daestrom wrote:

wrote in message

... Andersen says an average family of 4 puts about 2 gallons per day
of water (16.7 pounds) into house air. In an absolutely airtight house,
the RH would rise to 100% near windows with wintertime condensation.

ASHRAE says houses need 15 cfm of fresh air per full-time occupant, so
4 half-time occupants need 30 cfm at 0.075 lb/ft^3, ie 30x60mx24hx0.075
= 3240 lb/day of fresh air. January outdoor air in Phila has an average
humidity ratio wo = 0.0032 pounds of water per pound of dry air. If
minimal ventilation with no condensation removes 3240(wi-wo) = 16.7 lb/day
of water from the house, wi = 0.00834, and 70 F air at 100% RH has
w = 0.0158, so the house RH would be about 100wi/w = 53% with minimal
ventilation, or more, with a small efficient air-air heat exchanger
with outgoing condensation.

What do the numbers look like with an average outside dewpoint of 0F ??

Table 2 in the ASHRAE HOF says the humidity ratio wo = 0.0007875 at 0 F,
which would make wi = 0.00593 with a 38% RH above, or more, with some
outgoing condensation.

While the daily 'highs' here can sometimes reach 30F, the overnight low and
dewpoint of outside air is usually much lower for Jan/Feb.

Since many house materials (cloth, wood, paper, concrete) can store moisture,
it seems like a good idea to ventilate houses during the day in wintertime,
when outdoor temps are warmer. An exhaust fan might have a timer, as well
as a humidistat.

Seems like 'always' is a pretty risky statement considering some parts of
the country. Pellston MI is often one of the coldest places in CONUS, or
International Falls MN.

International Falls has wo = 0.0009 with an average 1.0 F outdoor temp in
January. Brrr.

How much air exchange happens when the door is opened eight times a day
(four people leaving for work/school and returning). Just wondered if you
have some data on that?

I have no data, but if the door's open for 3 seconds each time, that's 24
seconds total, ie 0.4 minutes per day. With 16.6x16ft^2sqrt(70F-0F)4')
= 4444 cfm when the door is open, we might move 0.4x4444 = 1778 ft^3/day
of fresh air into the house, ie 1778/24/60 = 1.2 cfm, averaged over 24h.

Airlock entrances just for the purpose of saving heat energy (vs mudrooms
that keep a house cleaner, etc.), don't seem worth the cost except for
department stores or very large busy families.

Nick