Two Vapor Barriers in Walls
I hired and architect who specializes in green building to design my energy efficient house. We moved in about two months ago. And now I’m afraid the design is completely flawed and we are utterly screwed.
We live in Zone 5. Our wall assembly has R-21 batt insulation (inside), a poly vapor barrier, Zip, R-12.5 exterior XPS, corrugated metal siding.
So, there is poly in the middle of the wall. We went over this detail a lot at the time. I know our architect consulted with multiple people about this. She said that it was designed so that the interior wall would dry to the inside and the exterior wall would dry to the outside. She said the exterior insulation was permeable and would breathe. That made sense to me.
Fast forward to now. I was doing a little research on indoor humidity levels in the winter, and I came across an article that mentioned that XPS decreases in permeability the thicker it is, effectively making it a class 1 vapor barrier.
So that doesn’t seem like it will breathe / dry to the outside.
I am sick to my stomach. Can anyone tell me how screwed I am? Is this an inevitable failure? Or just a maybe failure?
I have read about the external rigid foam ratios. I saw that in my climate zone, the rigid should be at least 27% of the overall R-value for condensation moisture control. If I did the math correctly, our rigid foam is 37% of the overall R-value. Is there any way — please god — that means the sheathing will never get cold enough to condensate?
I have read that XPS is very impermeable to rain. Our siding has built in air channels, which I’ve read is a good thing for preventing rain from getting inside the assembly. Is there a chance we are OK on the rain front?
That leaves leaks. I think we are just screwed there. I called the manufacturer and confirmed that the perm rating for our amount of exterior insulation is .65. Is there any chance that a little bit of drying is possible at .65 perm? Or does that mean it will definitely stay damp for long enough to cause a problem?
If we are screwed, what the heck are we supposed to do? I don’t understand how this happened, with all the information available to professionals on this website. So I’m hoping I’m wrong.
Thank you in advance.
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Replies
jeasto,
Here is my take:
You do have three very low-perm materials, but they are all sandwiched together - which is the best situation you could have. The problem with multiple vapour-barriers comes from them being on opposite sides of the wall, which limits the drying potential of the cavity.
Would it be better if there was no poly and some other material had been detailed as the air-barrier? Sure, but what you have is much like a roof with closed cell spray foam below the sheathing, and an impermeable underlayment above. As long as the Zip was dry when it was installed, I don't think there is any appreciable risk.
Hi Malcolm,
Thanks for your reply. What about condensation though? I'm afraid the Zip will get wet at some point due to condensation (if not a leak), and then it will never be able to dry.
I did the dew point calculations from this article (https://www.greenbuildingadvisor.com/article/are-dew-point-calculations-really-necessary).
When I used the mean temperatures for the coldest months (30.2, 24.9, 27.4), we were in the clear. But the thing I don't understand is this: where I live, we always have below zero days in the winter. Doesn't that mean condensation will *inevitably* occur on the Zip and will never be able to dry?
Or am I missing something? For example, does it take time for the dew point to "migrate" into the center of the wall?
jeasto,
Minus the poly, your wall is very similar to the one recommended by Joe L. and others as one of the best performing assemblies you can use. When we worry about moisture, the problem is the "condescending surface of interest", which in most walls is the backside of the sheathing. but with an adequately thick layer exterior foam (which you have), that surface is too warm to have moisture condense on it. The dew point is always going to be somewhere in that foam layer, where it does no harm.
So you are left with the possibility that the wall will take on moisture through leaks, and that the sheathing can't dry to the inside. That's maybe a concern, but much less than with other types of sheathing, as taped Zip is a WRB - and is itself a very low perm materiel, so you wouldn't get appreciable drying to the inside anyway.
I just can't get too worried about your wall assembly.
I truly appreciate your insight, Malcolm. It has eased my mind a little bit.
How the wall currently functions doesn't match the stated intention of my architect, which bothers me. Something went wrong, and that's hard for me to understand / digest as a homeowner, who thought she was paying for one thing and got another. That being said, the professional perspective you and others on this thread have given me is much appreciated.
I wish I knew I could use this website as a resource from the very beginning of my build.
I did go through my photos of the build, and it was very wet outside when the insulation was installed. It was rainy around that time last year. I'm hoping the Zip was dry though, when the insulation was installed.
Hi Malcolm,
I wonder if I could get your thoughts just one more time. Should I be *extra* concerned about humidity levels in my house because of where the poly is?
I have been combing this website for days for some best practices, but most of them are related to summer and/or RH, and I wanted dew point best practices. We are getting colder weather now, and I want to make sure I'm doing my best.
