Thanks for your comments, Nick!

wrote:
wrote:

The weather on the Pacific Coast from Northern California to British
Columbia tends to be warm and dryish in the summer but cool and damp
in the winter...

Maybe not. NREL says Seattle has these average temps and humidity ratios:

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

40.1 43.5 45.6 49.2 55.1 60.9 65.2 65.5 60.6 52.8 45.3 50.5 F

.0042 .0045 .0046 .0051 .0061 .0072 .0082 .0085 .0078 .0065 .0051 .0044

The humidity ratio w is the number of pounds of water vapor per pound
of dry air. It does not depend on the air temperature, and it doesn't
change much in 24 hours. The relative humidity is the number of pounds
of water vapor per pound of air divided by the maximum number of pounds
of water vapor the air can hold (at 100% RH) at a certain temperature.

I'm not sure how the absolute figures given above relate to the
experience that everyone who lives here has. Wouldn't RH be more
pertinent? I also doubt the accuracy of the figures. Ave. Temp. of
50.5 F in December? Nonsense, or a typo. Our own experience here, and
I don't think Seattle is too much different (although Seattle is
definitely wetter) is that it does not rain in August, and if it does
rain, the ground can be "bone dry" again in half an hour. In November,
on the other hand, it seems to rain constantly; your lawn will be a
quagmire. Water tables rise, and the moisture evaporates through your
basement walls into your living space.

I'd suggest that the moisture figures above reflect a) the fact that
warmer air is able to carry more water vapour and perhaps b) some sort
of filtering out of the effect of rain.

...So while for much of the continent, the season for dehumidifying
is the summer, on the left coast it is the winter.

That could be true in a Canadian airtight house with humidity sources,
unlike air-leaky US houses with energy-wasting winter humidifiers...

Airtight houses in Canada tend to be where the climate is severe. The
ideal place is Saskatchewan, which has hot summers and severe winters
with lots of sunny days. Airtight, passive solar, summer-shading
overhangs ... all work out well in Saskatchewan. There they have a
"continental climate" which would not likely need dehumidification in
winter.

Here in coastal British Columbia, construction tends to be more leaky.
And the case in consideration, our house, is a leaky house built in the
1930s. This thread is the result of an Energuide energy audit, which
resulted in the consultant telling us that we should make the house
more airtight, but that BEFORE we did that, we had to deal with the
humidity issue.


Dehumidifiers produce heat, therefore their energy efficiency is
important in continental applications. However, on the left coast,
they are used mostly in the winter, so the heat they produce is
mostly a slightly more expensive form of something you're going to
do anyway: heat the air in the house.

It's cheaper than electric resistance heat, with a COP of about 1.6.
You can measure this with a Kill-a-Watt meter and a measuring cup.

That's reassuring. We're also looking forward to the subjective
feeling of warmth in dry air at a temperature where we would feel cold
in damp air.

On the other hand, we may allow our rooms to cool below 65 degrees...

Good.

Any suggestions?

Turn on a small exhaust fan with a humidistat when the indoor RH rises to
60%. In January, w = 0.0042 makes Pa = 29.921/(1+0.62198/w) = 0.201 "Hg.
Indoor air at 60% RH and absolute temp T (R) has Pi = 0.6e^(17.863-9621/T),
approximately, and Pa = Pi makes T = 507.5 R or 507.5-460 = 47.5 F, so
you can dehumidify the house with an exhaust fan as long as the indoor
temp is at least 47.5 F. If you want to save more energy, take advantage
of weather fluctuations and hygroscopic house materials and do this less
often, only when the outdoor air is warmer and drier than average (during
the day) in wintertime and cooler and drier (at night) in summertime.

Summertime is never an issue here. I'm afraid that the most
hygroscopic house materials are the books. Don't want to store
moisture in our books. So it looks like you also prefer exhaust fans
to dehumidifiers. My concern is that the warm damp air gets replaced
by cold damp air from the basement or outside. I'd really like to give
dehumidification a chance before making another hole in the wall.

Finally, wouldn't a dehumidifier be more efficient if it took the
warm moist air in at ceiling level, then expelled warm dry air at
floor level?

Maybe not. Diffusion and convection make the water vapor pressure and the
humidity ratio of the air near the ceiling and floor about the same, even
though humid air rises.

By "humidity ratio", do you mean "relative humidity"? So, let's say
that the air at 70F and 70% humidity at ceiling level, but 65F and 70%
humidity at floor level. Something like that? Then I submit that this
could still be an advantage because of the dehumidifier's greater
efficiency at 70F than at 65F.

Nick