Make-up air in an older house
canada_deck
| Posted in General Questions on
I recently made a number of improvements to air seal the guest suite in my 1980s house. I think it was highly successful but now I’m faced with a dilemma. When I run the bathroom fan when the windows are closed, air is just getting sucked in through the remaining holes in the wall that are pulling in gross unconditioned air from who-knows-where (the attic, through the insulation, etc.) I’ve got one large opening in the wall where I have a submeter for the suite and when the bathroom fan is running, I can feel the air getting sucked into the suite with my hand. What should I do in this situation? The point of running the fan is to help ventilate the suite with fresh air (in addition to just using it when the shower is running.)
A few other things to note:
– I live in the Pacific Northwest near the coast. Summers are warm and fairly dry and we keep our windows open (No AC, rely entirely on fresh air from outside to keep cool). Winters are cool and very wet and we keep the windows closed.
– I’ve started monitoring moisture in the suite and it’s probably higher than would be ideal (65% plus). I don’t have much data yet.
– Our only heat is electric baseboards.
– The suite is about 600 sq ft.
I see a few options:
1) Crack open a window everytime I run the bathroom fan. This does seem a little counterproductive though – bringing in very moist air from outside to offset the moist air from a shower.I also think I will forget to do it.
2) Install some type of an automatic air inlet that opens every time the bathroom fan is on. Deals with the manual work but other than that, there are similar cons to the first option.
3) Install an HRV or ERV in place of (or in addition to) the bathroom fan.
4) Install a dehumidifer and rely less on the bathroom fan after/during a shower.
4) Install a dehumidifer and rely less on the bathroom fan after/during a shower.
5) Just leave it be.
Any thoughts?
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Replies
canada_deck,
Whether you open a window or install an air outlet, the make-up air is the same outside air. Fortunately, except for the summer when you have the windows open anyway, that outside air is much less humid than that inside once you warm it.
I know that seems counter-intuitive in the damp PNW, but you can easily confirm it with a cheap humidity reader. My indoor RH is currently 52%, while outdoors it is 76%. If I open my front and back doors, it will drop several points in minutes.
De-humidifyers will keep the whole house RH lower, but won't effectively deal with bathroom moisture. An ERV with boost function will do both, and improve your IAQ . With either of those, you will still have the same problem with make-up air unless you get rid of the bathroom fan.
If your house is tight enough that it de-pressurizes when you run the fan, an ERV might be a good idea. Otherwise I would just open a window.
Thanks Malcolm. If I was to look at adding an ERV/HRV, which would you recommend (HRV or ERV)?
canada_deck,
The PNW is the one region in North America where it doesn't make much difference. The choice may come down to that ERVs are much more widely available.
I'm in much the same situation. My 30 year old, semi-well air-sealed house is tight enough that a fan or the dryer will make my wood stove back-vent, and like you I rely on opening windows year round to provide ventilation, and avoid de-pressurization. Right now I can't get too excited about mechanical ventilation for such a small house, using so little energy to heat.
How about one of the fresh air inlets that has a course filter and a small damper? I have one for the same purpose. I found it at a local electric supply house.