I’m about to reroof a building I own in northern WI. shingle roof. currently has a kitchen vent hood and a bathroom vent fan both terminating through the roof – about 8′ above the respective vent fans. wondering about removing those vent terminations and running both of them about 20′ horizontally in the unheated attic space and out a gable endwall instead.
my thinking in favor of this is to minimize roof penetrations, which to me is always a good idea. I can see a possible downside of long horizontal runs in an unheated space with potential for condensation and/or frost. any thoughts as to which way to go with this tradeoff – and/or or there other factors to consider?
thanks – Roy
Replies
IMO with the bath fan I always like to see them go out the side. You can go 20' but when I run thorugh unconditioned space I use insulated flex duct. Cuts down on the condensation.
We are required to use hard duct for kitchen exhaust fans.
Kitchen is a different story. Thats standard (hard pipe for kitchens) because there is a potential fire hazard from the grease.
Sorry if I confused the issue. I meant insulated flex for the bathfan only.
Go out the sidewall. IMO less roof penetration is better. I use a inline fan in my attic to ventilate 2 bathrooms. The part from the vent grille to the inline fan is insulated flexpipe. After the inline fan I use a short section of insulated flexpipe before it enters rigid pipe that is sloped. If moisture is going to condensate on the pipe it will run out. So far I have never seen any drips coming out the side vent.
If you can, I would check the instruction manual for the kitchen hood, it will have a total number of "equivalent feet" the piping can run. It will be dependant on the size of pipe among other things.
Elbows eat up quite a bit of footage themselves. I do not know exact numbers myself and my guess is it depends on the CFM out put of the fan, I know duct size plays a factor as well.
Maybe somebody more educated in this will come by.
The bathroom fan will have the same sort of calculations to make.
A roof vent installed and flashed properly shouldn't really propose a problem.
What I always say: why put a hole in a perfectly good roof?