Post-Renovation Humidity Problems
NE_Sam
| Posted in Mechanicals on
We live in New England and renovated an old (1870s) wood frame row house last year (attached on one side). We tried to make it green and energy efficient, so we did closed cell insulation in all exterior walls and put in two separate ducted heat pump systems. There’s one heat pump and blower for the basement/1st floor, and a second heat pump/blower for the 2nd floor. We also installed an ERV in the 2nd floor system, which ties into the blower and also handles bathroom exhaust from two bathrooms on that floor.
The problem is that we now have very high humidity in the 2nd floor during summer months, regularly 65-71% RH according to the thermostat. I notice a musty smell that I think is wet drywall, and when I look at the blower in the attic crawl space I see condensation dripping off the outside of the main supply duct (even though it’s insulated). We could add a dehumidifier into this system, but the crawl space is tight and we might need to open the ceiling to get it installed.
So, some questions for folks here:
- Is 65-71% RH too high? It feels muggy and I worry about drywall being too wet/moldy.
- Was it a mistake to use an ERV for the bathrooms? Could we tweak the settings on the ERV to help get more humidity out (especially from shower steam)?
- What options might be effective that are less expensive/invasive than a central dehumidifier? Standalone dehumidifier in a bathroom? Changing to regular bathroom fans that exhaust to outside?
- If our heat pump systems are oversized, I’ve read online that this will reduce dehumidifying ability. Can we address this by reducing blower fan speed?
- Any other suggestions?
Thanks!
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Replies
Some ducted heat pumps can use what’s called reheat dehumidification - basically the air conditioning runs but so does the electric resistance heat too so the home doesn’t overcool. If your model has that, try it!
You can try this to find out how at risk you are, knowing just the RH is not enough: http://www.dpcalc.org/
I'd say anything over 60% is sub-optimal, and any time you have condensation is concerning. The condensation may not be directly related, it may just be that your ducts need better insulation.
On first blush I'd guess your AC is oversized, it's really common for well-insulated houses to end up oversized. Oversized systems tend not to do as much humidity removal. Just as a diagnostic I would try turning off the downstairs system, that will force the upstairs system to work harder.
+1 on turning down the downstairs unit and letting the 2nd floor unit do most of the cooling.
When I've had humidity issues, I've even had good luck running the main floor unit in heat mode an upstairs in dry mode. The extra bit of heat from the main floor unit kept the place from overcooling and was a surprisingly small additional load.
Sweating ducts mean issues with the vapor barrier over the insulation. Adding more insulation won't fix this, you need to find where the humidity is getting in and seal it up.
1870s medium tight????
Short of a full gut rehab call me skeptical. Did you get a blower door test and what is your ach50 number?
I say unless you have an ach50 under 1 turn off the ERV because it is bringing more moisture than your system can deal with.
Next try increasing 3° and or turning off the thermostat for the downstair unit this will force the upstairs unit to run almost continuously giving it the long run times it needs to remove more moisture.
Since you did the silly thing of putting your HVAC in the attic you have backed yourself into the corner and now you have little choice but to remove the insulation from the attic floor and replace it with more spray foam against the roof and condition the attic. Getting anywhere close to code minimum R values with spray foam will get expensive.
Paul’s reheat idea offends my sensibilities.
My thermostat has a dehumidify mode. You set a humidity set point and the number of degrees the system is allowed to over cool. Mine is set to 65% +3°. The set point is 78° so when the humidity is over 65% the AC cools the house down to 75°and stays off until the house gets over 78° and starts cooling again if the humidity is over 65% the AC cools the house down to 75°. The long run times and over cooling will reduce the humidity.
