Deciding Whether to Add Dehumidification
We doing a back to the studs deep energy remodel in Climate Zone 3.
Relatively tight (should come out between 1 and 2 ACH50). R19 walls R38 roof. Relatively high spec windows (U values c. 0.26).
The building is about 5000sq ft and will be served by a Mitsubishi VRF system with four ducted air handlers each serving an internal zone with relatively good internal separation. Zhender Comfo Q ERV.
The question we have is whether we’ll end up with humidity under control (target 45-55%).
Before demolition the interior humidity fluctuated broadly year round between 55 and 70, but obviously that’s not a good compare (as we were >15 ACH50, etc).
Thoughts welcome – should we add a dehumidifier? If so I’m reluctant to do all four HVAC zones – so better to do one in the largest zone (kitchen family room), or better to do upstairs (warm vapor laden air rises?).
thanks!
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Depends on other construction details.
Will you have a basement, crawl space or slab on grade?
If basement or crawl, how will you waterproof and drain the perimeter?
You stated "R-38 Roof." If this spray foam on the underside of the roof deck, or blown in insulation?
If spray foam, is it closed or open cell?
Same climate zone, 1.1 ACH, and other specs are similar.
Sealed crawl needs a dehumidifier even though it is waterproofed and has a foundation drain installed per the recommended details on this site. It particularly needs to run after a rainfall or when the outdoor humidity is over 80% which is often.
One attic is sealed spray foamed with open cell, with a vapor diffusion vent at the ridge. The humidity at the ridge area gets higher than I would like a few times a year. It would have been good to have a dehumidifier up there.
No issues in the main living space but I suspect the crawl dehumidification helps that. Another house that is slab on grade with an even lower ACH needs dehumidification in the living area.
Consider:
1. Your HVAC system - sounds like 4 heads with one outdoor unit. Maybe go to at least 2 outdoor units, so if one goes out you don't completely lose conditioned air.
2. Consider the filtering of the intake for your ERV; there is alot of pollen in many areas of climate zone 3 and that quickly clogs intakes.
Zone 3 does not tell us much about your local humidity.
If you are in a desert of west TX or CA you could be zone 3 and have zero need for a dehumidifier but if you live in a swamp, one could be a necessity.
Unless you are in a desert 45% goal seems unattainable without drastic options like cooling the air and then reheating the air to get your desired temp and humidity but that is exactly what a dehumidifier does.
It could be argued that a dehumidifier is a highly affective heater that produces more BTU per Watt than a resistance heater.
Consider just how much energy are you willing to burn in pursue of your humidity goal and is the goal reasonable and necessary?
Note an oversized HVAC systems do not run long enough to remove much moisture so get the smallest system you can. I think having mutable heads connected to one outdoor unit is a poor choice as the systems are unable to operate at the low speeds single heads systems do and multi head systems tend to be oversized.
My thermostat has a dehumidify mode that I can set the number of degrees I will allow the system to cool the house below the set point if the humidity is over the humidity set point. Let’s say the set points are 79°and 65% with 3° limit for dehumidification. When the system starts a cooling cycle it will cool the house to 68° and shut down and not run until the house is over 79°at that point it starts cooling if the humidity is over 65% it will over cool again on that cycle. 3° is not always enough to get to the humidity set point but is also enough below the set point to make the wife unhappy. So, I have to balance my goals.
Matt Risinger has several YouTube videos on this subject. If you live someplace where the outdoor humidity is often unbearably high while the temperature is reasonably low like Matt does cooling and reheating the air maybe the only option.
https://www.youtube.com/watch?v=rWWLW4nLNBI
Walta
Along those lines, the Daikin Quaternity minisplit is purpose-built to do that as efficiently as possible -- it has both a heating coil and a cooling coil in the same head. It's basically a large scale dehumidifier, with an external compressor and the ability to also either source or sink heat outside as needed.
I find it intriguing, but there seems to be a paucity of first-hand experience. I'm thinking of installing one in my summer house where the typical weather is cool but humid.
DC, please take one for the team and install one! I've been curious about these units for like 5 years and have only ever found 1 or 2 first hand accounts of anybody owning one. Would be amazing to have a reliable poster like you be able to provide an account!
What's holding me back is there aren't any Daikin dealers near me. I have to find out if the town will let me self-install.
It does stick in my craw a bit that the cheapest Quaternity, the 9K, is over $3K and a generic minisplit of the same size would probably be under a grand.
