We recently vented a gas clothes dryer through an attached and unheated garage, to an outside wall of the garage. We are located in Central Vermont. We used 30 ga. 4" galvanized duct and taped all joints (and elbows) with foil tape. we foamed around the opening where the duct left the house. The duct has 4 elbows and runs about 17 ft. total. Most dryers can handle this effective length. We used a standard 4" plastic vent hood on the wall of the garage. We wrapped the duct in 3 1/2" unfaced batts of fiberglass and boxed it in with pre-primed pine. 

The client then complained of cold air coming back through the dryer. The plastic damper on the hood often sticks slightly open. We changed the hood to the style with the floating plastic cup. That stopped the cold draft, but now there is ice all over the wall of the garage below the vent. I have also noticed ice dripping from one of the joints in the box. 

Has anyone found a good solution for venting a dryer through a cold space, which would- insulate to minimize condensation, not trap any moisture that might leak from the piping, would be protected from crushing or other damage, would stop back drafting into the house and would not promote ice buildup on the wall below it? 

Has anyone experienced icing up of the vent style above, where the air pushes up a cup to get out?

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If the pipe joints are dripping and you have a decent slope your joints may be reversed lapped. As for the ice on the siding, those vents direct the exhaust down along the siding. With a long run the exhaust may be cooling enough that it condenses immediately after leaving the vent. This vent is handling the moisture load from the wet laundry and from the combustion of the gas fuel.

Also, those cups vents seem to have a slightly higher back pressure, combine that with four elbows and you might be taxing the dryer fan.

BTW, I don't know that the plastic vent is appropriate for gas use. I don't know if there are code requirements but I always understood that only metal pipe and hoods were to be used on a gas dryer.


If you've insulated the pipe, sloped to the outside, properly lapped the joints, left a drain hole or two at the bottom of the vent termination...then I think you're just up against physics.  Physics wins every time. 

The vented dryer is not a sealed combustion or closed circuit appliance.  I assume the dryer is on the lower part of the home (unless the garage is on the roof) so in cold weather air will tend to leak in through the dryer.  I don't have faith in dryer vents having a good back-draft prevention - even if it has one initially, lint will gum it up and it will leak.  I'd be willing to bet that clothes left in the dryer get real cold real quick. 

Condensation is just going to happen in the pipe as Bill points out.  This is because infiltration is cooling the pipe in the off cycle and the cold pipe will wring a bunch of water from the exhaust before the pipe is warmed up and the condensation point of that steamy air moves toward the outside.  There's a lot of pipe to warm up and a single cycle might not be enough to warm the pipe to the outlet and re-evaporate the moisture (if it did, you'd be running the cycle much longer than you need to to dry clothes).

I think your best bet is to take a cue from the cold pipe in the garage and have the clients switch to a condensing dryer (or two condensing dryers if they need to dry more than one load per day).  Use the gas dryer at a different house where it can be vented directly through a wall.  Shortening the exhaust run won't get rid of the air leakage issue, but it should solve the dripping pipe thing.

older dryer could "push" more air than the new "energy star" dryer. I have put in line fans so to better damper in line so does not fill full of lint.  But thats not what you are asking.  I  used 4" PVC and drained to out side with slick sides it uses less fan HP to push out the air.  Same job the ac duct also dripped with water that was in attic but under 1" foam and 16" of fiber glass 

Maybe I am not communicating the nature of the problem in a very effective manner.

-To vent a clothes dryer (especially gas fired!) even BPI standards require metal ducting only, and not PVC. 

-The ducting must be insulated after it leaves the house in order to at least minimize condensation of all of that warm moist air as it passes through the cold garage (oh, by the way, the outside temperature has barely gone above 0 degrees Fahrenheit in the past few days). 

-Only a non-combustible insulation must be used, such as fiberglass, but air moves through fiberglass

-if the warm moist air from the dryer leaks out of the vent pipe, then it will condense in the fiberglass

-fiberglass insulation wrapped around a duct is very prone to damage, so it must be protected from damage

-if any of the moist dryer air leaks from the pipe, it will condense and wet the fiberglass; if the fiberglass is protected by any sort of sleeve or boxed in chase, the water will stay trapped in the fiberglass

-the typical plastic vent hood flapper is not very good at sealing the vent and often gets stuck at least partially open, especially when it has lint on it; the vertical style of vent "hood" creates a lot more resistance, or back pressure, and tends to direct the moist air straight down, which washes the wall with moisture more than the conventional hood. The louvered style seems to work a bit better. I considered an in-line damper, but I am afraid that it will clog with lint. Even when you religiously clean the dryer filter after every load, lint will still get into the vent. With the vent terminated about 8 ft. above the dryer, cold air is now going to pour down the vent pipe and into the house, much like water flowing down a drain. The laundry closet was getting frigid with the conventional hood, with its plastic flapper not always closing all the way. The vertical, push up against a cup, style stopped the draft, but presented so much more back pressure that the clothes were overheating and took much longer to dry. I think that the louvered style may do it.

-Yes, I realize that the total effective length of the vent here is 37 ft., which is just about the limit for the average dryer. All of the piping is 30 ga. galvanized pipe or elbows (4), with no flexible at all. The dryer is up against the outside wall, with no room for an in-line lint filter. 

-An in-line booster fan is not only a costly addition, but presents its own challenges. How do we insulate that to prevent condensation? How well will that fan work when it is very cold outside?

So, the biggest challenge right now is how to insulate an exterior dryer vent pipe to minimize condensation, and that seems to be only fiberglass insulation. Then, how do we also protect that insulation without trapping moisture that may leak from the pipe?