I finally found an ASHRAE document that said risk for condensation/mold growth in buildings is lower if you keep *dew point* at or below 60F (or 55F) during cooling seasons and at or below 45F during warming seasons. So I decided that was a good place to start.
Here's the thing: right now, my area has been in the 40s and 50s during the day and low 40s and 30s at night. We have also not turned on the heat yet, because the house is around 68 to 70 degrees still (though it was 66 when I woke up this morning).
When it first got cold, we had to run a stand alone dehumidifier for a couple days to get the warmer weather moisture out. (Unfortunately, where I live, it can go from 80/75 degrees to 45/30 degrees in a day or two . . . and then it might go back up to the 70s! Then it might snow. That's October.)
Anyway, we were trying to get things down to 35RH @ 70F and thought it would be easy, since the outside air has been so cold (and dew point low).
That has not been the case. And because of everyone's warnings about how inaccurate hygrometers are, it's possible that even when we get the hygrometers down to 35RH @ 70F (41.4 DPT), it could easily actually be 40RH @ 70F (44.6 DTP), and suddenly we are at the high end of the limit. And if it was actually 40RH @ 71F, suddenly we are at 45.5 DPT.
Is the solution to simply bite the bullet and get a (very expensive) professional grade hygrometer?
Is this something I am going to have to constantly worry about? The informational packet from my energy rater said to keep things between 30 and 50% RH @ 70 degrees in winter. 50% RH @ 70F is 50.5 DPT!!!! Is that really OK? This info is printed for anyone with a ZERH home (we exceed ZERH performance), so it's not tailored to my specific home.
I am hoping our struggles here are simply due to construction moisture. We have only been in the house a couple of months, and our rating company said it may take two heating cycles (or 18 months) to level off.
But I really thought the fresh air from our ventilation from our ERV (a Zehnder) would counteract any extra moisture, in the winter at least.
The dew points outside this week have been in the 20s, and when the dehumidifier is off, the humidity does not seem to budge much, even with constant ventilation. I don't understand, because in the hottest, most humid times, it certainly raised the indoor humidity, as expected. (Which was relatively easy to control with AC / dry mode from time to time.)
If it matters: our post-construction blower door test was .44ACH50.
jeasto,
No you are fine - and way overthinking this. The problem with the poly is that it leaves the sheathing no path to dry to the inside. It doesn't affect the risk of condensation in the wall. You have an adequately thick layer of exterior foam which keeps the poly and sheathing above the dew points you are worrying about. The only problem with your wall is it doesn't have much drying if the sheathing gets wet due to leaks. It is not otherwise risky.
What is a good target for winter indoor humidity then? I thought we all had to control it to avoid "accidental dehumidification" inside the house/walls.
One rule of thumb from this website is 30 RH when it's 30F outside, 40RH when it's 40F outside . . . but I'd rather not be as low as 30 RH if I don't have to be (feels too dry to me).
jeasto,
Sorry, I don't know what accidental dehumidfication is. Don't worry about the outside temperature. As long as you keep the inside RH from staying above 50% for prolonged periods you are fine. Find a comfortable RH - say 45%, and try to keep it fairy consistent.
jeasto,
Have you done much reading of Building Science Corp. articles like:
‣ https://www.buildingscience.com/documents/digests/bsd-controlling-cold-weather-condensation-using-insulation
‣ https://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-092-doubling-down%E2%80%94how-come-double-vapor-barriers
‣ https://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-026-they-all-laughed
‣ https://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-100-hybrid-assemblies#_ftn5
Admittedly your mid-wall poly is an unusual situation, but I would take solace in a couple things.
1) your ratio of exterior R-value to total R-value is a decent bit higher than needed for your climate zone (37% vs 27%). These prescriptions don't ask home occupants to keep strict interior RH regimens: the numbers are in the code and thus has some level of 'works for all' fool-proofness to it.
2) the coldest times of year, when the condensation risk is highest, ventilation will have a greater drying effect than it does now during a shoulder season (especially if you still currently have construction moisture being equalized). From the above linked article: "As outdoor conditions become colder, the interior RH drops because interior moisture is diluted by increasingly dry exterior air. This effect provides some protection against condensation, as the coldest week of the year is likely to coincide with some of the lowest interior humidity levels."
3) if a worst case scenario of extreme cold weather and high interior humidity happens and you do get condensation (on the poly), it can still dry to the inside from that face of the poly upon warm-up of the wall. I haven't done the math, but I'm guessing that at 37% exterior to total R-value ratio you are unlikely to ever get condensation at the poly.