Yep, I recently noticed that the Quaternity is massively more expensive than comparably sized Mitsubishis and Fujitsus, even.
> “Relatively high spec windows (U values c. 0.26)”
Sorry to go off topic but if R-3.8ish is what you consider a high spec window you should shop around some more. And in your climate you probably want to pay attention to the SHGC value too and keep that pretty low.
You're basically acting about predicting what your humidity will be. Predicting is what models are for. The most common model for predicting building performance is the Manual J, but it really cares more about extreme heat and extreme cold. But it does deal with humidity tangentially.
First, the assumption is that you're keeping water intrusion and similar sources of dampness out of your house. IE, your roof doesn't leak and you've sealed the basement against leaks and rising damp. Those sources can be fixed and should be fixed. Manual J expects two sources of humidity in the house. The first is "occupant behavior" -- this is everything from cooking and bathing and doing laundry to breathing and sweating. The model assumes this introduces 0.2 pints per person per hour. The other source is infiltration, even the tightest house will have some air leaking in and if the outside air has a higher moisture content than the interior air that infiltration will introduce moisture into the house.
Moisture is removed from the house by ventilation in the winter and as a side effect of air conditioning in the summer. Manual J will predict how much moisture the AC needs to remove, but it only concerns itself with the hottest days of the year. What you're asking about is a different question, what do the humidest days of the year look like, and are there going to be enough of them to be a problem?
I can tell you that interior humidity will be maximized on days when it is high humidity outside, warm enough that the air can hold a lot of moisture, yet cool enough that the air conditioning doesn't run. So maybe a day when it's 70F outside and raining and foggy, relative humidity outside is 99%. On a day like that, without a dehumidifier there will be nothing removing humidity from inside your house, any air that leaks in will be bringing in humidity and the occupants will be constantly adding more moisture to the air. If it stays that way long enough the interior of your house will go to 99% humidity.
What I can't tell you is how often you'll get days like that, and how long they'll last. I can't even think of a weather resource that would help.
I will say that in general well-sealed houses have very low infiltration rates. The standard model is that infiltration is driven by the temperature difference between indoors and outdoors, so on the days that humidity is a problem there is going to be low infiltration. So for the most part the humidity you're going to be trying to get rid of will be coming from occupant behavior. So if dehumidification is necessary, the answer to the question of where to put it is in the parts of the home with the most occupant activity. It's also true that moist air is lighter than damp air, so all other things being equal humidity tends to rise in a house. Warm air also rises, and dehumidifiers work better at higher temperatures, so they tend to be more effective higher in the house.
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Thanks! Really appreciate all the input here. Consolidating some responses:
* Construction is two stories over a crawl space, which is inside the conditioned envelope. Pango Wrap over clean crushed rock, with a 4” slab on top (in most places on top of R15 of Rockwool Comfortboard). Crawl space walls have 2” closed cell foam applied (up to the rim joist). French drains under the rock draining to a central sump evacuated to outside main drain. Exterior perimeter of the building has a french drain all around.
* Roof is hybrid, with 1.5” of polyISO under 2.5” of Rockwool under a TPO membrane (low slope roof) on top of the roof deck. Plan is for closed cell spray foam under the roof deck (know that folks have mixed feelings on this approach).
* Yes on the outdoor unit(s), will have two outdoor units serving the four indoor units.
* Nice idea on the pollen filter on the ERV intake. Thanks!
* Outdoor humidity is somewhat range bound in the 75–80% range year round (see attached).
* The HVAC is VRF (variable refrigerant flow), and i don’t believe that the indoor units are seriously oversized (but understand folks have different views as to this).
* Appreciate the thoughts on the windows. Yes I am sure i could have done better on these; SHGC is about 0.25.
Thanks for the input on location. Thinking about upstairs in the roof void, and then retrofit one in the crawl space if it becomes necessary down there. Does that make sense?
That humidity isn't really that high. August has your highest dewpoint, at 63.1F and 74% RH that's a dewpoint of 56F, that same dewpoint at 72F is 56% RH. That's very comfortable and healthy and isn't by itself going to cause humidity issues. If occupant behavior causes humidity to rise you could clear it by venting. The rest of the year the dew point is much lower, in January it's 44F which equates to 37% RH at 72F, which is even on the low side for comfort.
It looks like on balance you're slightly heating dominant, needing a small amount of heating most of the year, although I could see a house with some solar gain going cooling dominant. Looks like an ideal climate for passive solar.
I'm scratching my head trying to think where this is. An island off the Pacific Northwest?