Humidity in tight buildings – effects of windows?
We recently finished building a LEED Gold house in Southern California (very close to the beach) that has radiant heating (plumbed for future addition of radiant cooling), 4 HRVs, is very air tight (0.7 ACH50), has R-21 closed cell spray foam in the light gauge steel 2×6 walls with an additional R6 of rigid foam on the outside of the sheathing, and R-40 closed cell on the underside of the roof sheathing with an additional R10 of rigid foam above the roof sheathing (below the clay tiles). All seams were sealed with tape or liquid applied sealant, the whole house was wrapped with Tyvek, and the stucco guys also wrapped the house with 2 layers of grade D building paper.
Even with all of that, this summer’s heat and humidity has managed to find its way in to the house. Today averaged 81F with 63% RH outside (dew point of 67.2F), the inside of the house averaged 80.5F with 62.3% RH (dew point of 66.4F). Based on my research over the last two weeks, we’re seeing most of the temperature rise coming from glazing (despite the LoW-E coatings) and most of the RH coming from the HRVs passing outside air in to the house without dehumidification. There is also some latent load introduced from showers, cooking, breathing, and whatnot – but it is negligible compared to what is coming out of the HRV vents, as the indoor RH almost perfectly tracks the outdoor RH.
To deal with the RH, we are first going to swap the HRV cores for ERV cores. Our manufacturer says that the ERV cores will return 20% of the outside RH back to the outside, however they will also return 20% of the exhaust RH back to the inside – so we will likely need a one-time dehumidification of the house to set a lower base. If that doesn’t work, we will add in-line dehumidifiers to the ERV source air.
Once the RH is under control (40-50%), we will address the sensible load by installing a chiller and cooling the high-mass radiant floors to 2F above the dew point.
Now to the question:
With such an air tight house that is mechanically ventilated, where we’ve put so much effort in to sealing doors, windows, pipes, etc and controlling the humidity of exchanged air, what is the effect of opening a bedroom window or patio door for a full day? Does the humidity of the house shoot up to the outdoor humidity and then take some amount of time to go back down once the door or window is closed? Should we tie in door and window sensors to simply shut off the ERV in that zone while a door or window is open?
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Radiant heating and cooling do not help you with humidity control. In "typical houses" well designed forced-air HVAC system will do most of the moisture control. In your case, a dehumidifier should have been included in the design of the house. An ERV is NOT a good dehumidification strategy.
Here is a good read: http://www.healthyheating.com/Page%2055/Page_55_i_cooling_eq.htm#.WYcK9ITythE
There was no statement that we intend the ERV to be our dehumidifier... so, let me try restating what was in my post:
1. We messed up.
2. We are fixing the mess up by switching to ERVs to reduce the amount of outdoor humidity that is pumped in to the house and by adding dehumidifiers to deal with humidity in the house (the latent load).
3. If dealing with the latent load (via the dehumidifiers) doesn't make the indoor environment more comfortable, we will add a chiller to our radiant floor system to address the sensible load.
Back to my question:
Is all of this useless any time a window or door is open?
How large is the house, and how much ventilation are you creating with your four HRVs (soon to be ERVs)? It is possible you are over ventilating the structure.
Are your units balanced? Are you planning to install a whole house dehumidifier?
Who is advising you on your heating, cooling, and ventilation systems?
> a one-time dehumidification
Expect "ongoing, but minimal".
> high-mass radiant floors to 2F above the dew point.
Which, at the floor, will be a potentially mold inducing %RH. With a low mass radiator, a daily drying would prevent mold growth.
> what is the effect of opening a bedroom window
Speed depends on the CFM, but yes, it will rapidly cause the indoor dew point to approach the outdoor dew point. Don't distribute this air to conditioned areas of the house.
Also note that building materials store moisture - so high %RH in a room for a day and then closing the window will need even more dehumidification than you would expect from the air volume alone.
You question the value of all your HVAC and air sealing when windows are left open. Of course, at those times--especially if it drags on to hours, or days--all of that infrastructure is "useless." I built a house that's tight to 0.5 ACH50, but especially in this relatively cool Ohio summer have left my windows open for weeks at a time. One of my biggest concerns about "green building" is its fixation on isolating us from the outside. But for comfort, I'll slam my windows shut and turn up my Fujitsu when the dew point hits 70F, or the temps drop below 40, and then my tight house is a delight.
In your case, if you don't like the indoor humidity, I'd re-jig your HVAC to knock that down. Unless it's more comfortable to leave windows open. They're cheaper to operate!
Closed cell foam thermally bridged by steel studs is an even worse waste of expensive foam than in wood framing. That R21 wall would be performing at less than R10 without the continuous R6 sheathing (which nearly doubles the wall's performance.) Leaving the studs empty and installing 2.5-3" of exterior polyiso would have been higher performance at a lower cost (but the stucco guys hate thick foam.) Closed cell spray foam is one of the least green insulation options out there, due to both a high polymer per R, and the HFC blowing agents used. (The HFC blowing agents should be going away soon, and there is some low impact HFO blown foam options, but it's still more than 2x the polymer per R of open cell foam, which is blown with low-impact H2O.)