If you can't let it sleep, I suppose there is the option to install building sensors, but that is obviously an invasive operation.
In theory the dewpoint calculations are for some sort of 'average' and not 'at all moments.' In other words, the coldest days could cause some condensation potential outside the foam if prolonged enough, but on average there won't be enough wetting to cause issues when using the prescriptive ratios.
There would certainly be some delay between the outside temp dropping and the wall sheathing/poly interface actually getting to the dewpoint (and it depends on your interior RH of course.)
While the poly undoubtably slows drying of the sheathing towards the interior, it would actually further slow the diffusion of vapor from the interior towards the exterior, as it would typically be in winter. And while the XPS does not have a lot of permeable, it has some.
I think it is a sound assessment to say that the wall is really only problematic in the case of leaks. And leaks are likely to do in most any wall if left untreated.
Could you ask the designer why they chose to install the poly? It could be interesting to hear the reasoning.
Yes, will ask for clarification again and post what she says here. I know it was something that was discussed at length, and that the professionals decided it was best to do it.
My husband and I signed off on the approach based on how it was supposed to work. She explained that the interior part of the wall would dry to the inside and the exterior would dry to the outside because all the exterior layers would breathe (and the poly stopped inside water from going out and outside water from coming in). And I know "our climate zone" (5) was cited as a reason for having the walls dry this way. She actually assured us on multiple occasions that we were doing the best possible thing because my number one concern was moisture.
I, a layperson, did not think to confirm what the permeability of different insulations were. It actually didn't occur to me or my husband that insulation *could* be impermeable, let alone that different types had different impermeability.
The concept was certainly not explained as "your exterior walls will not breathe at all." So between that and some other things I've discovered, I'm guessing "mistakes were made" and the real-world performance of this wall is not what was intended.
jeasto,
"She explained that the interior part of the wall would dry to the inside and the exterior would dry to the outside because all the exterior layers would breathe (and the poly stopped inside water from going out and outside water from coming in)."
It's a sound strategy - even if you include poly. Where it went off course a bit was where the poly ended up. Has it been outside the sheathing there would be no concerns - although it all would have been better had some other layer been detailed as the air-barrier, and the poly omitted.
Malcolm is that consultant you wish you had explaining this stuff before you agreed to this set of plan. At least you are getting some amazing advice to set your mind at ease.
I agree with Malcolm when he says : The problem with multiple vapour-barriers comes from them being on opposite sides of the wall, which limits the drying potential of the cavity.
I think that you actually have a pretty good house that will serve you well. No house is perfect and many many times we see things we would do differently when we build the next one. So on the next house just omit the Poly on the inside of Zip sheeting.
Once you get
- your ICF insulation covered (I used corrugated galvanized metal that will eventually rust)
- the insulation in your attic covered up
- a dehumidifier in your attic (though Bill suggested just adding a return air in the attic)
You can have a glass of wine and find some piece of mind.
I admire how you are dealing with all these issues that you are “finding “ after moving in. You describe your issues really well. It is clear that you have been absorbing a ton of info. I am happy that you have a good house and that you are tying up those loose ends the builder left. They are little presents of sorts. :)
@mr_reference_Hugh,
Thank you so much for your encouraging words. I wondered whether someone on here had clocked that I seem to be limping from crisis to crisis. It has been hard to stay positive and not overwhelmed. (In addition to the "presents," my husband and I are finishing our permeable driveway ourselves and taking on the landscaping ourselves, so we have had plenty to do without this other stuff.)
Regarding the wall assembly, I have alerted my architect because I don't think it was her intention that the insulation be impermeable. But I do feel slightly better about it (except that I went back through my photos, and it had rained before the insulation was installed).
In case you are curious, there have been some developments on other fronts. First, we calibrated our hygrometers and confirmed that the attic's temp and humidity are basically the same as the living floor. Sometimes, the attic is a degree or two colder, which effects the RH, but the dew points are nearly identical at all times. So, we are hoping once our construction moisture is gone, there won't actually be elevated humidity. But we'll see.
Right now, we are hovering around 40% RH (and it's been in the 40s and 50s outside), but once it starts getting to be in the 30s consistently outside, we will aim for 30% RH and use portable dehumidifiers if we need to (before making a permanent decision).
We have also (very luckily) found a local foundation company who said they are interested in ICF-protecting project for spring (it is now too cold here to do it). We decided just to parge it with one of the ICF protection products, such as Tuff II.