A cold and clammy house is the classic complaint symptom for oversized equipment. Reducing the fan speed is a risky game in that it allows the indoor coil to run cooler that is fine unless it ever happens to get below freezing then the coil turns onto a ball of ice. The ball of ice will not remove much heat from the liquid refrigerant and it will not boil into a gas as normal. If the liquid gets back to the compressor, it could damage the compressor. When you have slowed the fan, everything is great and then someone block a vent or lets a filter get too dirty and starts the ice ball. Note slowing the blower will reduce the COP and increasing the operating costs
Walta
"Reducing the fan speed is a risky game in that it allows the indoor coil to run cooler that is fine unless it ever happens to get below freezing then the coil turns onto a ball of ice."
don't quote me on this, but I think I just read somewhere (can't remember where) that a lot of the heat pumps have sensors do adjust and won't freeze like this.
I have to chuckle a bit cus my house is runnin around 80% RH (no AC). Yeah it's not pleasant or ideal, but things aren't sloughing into the drain quite yet. For sure smells are stronger in higher humidity. You may also want to get an RH meter and see locally you have higher RH (like top of the ceiling).
Thanks everyone for some good responses. I realize it's especially hard to fight humidity in this current New England weather. With this system being only a year old, I do think it should be possible to adjust the settings to get better performance.
I'll do my best to respond to some of the helpful comments and questions.
I've looked back at my emails with the HERS rater, and he said that the system has variable fan speed so we shouldn't need to manually reduce blower speed for dehumidification. We did a full gut rehab, and did a blower door test, but we did not meet the 1.5 ACH requirement. The HERS rater said it was because of air flowing through the party wall with the attached house.
The attic has closed-cell insulation on the rafters and outside walls, but no insulation between the 2nd floor and the attic. So it is warmer than the living space but within the house's sealed envelope. @walta100 - can you explain why it was a silly decision to put the air handler and ERV in the attic? Does this still apply if it is within the conditioned, insulated envelope of the house?
Based on these comments, I think I will try setting the intermittent time control for the ERV to zero so that it only comes on when we push the button in the bathrooms. And I will try leaving the downstairs system off and running the upstairs full blast. We usually set the upstairs to 71 and downstairs to 75. Should I try downstairs turned off, and upstairs maybe around 68? That's colder (and more energy) than we usually want, but it would be a test for the dehumidifying capability.
I'd welcome other suggestions or questions. Thanks!
“@walta100 - can you explain why it was a silly decision to put the air handler and ERV in the attic?”
I think putting HVAC in an attic is silly because.
1 You are making the conditioned space larger and it will always require more energy to condition.
2 Depending on the slope of the roof the surfacer area of the thermal boundary is 20-50% larger so you must buy 20-50% more insulation cover the surface.
3 Moving the insulation to the roof line all but forces you to use expensive and ungreen spray foam. In fact, spray foam is so expensive almost no one is will to spend the money required for the code required R value.
4 All to often people will fail to fully condition the attic. By condition the attic I mean keep the attic at more or less the same temp and humidity as the rest of the home.
In short you spent more money to get a lower R value that leaks more energy.
What is done is done and the truth is the people that want HVAC in the attic are generally unpersuadableand I am tilting at windmills.
Walta
All of your points are about conditioning the attic. They can be extended to say that it doesn't make sense to condition the attic just to be able to put the HVAC equipment there. But if the attic is going to be conditioned anyway there's no reason not to put HVAC equipment in it.
Thank you Walta. I'm realizing now that we just didn't have anyone in our team (architect, GC or HVAC contractor) who was really thinking about this stuff and asking questions about the best design. Now we're stuck with it.
I have two seperate 3 ton A/C systems in my house. In the summer, I have things setup so that one of the two systems does the vast majority of the cooling work. The result is much longer run cycles on that unit, with the other unit only kicking in on extremely hot days when some additional cooling capacity is needed. The long run cycles help with dehumification, and are preferable to shorter run cycles. Total energy input is about the same either way, so don't think that shorter run cycles are more energy efficient -- they're not.