With steel framing and foam insulation the moisture buffering capacity of the structure is quite low. Opening the windows will only purge moisture when the outdoor air has a much lower dew point than the interior air, and will raise the humidity when the outdoor dew point is higher than indoors. Most tight houses need at least some mechanical dehumidification at least some days, but when outdoor dew points are high it's better to just minimize the ventilation rates.
Thanks for all the feedback, here are answers/responses to all the questions/comments I got...
1. How large is the house:
6,700sqft on the 1st/2nd floor, 2,400sqft (finsihed) in the basement
2. How much ventilation are you creating with your four HRVs?
4 Zehnder ComfoAir 200 units - 125cfm each. Air is pulled from laundry, bathrooms, and kitchen and fresh air is delivered to bedrooms, living rooms, etc. The unit was designed and commissioned by Zehnder to meet ASHRAE 62.2 standards (w/ continuous bathroom ventilation)
3. Are your units balanced?
Yes, the Zehnder rep adjusted all exhaust and supply registers during commissioning to ensure the units are balanced
4. Are you planning to install a whole house dehumidifier?
I'm currently looking at adding 4 - one Ultra-Aire XT105H per Zehnder unit.
5. Who is advising you on your heating, cooling, and ventilation systems?
The original designer of the HVAC system passed away during our 4 year construction. We built to his plans, but a lot of these systems were new to my HVAC installer. I am doing my homework before I hire a new HVAC installer or consultant.
6. "Which, at the floor, will be a potentially mold inducing %RH. With a low mass radiator, a daily drying would prevent mold growth."
Every radiant cooling article/document/etc that I read suggests 2 degrees above dew point for chilled floors, for example, see slides 15 and 24 of:
http://www.uponor-usa.com/~/media/Files/Technical%20Documents/CPS%20Trai...
Steel floor decking (Verco Deck) with 2" of gypcrete (tubes encased) with a mix of porcelain and natural stone tile on top... Even if we have a condensation problem, mold growth is unlikely. All radiant distribution lines are wrapped with insulation and a vapor barier to prevent condensation on the pex tubes while the cold water is in transit to the floor. If 2 degrees is not enough to prevent condensation, we can always change that delta. Where is the concern for mold?
7. "Speed depends on the CFM, but yes, it will rapidly cause the indoor dew point to approach the outdoor dew point. Don't distribute this air to conditioned areas of the house."
Thanks. We'll use the window/door sensors to shut down (or greatly reduce the cfm) of the HRV/ERV that serves the area with open doors.
8. "Also note that building materials store moisture - so high %RH in a room for a day and then closing the window will need even more dehumidification than you would expect from the air volume alone."
Thanks! My biggest concern is around parties where the crown is using the inside and the back yard - the patio doors would be open for 4-6 hours to allow people to flow. When the doors are closed and the hrv/dehumidifier comes back to life, I expect its humidistat to do its job and keep the system going until we get back to 40-50% RH.
9. "In your case, if you don't like the indoor humidity, I'd re-jig your HVAC to knock that down. Unless it's more comfortable to leave windows open. They're cheaper to operate!"
Thanks! Windows are our current mode of operation. It is ok, but not great - still a bit "sticky" feeling. I'm thinking we'll add the dehumidifiers and keep the windows closed most times.
10. "Closed cell foam thermally bridged by steel studs is an even worse waste of expensive foam than in wood framing"
There is no thermal bridging. The wall is 6" thick with staggered 2x4 steel studs (each one alternates touching the inside wall or outside wall).
11. "stucco guys hate thick foam
They complained endlessly about it.
12. "Closed cell spray foam is one of the least green insulation options out there, due to both a high polymer per R, and the HFC blowing agents used."
We used Demilec Closed Cell Heatlok Soy 200 Plus at R-7.4 per inch. Demilec uses Honeywell Solstice (GWP of 1) instead of HFC 245fa (GWP of 950-1020).
13. "With steel framing and foam insulation the moisture buffering capacity of the structure is quite low."
Interesting, I hadn't considered that there are very few materials in the house that will actually absorb and hold moisture.
14. "Most tight houses need at least some mechanical dehumidification"
Thanks. After all the feedback I'm thinking of adding the dehumidifiers before swapping the HRV cores for ERV cores.
Additional question: If I add the dehumidifiers to each HRV, is there any point in swapping the HRV cores for ERV cores? The dehumidifiers cost about the same as the new ERV cores.
> Where is the concern for mold?
Mold doesn't need condensation - long periods of high %RH is enough. 2F above the dew point on a high mass radiator will provide that. But radiant floors are efficient and you should be able to use much higher temperatures than that. Combine this with lower than normally necessary %RH in the room and you should be OK.
Parties can massively increase latent, sensible and ventilation needs.
Be careful about balance problems when adding a whole house dehumidifiers to a HRV/ERV.
"There is no thermal bridging"
The concept of "no thermal bridging" is sheer BS, even with staggered studs, but it does make the modeling a bit more complicated. Still spanning through an insulation layer is still a thermal bridge, and is still undercuts the performance by quite a bit.