I am asking the contractor to come back and cover the insulation, but I am not sure he will.
Awesome. Each time something comes up, we gain confidence that it can be addressed. This GBA site is so amazing to get help from people like Malcolm, Michael Maines, Martin Holladay... and the list really quite long. Soon you can start asking questions about house maintenance :)
I should have mentioned. If you are still worried about your ZIP because of potential future leaks that you talked about, I suggest you take a scrap piece of ZIP place it (off the ground) somewhere in your yard exposed to the elements. Just watch how durable that stuff is. It’s not plastic but that stuff is surprisingly tough.
And keep it mind that if you ever had a serious recurring leak in behind your siding for some reason, there are very few wall assemblies that would be able to handle that type of leak over several months/years. Your wall is better than most IMO and will stand up well.
I also remembered this video showing bulk water getting between ployions and ZIP.
This should also put your mind at ease a little more. I think.
https://m.youtube.com/watch?v=1Doeu7rzV7k
Hi Jeasto,
I think mr_reference_Hugh had a very good point about adding a dehumidifier. I would like to elaborate on this for the community. Aprilair makes some great residential sized units that can be ducted in line at the air returns before the inside air handlers. They have an on board humidistat which means it comes on when required. I think dehumidification is one of the missing links to all high performance, super insulated building systems. The moisture has to get out of the air and dehumidifiers are built to do this most efficiently. There is untold billions of btu's that get wasted on dehumidifying air with the air conditioner itself. In the winter, the warm air is carrying more moisture and no dehumidification is occurring. Old building systems overcame excessive air changes per hour with oversized HVAC systems. The inefficiency of the air movement itself prevented moisture, mold and rot problems. Less air changes per hour means that the indoor air quality should be managed with dehumidification, higher quality filtration and constant circulation of air. Without these two things in place we create the conditions for mold, rot, and sickness.
I was introduced to this when I had a build in 2016 of an all metal house designed with spray foam insulation--4" walls and 6" to the roof. No inside vapor barrier and sheetrock directly attached to metal purlins. I took it to the mechanical engineer and he designed the HVAC with an Aprilair unit. The customer reports that the A/C or heat doesn't run for 4 months out of the year and that the fans and dehumidifier are keeping the house comfortable in LA.
We see dehumidification a lot in commercial climate control systems because these get carefully designed and modelled for efficiency. Home building doesn't require this level of analysis. However, for the credibility of the industry and our own good names we should value Engineering analysis, especially where we are pushing the bounds of building performance and not just doing some run of the mill meet the code minimum cracker box.
Thank you for this.
We will install a whole house dehumidifier (we actually are looking into Aprilair!) if we have to but what I don't understand is this: Does it actually matter what my indoor humidity is if I keep the RH between 30 and 50%?
Many of the articles on this site led me to believe "yes." My takeaway (which could be wrong) was that ideal indoor RH depended on the weather outside. And that I could get condensation on my windows or worse if I don't fall within ranges, which differed in the summer or winter. Mr. Reference said he might recommend minimum RH in the attic at all times (though, that was when I thought we had elevated humidity. I have since found out that we don't really, after calibrating our sensors and looking at dew point instead of RH. I'm not sure if that changes his recommendation).
So far, we've certainly been able to keep things between 40 and 45% RH. If that turns out to be the case long-term, do I (or anyone on this site) need a whole house dehumidifier?
*Separately* I am thinking that we're going to find that a whole house dehumidifier might be sensible in our climate simply because the house seems like it's going to be resilient in the summer (e.g., not really need AC on most days) . . . in other words, for the reasons you describe above. :)
The portable models we have throw off enough heat that it starts to accumulate. So, in the hot months, we may only need dehumidification, but turning on the dehumidifier added too much heat. If a whole house models don't do that (I need to look into this), it likely would be worth it to us. We were going to wait the 18 months recommended to see how things changed (or not).
jesto,
"My takeaway (which could be wrong) was that ideal indoor RH depended on the weather outside."
I think this is worth unpacking because it appears to be at the root of your concerns. The idea that there is an ideal ratio of indoor and outdoor humidity and temperature to control dew point doesn't translate into advice that it should be maintained by constantly adjusting the indoor RH, and It isn't desirable to do that depending on what the outside conditions are for a variety of reasons.
- First because it simply isn't feasible to do, as over even brief periods outside temperatures and RH can vary so widely.
- It also isn't good for the interior of house, especially it's wood and wood based elements, for RH to fluctuate. The ideal would be to maintain a constant RH and indoor temperature all year round.
