In another thread (that grew quite unwieldy) the question came up about exhaust venting

Robert Riversong:  "the most foolish way to duct an exhaust fan is straight up through the roof"

John Brooks: "It seems to me that in a heating climate (or during the heating season)... It would be desirable to have the "passive NPP"  high. This would make the house more "negative" and would reduce (or eliminate) positive pressure near the top of the enclosure."

At this time I brought up the issue with condensation, wind, and dampers & that they should be vented out through a gable end if possible, if not then the roof  but never through a soffit. This was replied to with a "I did say "During the Heating Season"" so I thought maybe we should take a look at this

 

First, condensation is a big concern during the "Heating Season" as these fans are transporting humid warm air up & passing through a super cold area

Wind is a big issue which is one reason why chimneys have to be vented so high

The final issue is the dampers, if one was to get stuck open, you would have a nice big hole or open chimney transporting all that nice conditioned hot air straight out of the house 24x7.

 

So what say you???

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Hi Sean,

Building new vs a retrofit is usually a big deal, as new construction can and should do it right from the start.  So, what would be your best practice for new construction for a few different styles of homes.  Keep in mind that new homes can be extremely tight and in some cases a single exhaust fan can pull the entire home negative.  I'm not offering a suggestion as, frankly I don't have a clear picture. 

For retrofit, the soffit/gable/roof options can all have problems depending on many factors.  Most of the issues I am familiar with have been mentioned, but I'll add, I have see flexible bath vent ducts filled with nasty water.  I suspect the reason wasn't just the moisture from the shower, but from a slow and steady flow of warm inside air.  When it moves slowly it is almost certain to condense. 

Bud

I don't really see a difference between the two as bathrooms are placed where they make sense for the homeowner, not venting. The only time I might see if a bathroom can be moved or tweaked is for the water piping. 

Venting issues can be overcome with boosters, a remote fan (one at the gable instead of at the bath), or a host of other items depending on the need. Along those lines, the only time I really raise a red flag on venting locations is for dryers & stoves

I am not a big fan of flex for venting & rather see PVC used for bathrooms (not for roofs, but sloped runs) - dryers must be smooth metal & kept at or under 25'

As for pulling a house negative - that is a good reason for direct vent appliances & an ERV/HRV not being tied into the HVAC unit / using mechanical dampers. Even if the unit is not running at that moment, air will be allowed in to help offset the loss

Here is my take on how to do a retrofit through the roof.

Please advise should any of this appear incorrect, etc.

My god, those shingles : )

I am going to assume that you zip-tied the inner sleeve & hopefully used a tensioner - if so pretty good

Just curious, where is the second brace / arm, whatever one wants to call it that goes to the other truss or does this unit not have one? Oh & it appears you are missing a staple for the electrical (yeah I knew an inspector that was cruel about that)

Sean,

Forgive my ignorance, but what is a tensioner?

The other brace is obscured in the photo but trust me when I say it is there.

Also, thanks for pointing out the need for another staple.

Patrick

It gets the cable tie tightned up properly (i.e. tighter than you can get by hand & prevents it from being able to work loose)

here is one version http://www.cablemarkers.com/cabletietools.htm

Great, thanks Sean!

It is always interesting to see how our fans are installed.  This was a good install (except for the new color shingles...) - foamed edges, insulated duct, high rise roof jack, cable ties.  The fans come with three spreader bars - a short one for the duct end of the fan and two others for the other end that can telescope to both ceiling joissts or trusses.  It is generally OK to just use one of the long hangers to reach one joist, if you are 16" on center and as was done here, but it is a bit more stable if the two hanger bars are used to reach both joists/trusses, especially for 24" OC..

I would like to share some of the lessons learned by the Minneapolis Sound Insulation Program when it weatherized over 8000 homes near the MSP airport and installed continuously operating exhaust only ventilation in almost all of them.  We are known for our extreme winters up here, so the problems we experienced may not show themselves as dramatically in milder locations but the building science is the same. 

 

When you have a continuously operating fan with a roof termination it WILL melt snow.  The snowmelt will add to ice dam problems especially if it runs down onto a valley.  We learned pretty quickly to vent out a gable end whenever possible and always stay away from a valley.

 

When we installed airtight acoustic storm windows, that are often tighter than the prime window, it can lead to excessive condensation on the upstairs storm window on a two story home.  This makes since, right?  Air is leaking out on the upper part of the house will leave its moisture on a cold storm window.  And believe me, they knew who to call with this problem.  Our recommendation was to turn the exhaust ventilation system to high speed.  This almost always solved the problem.  Why?  It was a combination of lowering the humidity in the home (the cold air infiltrating is very dry and ventilation fan is usually in the most used bathroom) and raising the neutral pressure plane to at or above the upstairs window when the fan is at high speed.  Less pressure at the window results in less condensation.  If the NPP is at or above the window, the storm window will have very little condensation.  You are less likely to have attic condensation for these same reasons.

 

If you don’t have an airtight connection between the continuously running fans duct and the outside, you will have serious condensation and eventually rot problems with the roof decking.  You can also have a buildup of ice between the shingles and the roof termination.  We required our installers to use a roof vent termination with a collar on it.

Great insights for cold climates. Few in the industry, even in cold climate zones, appreciate the damage that ice dams can cause. For that reason, I am as much a proponent of attic/roof ventilation as for indoor ventilation to avoid moisture problems.

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