I suspect, as Ken points out, that this particular situation may be a losing battle. A condensing dryer would not be an option here. They have two small kids => lots of laundry.

I think you explained it pretty well & I do love the quip "even BPI requires" - it is so nice when when they actually recognize codes...

A few quick points - your outside damper is plastic which is all I was suggesting. The only requirement for metal is for the vent run, not the exhaust part which is located outside the building. The reason for this is fire & the biggest culprit besides the buildup of lint is not gas, it is actually electric.

The codes (M1502) require 25' long only for the effective length (though some jurisdictions like Alabama have bumped that that to 35' which actually matches G2439) - this is required to not only help prevent lint built up but also to allow for easier cleaning --- there is one exception & that is dependent on what the dryer manufacturer lists & specifies in its installation instructions

As for the insulation - there is nothing I know of that requires it on a dryer much less prevents one from taping the seams & spraying foam on it - there biggest concern is with a fire starting in the pipe & being allowed to get past it (i.e. melting a PVC run)

Now I do have one question on your clarification "The dryer is up against the outside wall" - if that is the case why are you not going straight through that wall & instead running it through the garage


wow 4 elbows - that adds 20 to the effective length which probably doesn't even include the flex line inside

as for the ice issue on the wall, the only thing I think of is do what they do in commercial & add a PVC 45 or 90 on the outside pointed down that sticks out a few inches (I know I just added another 2.5' - 5')

as for the ball system (well it goes if you do the above), while it seals off nicely, if it is to cold outside you will start getting water & ice build up (as for how much length that system adds - I have no clue)

I do have to ask, just how wet are the clothes they are drying as a newer washer should almost have the clothes dry before they ever hit the dryer - if there soaked going in you will have a ton of moisture trying to exhaust which will add to the issues

Brad, just curious, if the dryer is against an exterior wall, could you not install a Dryer Box directly behind the unit and eliminate the pipe run in the garage? Also, the vent caps from Seiho are the best ones I have found. They are around $50. I use them for all bath fans and dryers.


I have attached a photo of the original vent (brown vent hood low and to the right of the covered area), showing how the moist air goes up into the cathedral ceiling of that covered area between the house and garage. That moisture was instrumental in creating significant rot on the underside of that roof, including a structural beam holding up the bottom of the roof. We couldn't go straight ahead because the door to the garage was in the way. 

The second photo is the enclosure we made with the last side off. The vent goes out the wall, turns toward the garage door, turns up alongside the door, goes through the garage wall, turns and goes over the doorway to the gable end wall.

David, I don't see where a dryer box would be useful here, or on any outside wall where it would take up insulating space. I checked out the Seiho products. They all have to be double caulked in place and look difficult to clean the vent pipe. They do appear to be fairly heavy duty.

Sean, I agree that we are pushing the length. I cannot find any specific info for this older dryer, but I found a couple of sources that indicated that many dryers can operate OK at this effective length of vent. I did not build this mudroom or garage or place the laundry there. I have told the client about these issues and constraints, and I am trying to make the best of a bad situation. I do have another client with a somewhat similar situation. Their dryer vents through their garage with flexible aluminum duct running about 12 ft. It sags and has lots of dents in in it, as well as water. 

The washer and dryer are old (top load) and I have explained the benefits of going to a front load. If they do a stacked washer/dryer in that laundry closet, we could go through the side of the dryer, straight through the wall to the garage, up the wall, over and out, which would require only 2 elbows total. 


Brad, the dryer box itself is simply a way to cleanly route the flex connector from the dryer to the rigid duct. Yes, it displaces insulation but it prevents the flex from becoming a coiled mess behind the dryer, and it lets the dryer sit against the wall. It eliminates a problem with dryer installations. How many times have you looked behind a dryer and seen a piece of foil flex coiled up like a snake, full of lint?

Those aren't your immediate problems, but you could likely solve your problems by simply venting the dryer out the back wall behind the unit. That way, you wouldn't have a long duct run to cool the exhaust and eventually fill with lint that's stuck to the condensation, and you wouldn't have the exhaust coming out under that overhang, where the rising steam is apparently causing structural damage. 

Maybe the client didn't want the exhaust right behind the dryer, but the route that was taken here appears to be unworkable from what has been posted. What am I missing?

FWIW, I often furr out the wall behind the dryer to allow 1-1/2" to 2" of rigid insulation between the dryer box and the sheathing. No, not full insulation, but I think these boxes solve more issues than they create. I am remodeling a house right now where I installed the dryer and dryer box in 2005, and the flex is exactly where I put it 8 years ago. I can practically stick my arm inside the dryer from outdoors. Since the run is so short, the air velocity is high, the air temperature is high at the exit point, and there is almost no lint buildup.

Also, I do not caulk the Seiho caps to the duct or the wall. I know the instructions call for that, but I  want dryer caps removable for cleaning. I run the duct flush to the face of the siding, then caulk around the outside of it so there's no air gap into the wall. I install the Seiho cap on a block as shown in my pic, and there's virtually no gap between it and the block, so very little air leakage there. The flappers work very well, so again, very little air leakage. Owners don't complain about backdrafts and they perform well during BD tests. When there is lint to clean, the cap can be pulled off, and you can easily clean the entire duct (which consists of one elbow and a few inches of straight).


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other make them also.  If I under stand the ice forming,  You have high RH and it goes below the dew point and make ice.   You need more run time and less RH or more sealing so do not get below dew pt.


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