My guess is part of your problem is oversized cooling systems running short run cycles and not doing much in the way of dehumidification. The other issue might be air leaks in the attic and attic floor (upper level ceiling), and possibly also basement/crawlspace, depending on what you did in those areas. If you sealed up all the walls but missed a spot somewhere, that might also be part of the problem. Did you make any efforts to seal the rim joist area, for example? That is a classic leaky spot, as are upper level recessed can lights that protrude into the attic space.
Bill
Hi Bill - thanks, I think we will try running only the upstairs system for a while. The upstairs system is in an attic with just like 4-5 ft of height. The attic space is within the sealed, insulated envelope of the house. All outside walls and the attic rafters were sprayed with closed-cell foam, and we had it inspected by a HERS rater who said it was done well, but I can't be sure every leaky spot was sealed.
I expect we definitely have air leaking between the 2nd floor and the attic because there isn't insulation between them. We don't have can lights, we have wafer LED lights, but there are still holes for them. So my thinking is that the condensation on the supply duct is because there is so much house humidity making its way into the attic space and then condensing on the cool supply duct (there's also condensation on the *outside* of the condensate drain pipe, since that water is cool on the inside).
Any other suggestions? I'll try running the 2nd floor only and report back if that changes. I might have to put a box fan at the top of the stairs to get that cool air down to the main floor.
Note that insulation isn't an air barrier, so lack of insulation in the attic floor is a seperate issue from air sealing. If you have spray foamed the attic ceiling (the underside of the roof), which is what it sounds like you had done, then the attic is inside the conditioned envelope as you mentined, and you wouldn't need to worry about insulating or air sealing the barrier between the attic and the house.
Humid air rises, so moisture tends to collect in the upper part of the home. If you have limited air circulation in the conditioned attic space, it would make sense for it to get pretty humid up there. If only the lower level A/C system has been running, then that unit won't be efficient at removing humidity in the upper levels. Try running your upper level unit only for a while and see if things improve -- I think they probably will.
BTW, if you continue to have issues that may be related to air leaks, a blower door test is a good way to find those leaks so that you can seal them.
Bill
Hi Bill, thanks!
When I said insulation, I meant spray foam. I agree with you that the attic is inside the conditioned envelope, and there is really no insulation (foam or fiberglass) between the 2nd floor and attic.
There is pretty much no circulation in the attic space. We have been mostly running the 2nd floor system at 71 and the main floor at 75. I'm going to turn off the main floor and set the 2nd floor even lower to try and dehumidify.
We did a blower door test post-reno, but couldn't hit the 1.5 ACH target. The HERS rater said it was because, even with spray foam all around the outside envelope, there was still the unsealed party wall with the neighbor. He put a 2nd blower on their door to try and counterbalance, but their house (no reno) was too leaky and he couldn't maintain any pressure on their side.
Come to think of it, I believe the attic space might be completely open between the two units, so the attic is probably also impacted by the neighbors who do not have a central ducted system at all.
>"Come to think of it, I believe the attic space might be completely open between the two units, so the attic is probably also impacted by the neighbors who do not have a central ducted system at all."
That could be your problem right there. If you're newly renovated, nicely insulated, and well air sealed unit is connected to their leaky unit through the attic, that will cancel out nearly all of your efforts to air seal your own home! I would put some kind of air barrier between the two attic spaces if possible. This air barrier would likely look like a studwall with plywood or drywall on one side only. Assuming the other side of the attic isn't insulated, you would also need to insulate this "air barrier wall".
Bill
I probably shouldn't be surprised any more, but it never ceases to amaze me how many people involved with residential construction just don't get the concept of a building envelope.
With the insulation on the roof, the attic is and should be part of your conditioned space. That means it also needs to be conditioned.
If you are getting sweaty ducts up there than it means that there isn't enough conditioned air supply to the attic. Check the supply vent up there and make sure it delivering at least code required CFM. If you are getting the right amount of CFM but the attic humidity is still high, you might have a very large air leak up there which would be worth to hunt down.
