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Old 02-03-2011, 11:38 PM   #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, 04:04 AM   #2
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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, 09:01 AM   #3
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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, 10:34 AM   #4
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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.
"Cut it twice and it is still too short".
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