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Old 02-04-2011, 12:38 AM   #1
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What do you do about negative pressure?


My interest is mostly in trying to avoid pulling cold air in at this point. I suspect I have negative pressure, but I don't have any measurements to confirm this and I don't think it's causing any CO2 problems as I'm not tripping any CO2 detectors.

I live in Michigan and have a house built in 1917 where it's been in the process of renovating. I gutted the second floor and redid the roof, the roof is done although I needed to put in soffit vents which I did not get done before winter came along. I have most of my insulation up, I'm near ready for drywalling the second floor. Today I caulked and weatherstripped the 2 exterior doors, windows are well sealed newer windows (5 years old or less).

Is this mostly a matter of getting through the insulation, or is there any other work I need to do to balance the air pressure? The furnace is somewhere around 7 years old, it's a newer 83% efficient unit, no AC yet, it's in the crawlspace.

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Old 02-04-2011, 05:04 AM   #2
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What do you do about negative pressure?


If your crawlspace isn't sealed, then your house is probably not in a negative pressure. You get lots of infiltration from a crawlspace.

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Old 02-04-2011, 10:01 AM   #3
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What do you do about negative pressure?


Well, that's where I'm heading. The crawlspace is most definitely not sealed. It is where the furnace is located, and there is a register on the plenum to let heated air into the crawl space. It is an 80% efficient unit, so it is not sealed combustion.

I will be insulating the crawlspace walls, it's part of the next round of projects I will soon be starting, along with plastic sheet over the dirt, Tyvek air barrier on the inside before insulation goes on, adding footings, piers and beams.
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Old 02-04-2011, 11:34 AM   #4
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What do you do about negative pressure?


As long as the furnace can get adequate combustion air then your are safe and can insulate to the max. May need to have a vent/combustion air pipe or grill from the outside of the foundation to let air into the crawlspace for combustion. How old is the furnace? May want to replace it with a sealed combustion high efficiency one and eliminate that problem. Still needs some heat down there/at least 40 degF so the condensate drain does not freeze.
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