- Perhaps most importantly, we condition houses primarily for the occupants, setting conditions for health and comfort. A house which runs into trouble when it experiences a constant reasonable indoor RH and temperature has a fundamental flaw in the way it was designed or built. I don't think you will find any advice on GBA suggesting otherwise.
Hi SamNC123,
I am returning to this thread because we are finally trying to get an AprilAire whole house dehum installed. There seems to be some question that these units cannot be installed with a ducted minisplit system, which we have (a Fujitsu).
Have you heard of this?
I am a bit surprised because I have seen advice to add a dehum in multiple places and no one has mentioned that ducted minisplit systems don't work with it (and these systems seem common in energy efficient homes). My HVAC contractor is trying to contact Fujitsu to understand if there is an issue.
I tried to contact AprilAire (because they website says the product works with almost any system that uses ducts), and the chat person said the units usually don't work with "minisplits" because they don't have the right ducts and would not confirm they understood I have a *ducted* system.
Hi Malcolm,
"- Perhaps most importantly, we condition houses primarily for the occupants, setting conditions for health and comfort. A house which runs into trouble when it experiences a constant reasonable indoor RH and temperature has a fundamental flaw in the way it was designed or built. I don't think you will find any advice on GBA suggesting otherwise."
I hear you. I have maybe read too many articles about cathedral roofs that fail. I have to admit that they were often built decades ago. Of course there are lots of articles about SIPs roofs also. I do think that jeasto has a well built house and there was considerable thought that appears to have been put into it. The issues she is managing now are maybe not uncommon with taking over a new house after construction.
I myself built in two spots for in wall dehumidifiers in our place and I plan to add them later (cost constraints for now). Part of it is that we have humid hot summers but cold dry winters. In the shoulder season, it can get quite humid and colder so we can't run the A/C to dehumidify. I like the idea of being able to control the RH levels in the house during the humid seasons without relying on an A/C.
Part of it also is that I don't really trust the installers of the spay foam insulation to be able (as regular human beings) to be able to see all the small holes or gaps that the spray foam might contain. No matter how qualified they are, no matter how much they care about doing a good job, my own understanding is that there is a documented history of spray foam not self-sealing the way the industry claims it does. My suggestion to anyone with a conditioned attic with a spray foam under the roof deck would be to control the humidity - in the conditioned attic. It just seems like a small cost to manage the potential risks when we build an air tight well insulated home. BUT, it is obvious that I could be wrong. I am just one person.
And as mentioned before, I read your replies and learn a lot from you. I admire your level of knowledge and dedication to helping people on GBA Q&A.
Hugh,
No you are quite right. I'm certainly not suggesting that maintaining a reasonable (50% or less) interior RH isn't important. Excess indoor humidity can have a big effect on how building assemblies work. I'm more getting at jeasto's contention that interior RH should be set or varied based on outside conditions. I don't think that makes much sense.
Needing a dehumifier in the humid summer months or shoulder seasons and being concerned about condensation during the coldest times of the year (when outdoor air is also very dry) are two different things. To address a concern, you need to be clear about what the concern is. At this point, it's not clear to me the actual and precise concern.
You won't find a magic RH number to live by, as Malcolm says. I suggest reading the Building Science Corp. articles inlinked for more in depth discussion on how the exterior foam ratio numbers were determined and the effects of interior RH. 35% RH gets tossed around in one of those articles, but you should note they do NOT imply people with these assemblies need to go around dialing their houses to precisely 35RH.
Well my own concerns with spray foam assembly and the document issue of the product not air-sealing (or vapour sealing by consequence) as the industry advertises. There are many articles about this. Here is FHB which I believe is associated with GBA. I believe that we need to consider these well known problems when dealing with spray foam or just avoid spray foam to the extent possible.
https://www.finehomebuilding.com/membership/pdf/170903/021262046-SprayfoamProblems.pdf
There is no mention of keeping RH at 35 but I don’t see many solutions being proposed to mitigate against the proven risks of poor spray foam insulation.
As mentioned, I believe that this house was well designed by thoughtful people. The owners is diligently asking questions to make sure she has all the right information to benefit from and to protect her asset. She is very thoughtful in her assessment. I am just sharing my point of view. I already endorsed Malcolm’s response many time. Not sure that I have done anything wrong here.
mr_reference_Hugh,
My response was to the OP and had nothing to do with spray foam. I felt that a degree of specificity was needed in terms of what the actual concern was.
