Replies

1. 
   onslow | Apr 02, 2017 12:05am | #1

   Hello Mark, I live in a snowy area CZ6 and have had up to 20" of snow on a low slope roof. I opted not to do any roof vents for several reason related to the pitch and insulation details. However, I also have some background dealing with a friends straight vertical range hood vent. The main irritant was the vague endless chill by the stove. The hood was subject to a lot of cold air drop like a colonial fireplace and even the best butterfly damper I could find only tempered it. Frost or snow plugging wasn't much of a problem, at 1200cfm not much stayed in the house or on the pipe cap. A circular cap design and 30" over roof deck helped too. However, condensate on the very chilled 6-8' of pipe above the insulation deck took it's toll over the years creating pinholes and crusty trails. Fortunately, there did not seem to be enough condensate flow to reach all the way back to the hood. You sound like you may have similar conditions concerning you.

   In light of the above; can you find a good path to a soffit or side wall. All but one of my vents exit just below the soffits thanks to the roof truss's "energy heel" which provided almost 12" of clear space between the top plates and the soffit . The top chord element extends past the wall line like a rafter tail and my soffit panels go direct to tail. No return framing was done. More traditional framing that results in the closed triangular space which can be a bit more work to deal with but done properly will let you vent just fine. The vent caps are a bit different from wall types.

   In either case it is important to connect 4" bath or 6-8" kitchen pipe to a vent cap sized to allow full flow. The ones that look like ordinary dryer vent caps often don't have enough free area. The single flappers types don't open as much as needed to really flow. Insect screening compounds the issue. So far the rectangular plastic ones I selected have worked well with no frosting or grease drool. They have a cross section about 2 1/2 x10. Yes it does have a single flap, but so far no signs of problems.

   There is an alternate in-line back flow stop that uses a fabric insert much like a wind sock. No chance of sticky flappers and absolutely full cross section for flow, but the reviews I read didn't encourage me to bury one in the attic where it would be hard to reach. Also, I suggest you go up in cfm if you have long runs to make, go up a size of pipe to keep it quieter and maintain flow rates. All my bath fans have a 4 and 6 inch hook up option. I went with six inch and wish I had bumped up the cfm as well.

   I connected fans using flexible only at the first joint to dampen vibration transfer, then solid pipe. Solid pipe becomes most important closest to the wall line where the temperatures will shift the most. My thinking is any water/grease condensation will likely occur in this area and pitching the pipe down hill to the vent cap should keep the puddling seen in flexible pipe at bay. Pipe is set seam side up, all joints and connections sealed with metal foil tape and male joints pointed to the outside. If you can, plumb your air much like a wet vent for sewer. Come off the fan with flex to a high start and slope down hill all the way to the wall or soffit.

   I don't have insulation on the ceiling side due to my roof design so admittedly I had a nice clean way through the trusses. In my case, the pipe is room temperature all the way to the exterior wall. The slope also creates a bit of a thermal incline impeding back flow when cold outside. I think this is where the goose neck idea comes from, though I would suspect puddling of condensate in the down bend just before it exits the roof. I can't say I have seen a formally designed goose neck, just lots of bad flexible and marginal solid vent work.

   In our area, seasons of very high (30-45mph) winds create enough pressure drop to pull air out the vent lines. For the kitchen hood I have a second butterfly damper to keep the suction losses down. The bath fans have a integral dampers, but the runs are long enough that I don't ever hear them popping from the wind.

   Ice dams or moisture point to grossly faulty installation practices such as dumping a bath vent into an attic or a soffit. It is done all too frequently and I have encountered one condo where all the bathrooms vented directly to the attic with nothing more than pan vents near the ridge to relieve the moisture. Astonishingly, I learned that all the units formerly had roof fans that were removed during a major re-roofing. Never rely on the venting provided for the roofing to carry a bath or kitchen load. Always make a clean path to the outside atmosphere.

   Hope this doesn't overwhelm the question.

2. GBA Editor
   Martin Holladay | Apr 03, 2017 05:53am | #2

   Mark,
   Roger has given you some very helpful, detailed, experience-based advice.

   In general:

   1. Through the wall is better than through the roof.

   2. You want a range hood exhaust fan with a very low cfm rating if possible. (To learn why, read this article: Makeup Air for Range Hoods.)

   3. Some Passivhaus builders avoid the use of a range hood exhaust fan. Instead, they install a recirculating fan that includes a charcoal filter. This approach works best if there is an exhaust grille in the kitchen ceiling (far from the range) connected to the exhaust port of an HRV. Note that this approach isn't accepted by all code officials, so if you want to try this approach, you may have to negotiate with your building department.

   4. The need for a range hood exhaust fan depends on the cooking habits of the occupants.

   -- Martin Holladay