wrote:

Thanks for your comments, Nick!

You are welcome.

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?

No. That varies with temperature.

I also doubt the accuracy of the figures. Ave. Temp. of 50.5 F in December?
Nonsense, or a typo.

Oops. A typo. Shoulda been 40.5, with 35.8 and 45.1 average daily min and max.
I rechecked the rest of the numbers, which look OK.

...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.

Maybe that's where your water vapor is coming from. A 1930s house with
no vapor barrier under the basement floor?

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.

Dunno about rain. Warmer air can carry more water vapour.

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.

Airtight houses need dehumidification in wintertime because they contain
humidity sources, people breathing and showering and washing floors and
cooking and so on, as well as damp basements. A perfectly airtight house
would let the indoor RH rise until condensation happens on the indoor
surface of windows.

...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.

Sounds like you have a major indoor humidity source.

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.

That's backwards :-)

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.

Also concrete, wood, paper, fabric, and so on.

Don't want to store moisture in our books.

The RH might range from 30 to 60% with no damage.

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.

Cold air tends to be drier than warm air. You have my numbers, which you
seem to have ignored. It isn't hard to make a hole in a window. You may
already have an exhaust fan in a kitchen or bathroom.

By "humidity ratio", do you mean "relative humidity"?

No. I already explained the difference. You seem to have ignored that too :-)

Nick