I just wanted to make sure I have a "safe" level of indoor humidity at all times to preserve the structure of the building. I had two apparently wrong thoughts: I thought indoor humidity suggestions were different based on the season, and I thought my wall/roof assembly put us at increased risk for damage (and that a lower indoor RH would reduce risk).
It seems like my takeaways should be:
(1) Consistency pf temperature and RH is a worthy goal
(2) As long as I stay under 50% RH, the structure should be fine
(3) I should make sure my attic is also maintaining 50% RH or below
(4) If the attic isn't, or if I find maintaining indoor humidity is a bother in summer and shoulder seasons, I should look into a whole-house dehumidifier to take control of it.
This is worth a read if you haven't already:
https://www.energyvanguard.com/blog/what-is-the-best-indoor-relative-humidity-in-winter/
I think your takeaways are fair, though in my opinion the point about interior RH is not to keep it constant per se but rather that your assembly is robust and you don't need to stress about hitting precise benchmarks (like keeping it at 35% 24/7 througout the winter. I think allowing RH to shift seasonally to some degree is expected, and is fine or even good as winter ventilation will naturally lower RH. I honestly don't know, but to me it sounds like a constant 50% rh in winter would be a TAD high (but maybe not in your CZ). If you really want to dial this all in it is possible to do dewpoint calculations for your specific assembly and climate. Prescriptive code in theory makes it so you don't have to, but you can.
maine_tyler,
50% RH is an old, leaky, cold climate house might well cause problems - or in a new one which had incorporated risky building assemblies. I don't think that's the case with jeasto's, and I don't think we should use assemblies that can't take that level of RH.
That we can or should rely on occupants to consistently keep very low levels of RH seems a bit problematic. Rather we should design buildings robust enough that they don't see failures at RH levels commonly experienced in houses.
Where does 50% come from? (Meaning how did you choose that number in this case)
There may be a slight difference in our perspectives based on our respective climates. If I understand, temperatures are pretty mild and much more consistent where you live. Here, regardless of whether a building can handle it or not, 50%RH would not be considered 'very low' for winter. Part of that is indeed the leaky houses, but also just by virtue of having really dry exterior air, so even controlled ventilation will dry houses down.
From everything I can remember reading, 50%RH is on the upper end for cold climate recommendations (upper end not meaning there WILL be problems, only that it's going to be getting flirtatious in some cases-- and its undoubtedly conservative when talking well-built assemblies). CZ 5 is maybe not all that cold anyways, and this assembly is robust... but still, what is the recommendation for keeping steady 50%RH based on?
maine_tyler,
50%RH is often cited as the upper end of the acceptable range. So for instance jeasto's energy rater told her to keep it between 30% and 50%. That's the same top end of the range which optimizes occupant health Allison Bailes quoted in his blog on the subject: https://www.greenbuildingadvisor.com/article/humidity-health-and-cold-climates.
I'm not suggesting aiming for that, but rather that a well built house, like jeasto's, should be able to maintain that level of RH without experiencing problems.
The back and forth between you and Malcom has been helpful here.
For what it's worth, I'm not aiming for 50% RH in the winter. I'm hoping to be able to keep things between 35 and 40%RH. But it is comforting to know that if it went up to 50% RH temporarily, it's not the end of the world.
Since this is our first cold season in the house, we are trying to see what our home tends to balance out at "naturally" (knowing that we still might have construction moisture drying out).
We have had to run our stand-alone dehumidifier in this current shoulder season (which has had wildly fluctuating temperatures) to keep things at 40% RH . Ideally, I want to see if we can mostly do away with it. But I was uncomfortable doing that until I understood "ideal" winter RH.
We have not yet had super sustained cold temps -- and we have not had to turn our system to heating yet -- but I have so far been surprised that, for example, a week of low dew points does not seem to bring the indoor humidity down, with the ERV going constantly. I know there is some heat and humidity exchange there . . . but for now, at least, it doesn't quite make sense to me.
Jeasto,
The outdoor air dewpoints may be lower than summer, but if you are still not heating the RH is probably somewhat more relevant. In other words, if outdoor RH is medium/high and you bring that air inside AND don't heat it, you won't drop your interior RH.
The other thing is that building materials will have some lag in that they will continue to give off moisture even if the air has begun to dry. It's still pretty early in the season. I live in a moderately old, moderately leaky house in maine and even with the heat coming on I'm not yet below 50% rh for the season.
And of course cooking, showering, breathing, etc. all add moisture load.
Thanks for this insight! I was wondering if it was partly to do with us not heating the air yet.