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Old 01-31-13, 10:24 AM   #13
stevehull
Steve Hull
 
Join Date: Dec 2012
Location: hilly, tree covered Arcadia, OK USA
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AC and gang,

Good information on ERV vs HRV graphics.

First off, homes rarely have any CO2 buildup as the inherent ventilation of a home is far more than a school, office or other work space. This is because the ratio of surface area to living space is so large in a home.

Very, very tight homes still have an air turnover (with no HRV/ERV) of about 1 air change per day so CO2 is very rarely a problem in homes. With regard to "rest" recall that 6-8 hours is sleep (even less CO2 output than rest) and that few really do large workouts in the home. But we do have dishwashers, showers, and latent vapor coming off our lungs which all contributes to significant water vapor loads.

All that said, I also assume that a home has properly vented hot water heaters. furnaces, etc (if using propane/nat gas).

On the issue of ERVs moving water vapor. Yes, they are bidirectional if there is a large dew point difference between the inside and the ambient outside.

Air and water vapor permiance are important concepts. In very cold climates, the issue of water vapor permiance into walls can become very problematic. If sufficient water vapor goes into walls, there is the infamous critical freezing point in the middle of the insulation. If the insulation can become frozen and then water soaked (cellulose), then it turns to mush.

In northern climates, I far prefer to have a 6 ml poly sheet on every wall, just underneath the drywall, walls ceilings and lapped like you are building a boat. I also prefer to put a closed cell foam layer on the cold side of the wall and a water non-permiant insulation inbetween. Yes, foam is great (expensive), but fiberglass batting (R18-R20) is very acceptable if well done (properly fitted into cavity).

Obviously ANY air infiltration around electrical boxes must be sealed. The poly sheeting prevents water vapor going into walls and freezing there.

Older homes are a real pain as studs are not 16 OC and custom cutting fiberglass batts becomes very tough. Over stuffing in nooks/crannies is easy to do and almost destroys the insulation value.

In southern states, the opposite is true with the vapor barrier on the outside of the home as dew points are almost always higher there.

The use of ERV's can help a LOT with water movement as in the winter it will increase relative humidity in a home and then decrease it in summer (to a lesser extent).

HRV's don't move any water except that which may condense out on the cold side of the HRV.

But all that said, I still have a dehumidifier that kicks in here (central Oklahoma) on humid May days when there is not a large heat load, but showers, dishwasher, etc all add a lot of water vapor to the house.

I do encourage all to have a home humidity meter and to never allow it to go over 60%. I prefer to have it never go above 55%. In the winter, high humidity (in no wall vapor barrier) will accumulate in the ways "sogging" the insulation and in the summer it will contribute to mold build up in same.

AC, just get some "raw data" from the CO2 sensor ($10 is worth playing with!) and if you need help, feel free to call.

Great discussion!

Steve
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