High Humidity in Apartment Above Garage
70% Humidity
We recently moved into our newly constructed garage with an apartment above it. The building is pretty tight but I do not know how tight yet. Blower door test tomorrow. Both spaces are heated/cooled with one Mitsubishi HP.
The apartment is very comfortable with the thermostat set to 68. It’s been cold here lately, with nighttime temps in the 30’s. We love the consistency of heat the HP provides.
From day one I noticed the humidity reading on the thermostat to be in the low 70’s. After running the system for a few weeks now, it’s still at 71%. I understand it’s new construction and materials are “drying” out, but those readings seem pretty high to me. Today the outside humidity is 49%, 71% inside.
What’s going on here? Should I be concerned? There is no HRV or ERV.
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Replies
nynick,
Almost all heat sources are closed systems, and are independent of how the house is ventilated. Every house needs a ventilation strategy to deal with both air quality and humidity. If you don't have an ERV/HRV, you need some other form of mechanical ventilation, or to open the windows.
Thank you Malcolm. So I assume the houses of the past were so leaky, they ventilated themselves, so the tighter the house today the more need for controlled ventilation?
Could I retrofit my Air Handler to dehumidify or handle an HRV? You would think this would be a common issue these days, so HVAC guys would build the systems anticipating humidity.
nynick,
Yes it's one of the prices we pay for more energy efficient houses. Old leaky ones experienced enough air changes to keep humility down in most climates. Every new house should include mechanical ventilation, and most building code now require it. It can be as simple as a constantly running bath fan.
When you ask "Could I retrofit my Air Handler to dehumidify or handle an HRV?" I'm a bit confused by what you mean. Can you describe what air handler you presently have?
Does it feel really humid? The reason I ask is that I've found a few indoor humidity sensors to be wildly inaccurate.
Assuming this one is accurate and that this is a "house drying out" issue, you could just get a portable dehumidifier and see if it does the trick before investing in a system.
The other thing to consider is your lifestyle choices, as in lots of potted plants, long steamy showers, not using the exhaust fan or range hood. That could make any house humid and if that's the case then you should look at a more long term solution.
andy,
Just dehumidifying can work in unoccupied areas like crawlspaces, but in a tight house you still have the problem of air quality. Nick needs ventilation - mechanical or natural.
Malcolm, I completely missed the "There is no HRV/ERV" part so yeah, you're right...GET SOME VENTILATION!
...and then, if there is still a humidity issue you might want to get a standalone dehumidifier.
It doesn't really feel that humid...just comfy. The windows are triple glazed and don't steam up. We have very few plants in the house. No long showers but we're not really big bathroom or kitchen exhaust fan users. The house has zero drafts.
There's just two of us living here.
You might start with getting additional RH measuring devices. There is a good deal on Awair units right now: https://www.getawair.com/. They are probably more accurate at measuring RH than ultra-cheap meters, and they also measure CO2, PM2.5 and VOCs--the other reasons you should consider ventilation.
I have a vague memory of trying to convince you to install ventilation but you didn't want to. Perhaps I'm thinking of someone else. Ventilation is required in the IRC for homes that test at 3.0 ACH or tighter, and that's the worst house you can legally build in most places.
Wasn't me. I would've done so if I thought it was necessary.
Worst case, I could have the HVAC guys come back and install an HRV/ERV. We have a mechanical room with an outside wall, plenty of electric and the supply return vents right there.
I'll go buy a cheapo humidity monitor just to check the thermostat accuracy.
It is an ADU, I would not add more equipment to it as the operating savings will probably never pay.
Construction moisture needs to be dealt with, best is a simple stand alone dehumidifier. This will bring the indoor RH down.
For fresh air, crack a window or run the bath fan.
Akos,
"For fresh air, crack a window or run the bath fan."
My bet is either of those will also drop the humidity 10 points.
Ventilating during heating season will dehumidify. Let's say outside air is 49% RH, 45F. That air warmed up to 72F will be at 22% RH. Just leaving a bath fan running should dry things out.
Thanks everyone. Opening a window isn't really an option right now...it was 39 degrees this morning. We got the kitchen exhaust hooked up yesterday, so we'll be using that now. I ran the bath exhaust this morning during my shower; I'll be doing that everyday now. And we'll pick up a monitoring device of some sort soon, just to double check the RH level.
We're having a blower door test done this morning as part of the build. I'm gonna guess 1.5 ACH. I'm excited to see how we do.
Nick,
You haven't been using either your kitchen or bathroom exhausts?
The kitchen was just hooked up and by habit I haven't been using the bath fan Wife uses it though.
You really do need some form of ventilation in there. If you do monitor for VOC and CO2, you'll find those levels are likely very high. As you have an air handler, retrofitting an ERV to the supply and return of that unit in your mechanical space should be pretty straightforward. What climate zone are you in?
Zone 5, Coastal CT.
Blower door Test came in at 2.11...pretty good.
Duct leakage test failed. At least they're leaking into conditioned space. Now we have to find the leak(s).
I'll get one of those monitors Michael suggested.
I purchased a cheapo thermostat with humidity readings yesterday. It's trailing the HP thermostat by 12-15 % points. The cheapo is reading around 60-63% RH and the HP is reading 70-75%. FWIW we live directly on the water in CT. It's very foggy this morning and my phone app says the outdoor RH is 96%!
The installer is coming back Monday to assess why we failed the duct leakage test and to give us an estimate for an ERV. Clearly the HP thermostat is reading high but we still need to address what and why.
I'm just up the coast in RI and this morning it was 50F and 97% RH. But that's a dewpoint of 49F, that air warmed to 72F would have a RH of 44%. Which is just a smidge on the dry side. So even though it feels really damp out, in a heated home ventilating would lower the humidity.