Indoor Humidity – dew point & monitoring condensation
我住在科罗拉多州,带5,our winters frequently drop down to 0-20F and outdoor humidity levels might be in the teens.
I recently figured out my chronic congestion gets about a hundred times better at 45% humidity. Despite all the sealing & insulating we’ve done, the plants and three dogs, our house drops down into the twenties in winter, sometimes even the teens. Because this is a retrofit, I probably can’t hit 0.6ACH. I’m around 1.5ACH right now, and the next sealing & insulating step will require ripping off the roof, so that’s on the back burner.
So, I put in a humidifier. It’s got an outdoor temperature probe that limits indoor humidity based on outdoor temperature. I’d like to get the most out of it, obviously without rotting the house. The house dries to the outside, no vapor barrier. The attics will always be properly vented and all the equipment is within the conditioned envelope. The weak point for humidity is probably the ordinary R-13 2×4 walls (rigid foam under the siding may never happen, the clapboards are in good shape).
Anyway, so the questions:
——乔声称“很少有理由使用midifier if the RH is above 25%. If you feel you must use a humidifier, measure the relative humidity in the area receiving the humidified air. Never let the humidifier raise the relative humidity above 35%.” Does that assume a certain standard indoor temperature? What really matters is moisture content, e.g. dew point, which means I can safely have a higher humidity if I have a lower thermostat set point, right?
– Assuming there’s nothing really stupid in the house, are there particular places I can monitor to reassure myself there’s no condensation? For example, would frost be likely to show up on the underside of clapboard near the gaps in the OSB? Or is condensation on windows a reliable indicator of condensation in the walls?
通常我渴望做“正确”的事情,快乐to go to extremes, put on the extra sweater, and all that. But I also live in a cold, semi-arid winter and value the ability to breathe easily.
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Ah, and I can balance my ERV to produce a slightly negative net pressure, in hopes of drawing dry air in through cracks instead of leaking moist air out through those same cracks. Is this generally effective & worth pursuing?
The catch would be, while we have radon mitigation, a negative pressure is not a desirable thing from a radon perspective.
Yes, more negative building pressure will reduce moisture problems in the walls/ceiling, allowing higher humidity. It won't effect window condensation.
Most (but not all) of the areas covered by the radon mitigation system will be much lower pressure than any reasonable house negative pressure. So you should see little increase in radon.
Where to measure moisture to monitor for a problem is a good question.
Pshyvers,
You are embarked on an experiment, but the reason you are walking this risky path is a medical issue. Good luck.
Certainly if your windows are dripping, that's a bad sign. Hidden condensation is hard to predict; it usually follows exfiltration paths, which are specific to each house. My advice is to minimize the use of the humidifier if you care about your house.
Depressurizing your house may help, but in many cases, running an exhaust fan just moves the neutral pressure plane upwards, without stopping stack effect exfiltration.
We live in Ottawa. Technically a cold/humid climate, but there's no such thing as humid when its below freezing outside (relatively speaking, of course).
We live in a two story townhouse of very average build and experienced the same thing you have; extremely dry winter time air. I installed a humidifier when I did our hardwood floors, and it has made the house much more comfortable. Up until last year I kept the RH at about 35% until it got really cold, then I'd have to reduce RH to the high 20's to keep our average dual pane windows from sweating excessively.
Last year I installed heat shrink film on our windows and it made a big difference in how much humidity I could maintain before window moisture became an issue. Even in really cold weather (well below 0F) I was able to keep RH in the mid-30's with no issue.
Then I started researching our new house build and wondered if keeping our humidity that high was wise in what is surely a very leaky house.
In the past couple of months I blew in new attic insulation and foamed the sill plate area of our mechanical room. During both upgrades I got a good look at the structure of the house when pulling back and removing the original insulation and vapor barrier. I can happily report that keeping increased humidity in our home does not appear to have compromised the structure, though I do admit that I didn't see inside our walls, only the basement rim joist area at the back of the house and the top of the walls and ceiling around the edge of the attic.