If your units are modulating heat pumps, there is no issues with turning the fan lower. I've have even adjusted the static pressure setpoint on these to get even lower CFM than the low setpoint to get better humidity removal. At the lower CFM, the supplied air will be colder though so you have to get the attic conditioning sorted out otherwise your ducts will be a soggy mess.
This has been really helpful troubleshooting discussion. As best I can tell from looking back at pictures we took during construction, it looks like the attic space has roxul insulation between us and the neighbors but nothing that would be airtight. And the attic is definitely not conditioned.
Akos, you suggest checking the supply vent up there, but right now we have no supply vent into the attic at all. So we have both a leak and a lack of conditioning. Unfortunately it will be very tough at this point to add a barrier between the two units.
Any suggestions about how much of this is worth bringing up with the contractors who did the work? I now realize that they weren't particularly knowledgeable about getting these things right - though neither was I!
I think of your earlier comment:
"We did a full gut rehab, and did a blower door test, but we did not meet the 1.5 ACH requirement. The HERS rater said it was because of air flowing through the party wall with the attached house."
He's the guy who should have noticed that, um, there's a great big hole in the attic. The party wall in the finished space is probably decently sealed, interior drywall or plaster usually is.
The next question is, do these houses have a shared basement? What's going on down there?
Thank you! The basement is fully separate between the two houses, no airflow issues there.
I'm not sure what to do about the airflow in the attic. The space is too cramped to do any work up there without opening the ceiling, it's really more like a crawl space above the 2nd floor (it's under a slightly sloped flat roof, not an A-frame roof). At the highest point it's only about 4ft, then down to 1.5 ft at the low end. Do you think it might be adequate to spray foam on top of the roxul more easily? It won't be easy to put drywall up there.
Is the Roxul supported by any kind of framing? You could probably make an adequate air barrier out of Tyvek or similar, but it would need to be fastened to something and then taped along the seams and edges.
If there is framing you could cut plywood into 16" strips and screw it over the roxul. Then tape the seams. If the longest is only 48" you can fit that through any opening a person can fit through.
It seems to me at this point you have no choice but to extend the parting wall to the roof line and make it your air and thermal barrier by making it air tight and insulating it with the same R value as your roof.
Also, you will need to connect the attic to the rest or your home so the attic is more or less the same temp and humidity as the rest of the house. Consider moving the return vents to the attic and adding transfer grills to connect the attic to the rooms below.
When you bring the attic into the conditioned space it is a good idea and often a code requirement to cover the spray foam to delay it from burning in a fire often with a special paint or drywall.
Walta
Thanks Walta. I'm copying my comment from above, and I'd welcome any suggestions on how to seal off the attic from the neighbors:
I'm not sure what to do about the airflow in the attic. The space is too cramped to do any work up there without opening the ceiling, it's really more like a crawl space above the 2nd floor (it's under a slightly sloped flat roof, not an A-frame roof). At the highest point it's only about 4ft, then down to 1.5 ft at the low end. Do you think it might be adequate to spray foam on top of the roxul more easily? It won't be easy to put drywall up there.
Find skinnier contractors....
As contrarian mentioned, tyvek etc would airseal, but it occurs to me that fixes the HVAC issue, but not things like, umm fire
I think I would like a nice sealed up 5/8 sheetrock wall up there between me and my candle burning chain smoking neighbors.
I think even my slightly paunchy self could work in a 4 foot most of the way down to 1 1/2 foot space. Although a slightly skinnier version of me did almost get stuck when I wiggled into a tight space to stuff in some missing insulation and realized backwards is harder than forwards.......
anyway, precutting a roll of tyvek and armed with a staple gun sealing tape and a razor knife, pretty sure it could be done.
Since there must be a hatch to get there, one could figure out a way to get 4x4 and smaller pieces ofsheetrock up there if one decided it was required
+1 on 5/8. Here it is code required when doing this type of retrofit.
Might feel like a big job and won't be fun in this weather but once you start it doesn't actually take all that long.
For attic conditioning, most code require 50cfm/1000sqf. Only add this in once you seal up the attic though.