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Community and Q&A

Heat loss and condensation through mechanical penetrations in the building envelope

Tom Ruben | Posted in Mechanicals on

A great deal of space here is dedicated to building air tight well insulated homes. This of course is good but what products can minimize the heat loss and vapor problems that arise from the penetrations in the building envelope by mechanicals? For instance how can a dryer vent be gasketed to an R of more than 1. Likewise how are the bathroom and kitchen vents handled to improve efficiency? Also concerning the dryer vent what is done with the moisture that can form when the moist warm exhausted air hits the cool outside air?

When we consider the whole R-value of an assembly that is fairly air tight and well insulated with high R windows aren’t these penetrations the limiting factor in the design? Lets assume the vents are properly installed so that moisture is not entering around the vents. What products and methods can substantially boost the performance of these mechanicals through the wall and roof?

Thank you,

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  1. GBA Editor
    Martin Holladay | | #1

    All of the issues you brought up are regularly discussed by Passivhaus designers.

    The best solution to the dryer vent problem is to choose a dryer that does not require venting -- in other words, a condensing dryer. For more information on clothes drying options, see Alternatives to Clothes Dryers.

    Bathroom exhaust fans are usually replaced in Passivhaus homes with an HRV that pulls exhaust air from the bathroom, the laundry room, and sometimes the kitchen ceiling.

    Range-hood exhaust fans are usually eliminated (if the local building inspector allows this option). Passivhaus builders install a recirculating range hood that draws the stove-top fumes through a charcoal filter. Further exhaust occurs when the HRV has an exhaust grille in the kitchen ceiling. (Usually this grille is located as far as possible from the stove, to limit the negative effects of grease.)

  2. Tom Ruben | | #2

    That is great news on the ventless dryer. Long runs of even solid pipe collect lint and the flaps never fully close.

    I hope Passivhaus has reinvented the charcoal filter cause it don't work so well on those microwaves installed above stoves.

    In a related note, why has code allowed the use of ventless gas heaters. We are building a home and I am renting a nearby house with these heaters until ours is under roof. I can tell you for certain these heaters emit harmful fumes. Fortunately our lease was from April to September because I wouldn't have spent a winter in that home. My guess is a strong lobby by oil and gas has prevented code authors from addressing this issue.

  3. GBA Editor
    Martin Holladay | | #3

    You're right -- the use of unvented gas space heaters is supported by a powerful trade organization and lobby, the Gas Appliance Manufacturers Association (GAMA).

  4. Aj Builder, Upstate NY Zone 6a | | #4

    Better to go vented for sure. But travel the planet in person or via National Geographic and it seems like at least 6 billion of us live cooking over open fires inside mud and straw homes that are the size of a typical master bath in my neck of the planet. And I haven't yet seen a TV special documenting widespread lung disease deaths due to such. Might have missed the show though.

  5. John Klingel | | #5

    "...what is done with the moisture that can form when the moist warm exhausted air hits the cool outside air?" •• Not an issue in Fairbanks, so I doubt that it is of concern where you are. The only frost I ever see is on the gravel or the car tire.

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