While researching the issue of moisture in walls a recurring theme seems to be that most/all cold climate walls get damp/wet, but as long as they dry thoroughly in the spring then the moisture doesn't tend to lead to mold growth. (Someone please correct me if I'm wrong!) Perhaps localized moisture from exfiltration could lead to higher local moisture content and be an issue?
What type of house do you live in? If you are in a slab-on-grade bungalow the stack-effect that Martin mentions will be greatly reduced over what it would be in a two-story with basement, in which case the ERV depressurization method you describe could be effective.
Our new build will be as tight as possible with good triple pane windows, and one of the major motivating factors is being able to keep our indoor RH at a decent level in the winter. I'm optimistic we'll be able to keep it at around 40% or so without window moisture issues. I don't want to live in a dry house ever again!
If this place is really testing 1.5ACH/50 and running dry indoor air in winter I'm going to hazard that the heating system is a ducted hot air system with either:
A: An unbalanced duct system inducing high room-to-room pressure differences when operating...
...or ..
B: ...a outdoor air intake on the return ducts, drawing in excessive ventilation air.
It could be both.
If any of the ducts are in the attic and leaking it could be even worse levels of air handler driven infiltration.
Fixing the duct balance issue and closing off (or at least turning down) any ventilation air schemes would be the first order of business. Energy Star homes should have
It's a two-story with basement, so the stack effect is reasonably strong, I measured the old combustion air vent in the basement flowing something like 100cfm+ before I removed my open combustion appliances.
I had also heard that as long as the house dries to the outside quickly when weather warms you'll probably be OK. Considering our climate is so dry, and the house has no vapor barriers & a vented attic, I am thinking the shell will dry quickly.
I'm still curious about the dewpoint approach. For example, if your house is at 72F, and you follow the rule of "no higher than 35% Rh" then your dewpoint is 43F. If you then lower the temperature to 65F, your dewpoint remains at 43F but your Rh is now 45%. With the addition of a light sweater, this would be more comfortable to me, while not increasing the moisture content of the air.
One day when we replace the roof & seal up the rest of the attic floor, I should try to measure the final pressure differential.
Learning about green building sure does make building one's own house seem like the only way to get all of this right!
Dana, 1.5ACH is a bit of an estimate I'll admit, it tested at roughly 900cfm50 (2.3ACH) before I demo'd the 6" combustion air vent, which should flow roughly 200cfm50, bringing us to ~700cfm50, plus a little more work, so I estimate 1.5-1.7ACH. Apologies if I've taken too many liberties with my estimates, I'll get it tested again at some point here.
It is indeed a ducted forced air system.
A) is an interesting line of question, and not something I've explored. I'll see if I can measure the differences. There is definitely some temperature imbalance between rooms, though I've been chalking that up to an oversized furnace & the "far bedroom" problem, plus different levels of insulation.
B) As mentioned in passing I have an ERV, which has been providing roughly 65cfm. It's an EV90 so it does not have the very highest latent heat recovery, but it's better than a simple outdoor fresh air intake. We're experimenting in reducing the runtime.
No ducts in the attic, all within conditioned space in walls & floors. We've got large return vents in every room, though the returns are built as cavity returns & panned joists. (Supply side is all 6" rigid duct, looks well sealed) I'll explore the room pressure difference question, thank you!
Most <$150 hand held manometers have 0.01" resolution, which is good enough for finding the most egregious imbalances. Any measurement greater than 0.01" would be a "fail" for Energy Star levels, but getting it down to the 0.02" range would be a good start for most systems.
You can easily test the effect of no HVAC induced room pressure differences by leaving all the interior doors open.
You are correct that wall/ceiling moisture is actually influenced by interior dew point, not indoor %RH.
Any decrease in building pressure decreases ex-filtration (caused by stack effect or otherwise).