Thanks again.

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 40.5 F
(corrected figures)
.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.

The temperature does vary.

...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?

The water vapour could be coming from the basement, yes, it is likely.
There is no stairway for the moisture to come up to the main floor,
just the furnace and a few small holes for plumbing. Yet rise it does.

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.

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 :-)

Is it? In places like Saskatchewan, they make a joke about the temp
being minus 40 but tolerable because "it's a dry cold". You can be
sure that when those same people retire to British Columbia, they often
feel (subjectively) colder even though the temperatures are oh 40 C
warmer, because of the dampness of the cold. And the most harrowing
tales of winter misery are not from the Yukon, but from houses in damp
England.

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.

Books are made of paper. If the moisture condenses, it can easily
evaporate from wood or fabric, but not so easily from books in
bookcases against a wall.

Don't want to store moisture in our books.

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

That shows that the RH is frequently above 60% because the books in my
office, next to the wall, do get a touch damp.


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.

I questioned their applicability. I remember about 25 years ago I
wanted to get an old house in Ottawa insulated. The main choices at
the time were blown in cellulose and urea formaldehyde. I was dubious
about the UF, so I asked the contractor to provide "proof" that it was
OK. Surprisingly, they provided lab studies which "proved" that UF was
OK. I looked at the figures and the methodology, and concluded that in
the real world there was no way that UF foam could be blown the way it
was in the lab, so I went with cellulose. A couple of months later,
the issue blew up and I luckily didn't have to take the contractor to
court or demolish the house etc.

So I'm not sure what your numbers mean in this context. It can rain
zero days in August, and 29 days in November, yet August is
considerably damper than November. I can only think that those figures
somehow have the rain column shunted off into another category. Also,
the temperature inside the house does vary. In summer, we allow the
temp inside the house to go up to 76 or even 80. It is still
comfortable; if not, we can go outside under the grape arbour. At
other times we might let the temperature fall to 65 or even 62 and put
on a sweater. Those are the times when the vapour might condense out,
and those are the times we might need the dehumidifier.

It isn't hard to make a hole in a window.

I hadn't thought of that.

You may already have an exhaust fan in a kitchen or bathroom.

Kitchen.


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

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

Ooops, sorry, I have a selective memory. Sometimes it doesn't cotton
on to terminology. Maybe I'll remember now.

But this is interesting ... the air at the top of the column might be
70F, with an RH of 50%, but at the bottom of the column 65F with the
same "humidity ratio" and therefore a higher RH. The warmer air might
not activate the unit's humidistat. That is consistent with the
observation that in the morning when we wake up, most of the
condensation is at the bottom of the window; the top may be clear. It
might also explain how the basement is a source of humidity to the main
part of the house: warm damp air rises through cracks, but never has a
chance to settle back down into the basement. And finally, it makes me
think that this old house might be more air-tight than I thought it
was. The wall construction is peculiar, as it includes 1" thick
horizontal planking. Maybe that traps a bit more of the moisture which
would otherwise escape.

More dehumidifier investigation: Local stores (to Nanaimo, BC):
Zellers and The Bay don't sell them. Wal*Mart has a Fedders unit whose
model number (D4989M) does not match any specs on the Internet. The
closest source for Bionaire in Canada is a dealer in London, Ontario
(over 2,000 miles away). No local refrigeration outlet has a
dehumidifier in stock, though one said he could order a Friederichs D30
unit for $cdn488. As reported earlier, there seem to be at least
three manufacturers (Bionaire, Soleus, Surround) who produce a small
dehumidifier with a noise level around 40 dBA and some sort of
anti-frost protection so that the unit can be allowed to operate at
temps of 65 or lower. The latter two seem not to be available in
Canada.

--
Jonathan Berry