around it and secure with plastic cable ties? Should I take the Kraft paper off or which way should it be oriented.
Thanks in advance for any help.
Replies
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I find flexable aluminum ducting some what flimsy. I would be concerned about its ability to support the weight of insulation over any distance. Think I would put it on with the Kraft paper facing out and secure it with duct tape. For 4" bathroom vents I like to use Schedule 30 PVC sewer pipe and fittings (the light weight stuff). The inside of a 4" pipe will slide over the 4" plastic fitting on the vent box (you may need to slightly bevel the pipe end with a file to get it to start). It can be glued in place (use a small amount of glue). I use a 90 degree fitting to turn up and carry the vent into the attic. The fitting on the vent box is also flimsy so I support the pipe with some sort of mechanical fastener into framing (you can break off one of the plastic fitting's snap tabs if you are too forcefull). Since situations vary you have to be adaptive (bolts, plumber's metal tape, etc). I think the smooth insides of the PVC pipe reduce friction (over flexable duct). However, at some point, you are going to have to transition to flexable duct if you vent through the roof. You can make a sheet metal transition from flashing material and hold it in place with screws. In my area, the common practice is to vent into the attic.
*There is actually a fiberglass sleeve out there that will fit over four inch duct. Be careful to think about where any condensation that forms is going to run, and of course where the damp air is going to be dumped.
*Can you run it out under the eave and pitch it so the condensation runs out? I use the pvc too.
*a lot of good ideas.......to recap: smooth wall 4" pvc or 4" rigid metal duct and adj. elbows, sloped for condensation to drain out; Never vent directly into attic but at least extend and hang/support vent pipe to gable vent or roof or ridgevents, or down to soffit vent (however be forewarned that mildew and mold will probably build up in the soffit areas especially on the north side of house) I have used the insulated HVAC flex duct, comes in 25' lengths in 5" dia. to easily slide over your duct run and around bends! DIERKS
*I think the insulated HVAC duct approach is the best since it can combine good quality (i.e. low air flow resistance) duct with a easily sealable vapor barrier. The vapor barrier will deter condensation during the summer when the duct is cooled by exhausting conditioned inside air. The only caviate is that sharp bends must be avoided.The PVC approach has the advantage of providing some insulation but my reservation is that the available elbows have a very thight radius of curvature (about equal to 1/2 the duct diameter) which drasticly increases the air flow resistance and bath fans are notoriously weak. The minimum radius of curvature along the center line of the bend should be limited to 1.5 times the duct diameter and preferably 2x. If I was going to use PVC, I'd make 90's using two 45's.
*The PVC duct sounds like a good idea. As noted in the previous post standard elbows will impose a possibly unacceptable pressure loss. Large radius elbows are available from some manufacturors. Another possibility is the PVC duct now sold for dust collectors. The elbows are wide radius. Note that I am not recommending PVC ducts for dust collection.
*Jim: I assume you don't recommend PVC for dust collection for fear of things going BOOM! (Static charge on the plastic and, potentially, an explosive mixture of vapors or dust). My concerns in shop/home use for explosive mixtures in dust collection/shop vacs are: organic vapors (gasoline, paint thinner, finger nail polish remover, whatever), very fine saw dust from fine grit sanders or high speed shapers, powdered sugar or wheat flour. Try those last two or non-dairy creamer, sprinkled over a campfire and you'll appreciate why grain elevators blow up sometimes. -David (PE)
*Blow the duct full of dense packed cellulose! Or is it Icynene? Thats how everything gets insulated around here.....I think????
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Greetings:
I recently installed a bathroom exhaust fan. I used flexible 4" aluminum duct to the "exhaust damper" on the roof. The duct runs in my "attic" above the bathroom ceiling (two story home). This is non-usable space and it is well ventilated with ridge and soffit ventilation so it does get cold in the winter. Can I just wrap a piece of leftover R-13 insulation around it and secure with plastic cable ties? Should I take the Kraft paper off or which way should it be oriented.
Thanks in advance for any help.