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Community and Q&A

Adding mechanical ventilation to a leaky house

mfredericks | Posted in Mechanicals on

I have an older home that I know leaks a lot of air, and I’m wondering if adding mechanical ventilation would improve the indoor air quality.

When i first bought the house, it tested over 10 ach/50. I have since improved the basement by air sealing the rim joist, service penetrations and the slab to foundation wall cracks etc. The house was last tested around 8 ach/50 so this helped, but obviously the rest of the house is still very leaky.

My wife and I both experience allergies in the house, and I suffer from asthma. During the winter we get lots of condensation on our windows (mostly single pane with aluminum storms) and I’m worried about the quality of the air because there is no bath fan or kitchen exhaust fan, and we heat with wood.

Would adding a ventilation system like a pair of Lunos fans help reduce window condensation and improve the indoor air quality? Or would an exhaust only system be as effective given the home’s air leakage will continue to allow lots of air in through the walls and attic anyway?

I appreciate any thoughts. Thanks

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  1. Expert Member
    Dana Dorsett | | #1

    The window condensation issue may be related to outdoor air leakage into the space between the storm window & old single panes, if the condensation is on the interior glass. If it's on the storm window it's air leakage from conditioned space into the space between the storm & original.

    Active ventilation will lower the humidity levels in the house, maybe to a level where the dew point of the air is lower than the temperature of your windows, or maybe not, unless the ventilation rates are quite high. Not-so-high-R houses heated with point source heating like wood stoves will usually have some rooms running 10F, maybe even 15F or more cooler than the rooms that freely convect to the wood stove space.

    In a house that leaky an exhaust-only approach will dry the house out, but depending on where the air is being drawn in it won't necessarily improve indoor air quality. If it's pulling a large fraction of the air through a moldy crawlspace or from a crack in the basement slab it may be making things actively worse. With balanced ventilators you have at least some control where the bulk of the ventilation air is coming from.

  2. GBA Editor
    Martin Holladay | | #2

    To help answer your questions, it would be useful to know your location or climate zone.

    Unlike Dana, I seriously doubt that the condensation problem you describe has anything to do with air leakage at the windows.

    Considering your blower-door numbers, and the fact that you heat with wood, you really shouldn't have high indoor humidity. I'm guessing that the most likely cause of your problem is a damp basement or damp crawl space. By any chance, does your home have a basement or crawl space with a dirt floor?

    If you have a damp basement or damp crawl space, you should address those issues. Here are two articles that you may want to read:

    Fixing a Wet Basement

    Building an Unvented Crawl Space

    If your basement or crawl space is dry, the next step I would recommend is to install a bath exhaust fan and use it regularly. For more information, see Bathroom Exhaust Fans.

  3. ntisdell | | #3

    To help pin point your problems and get a good diagnosis - I always recommend picking up either a couple cheap temperature/humidity meters off of amazon for like $9-10 (AcuRite) - otherwise even better is a weather station with a remote temp/humidty sensor. Have one on main level and another in the basement. Or do you already have some humidity and temperature data you could share?

    Makes it very easy to get high/low humidity, track behavior vs humidity, etc.

    This early in the winter - homes seem to hold some humidity - so the first few cold snaps can leave windows looking troubling. Drywall/wood/blockwall can act like sponges and early winter weather is also very up and down with temperature. Eventually after prolonged dry winter outside air - things dry out and often will get too dry.

  4. ntisdell | | #4

    Because you have asthma do you have a humidifier to keep humidity consistent and in very comfortable ranges? That will fight any effort to ventilate further.

    Seems like from what i have read regarding my daughters light asthma is that high or low humidity is a problem as well as very cold air. Triggers of course are numerous - and vary by person it seems. Smoke/dust is definitely a common one.

  5. mfredericks | | #5

    Thanks for the comments, I should have mentioned I'm in Nova Scotia, Climate Zone 6.
    Nick, humidifiers do run occasionally during the winter, partly for my asthma but also to help humidify the seriously dry air that's common in such a leaky house with a wood stove burning all day. I'm aware running humidifiers is not a great idea but in our house it seems that comfort will often trump my explanation of building science!

    The humidifiers would explain the condensation on the windows. I don't have any humidity data, although I've noticed the windows on the second floor are always much worse than those on the main floor. At times, the upstairs windows are partly covered in ice. These windows are also the draftiest and in the worst shape. I'm currently building interior storm panels to try on the inside of these windows to see how they perform.

    My basement has a crawl space connected to it where an addition was built. This used to have a dirt floor but I've covered this with poly and insulated the crawlspace walls. This crawl space is now quite dry. I'm pretty sure the the basement slab was poured without any moisture barrier under it, and likely contributes to some humidity too. Otherwise the basement is tight and fairly dry and the few windows in the basement do not get condensation on them. These basement windows are sealed shut and are 2 sashes of single pane glass, and we've never noticed any condensation on these.

    As Dana points out, with an exhaust only approach the make up air could be drawn through an undesirable part of the house and since the basement is well air sealed (confirmed on the last blower door test) the make up air would probably be coming through the next lowest part of the house like the dusty walls or maybe the attic. The home is a cape style house so the upper floor is poorly insulated and has no air barrier. From where I've poked in, the area behind the knee walls is super dusty. This roof is not vented along the eaves, but there are 2 gable end vents.

    My thought was that with a pair of lunos fans, we could have a balanced system and wouldn't be drawing as much air through potentially dusty parts of the house. If I used a exhaust only system, it seems this would be made worse.

    What I'm wondering is whether it makes any sense to pay for these fans/the electricity to run them, given how leaky the house is? Would the effort to control the air source with mechanical ventilation just be a waste, when there are so many drafts and air leaks?

  6. GBA Editor
    Martin Holladay | | #6

    I don't think that your house need more ventilation -- except perhaps in the bathroom, where an exhaust fan always makes sense.

    The reason that you have condensation on your windows is that you are operating a humidifier.

    If you want to reduce your humidifier operation -- and you should, although it's not clear that you want to -- the most important thing you can do is to perform air sealing work in your attic and basement.

  7. ntisdell | | #7

    Hmm, well it sounds like you can definitely use at least one hydrometer to get some quantitative data to help regulate and monitor humidity.

    There is definitely no point in adding air exchangers to reduce humidity if you are going to add it back in. It does give you some flexibility for drying out home when you see a big cold front coming (...cracking windows also can fix this in the short term without elec or install cost).

    If you are concerned that your air quality is lacking due to a tight home or polutants (ie fire biproducts) then that is a different issue.

    You spend most of your time in the bedroom, I would recommend window film insulator kit in the bedroom which will eliminate frost/dew. Then you can have a local humidifier in the bedroom without upping the entire home.

  8. Expert Member
    Dana Dorsett | | #8

    When running humidifiers in a cool climate it's nicer to the house (and the springtime mold-spore count), to keep it between 30-35% RH @ 21C to limit the amount of moisture build-up in the walls. The "healthy-comfortable" range for humans is 30-50%, but the higher the wintertime humidity, the more moisture issues you'll have with the house.

    Humidifying to 45-50% @ 21C would result in copious window condensation too.

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