The description in the original post changed from "... light gauge steel 2x6 walls ..." to become in response #7 "The wall is 6" thick with staggered 2x4 steel studs..." , which is confusing, to say the least. Is it actually 5.5" (a nominal 2x6 depth), or is it 6".
Since either 5.5" or 6" of closed cell foam would be well north of R30, are we to assume that the original R21 descriptor was a calculated "whole-wall R" that accounted for the thermal bridging? 5.5" of Demilec's HFO blown 2lb foam would be R40, not R21. So effectively the thermal bridging of the steel cuts the performance in half, assuming that was a whole-wall R calculation.
Mind you, half-pound open cell foam, which runs about R21 at 5.5".
Steel studs above 2x4 are true dimensions, not nominal. 2x4 are available in both true and nominal. The wall was sized for 2x6 (true) but was installed as staggered 2x4 to avoid (or minimize) thermal bridging. I can say that when I use my FLIR camera on a west facing wall in the afternoon (direct solar load), I see no visible thermal bridging - it is one solid color. On my old house (less than a block away) that was built with traditional 2x4 wood studs, the thermal bridging is obvious in the FLIR camera.
The closed cell foam we use is R-7.4 - we sprayed 3", so it is roughly R-21 of spray foam PLUS the rigid. It would be absurd to fill the entire wall cavity with closed cell foam in SoCal.
So, you have a 3" path through the steel studs into an empty ~3" deep wall cavity?
If the steel studs are 16" o.c. that R21 portion of wall is operating at about R7-R8 after thermal bridging, maybe even less due to the heat sink effect of the additional steel that extends beyond the foam into the cavity air. With more infomation it could be better modeled with THERM, but the laws of physics are self-enforcing. The thermal conductivity of steel is orders of magnitude higher than closed cell foam. e fact that the steel protrudes 1" into the air space and has a 90 degree bend for another 1.5" of steel means it's providing a lot more highly conductive surface to the cavity air than if the studs had been completely filled. Fully encapsulating the stud & stud edge with an inch of foam would have been a cheap performance enhancement.
The continuous exterior R6 + stucco cladding is enough to mask the shadowing of the studs on an IR image unless the interior to exterior temperature difference is quite large. The effects of direct solar heating of the cladding only lowers the contrast that would be seen.. It will be more visible in an IR image during a cold winter night for higher delta-T without the interference of the sun than at the modest summertime delta-Ts with solar heating of the surface in progress.
With a staggered stud configuration on full 6" plates with full 4" steel 2x4s the performance of the wall would roughly double if instead of the 3" of closed cell foam a full fill of half-pound open cell foam were installed. The center cavity R would be the same R21-R22, but there would be R7+ of additional thermal break at the stud edges. The total amount of polymer used would be half that of 3" of 2lb closed cell foam, and the cost of the foam would be about 30-40% less.
There's simply no point to putting 3" of closed cell foam between the outer course of steel studs.
As a retrofit, installing 3" of cellulose in the empty wall cavities would provide some hygric buffering and a modest amount of thermal mass, while delivering that R7+ thermal break on the studs that's currently missing.
Thinking about whether to spend money on ERV vs. on dehumidification doesn't make much sense. ERV only matters (vs. HRV) if you already have the inside dehumidified--it helps keep the dryness inside once you have achieved it, with A/C or dehumidifers. If you have a climate where the summer humidity varies frequently, and you manage things actively, looking at the outdoor dew point forecast and adjusting ventilation rates and opening and closing windows accordingly, you might be able to get the indoor humidity sufficiently low compared to outdoor that you'd get some benefit from ERV vs. HRV, but that's not very common.
What is your planned configuration of the dehumidifiers with the HRV? The air flow specs on the dehumidifiers are more than the HRVs, so I assume you'll be recirculating air in addition to dehumidifying incoming air. I suspect you are specifying a lot more dehumidification than you need.
If you are interested in dehumidification more than cooling, radiant cooling is a strange choice. I'm not sure what kind of chiller your planning to use and whether it's installed, but you might consider low-profile quiet fan coil units such as from Chiltrix and HTP as a way to use chilled water to get both cooling and dehumidifiication.
http://www.chiltrix.com/chiller-fan-coil.html
http://www.htproducts.com/fan-coil.html
Edit: You could also use the chilled water to chill and dehumidify incoming air from the HRVs.
Charlie -
It isn't so much ERV vs dehumidification, this group convinced me I should have had dehumidification from the start and I'm aware that the ERV is not a substitute for dehumidification, so it is HRV + dehumidification vs ERV + dehumidification. It sounds like the addition of a dehumidifier makes the HRV/ERV question moot.
I submitted a request to Ultra-Aire over the weekend to see if they have a suggested configuration, as they only show stand alone and tied to central A/C. I haven't heard back yet (we're barely back in to the work week), but I'll let you know if/when they respond. For my current thinking, I'll use a few terms from Zehnder's manual (see page 5 of http://zehnderamerica.com/wp-content/uploads/2014/11/TS106_CA200_8Seiter_V11-2014_EN_US_screen-Edited-NWE-09-12-2014.pdf ), hopefully they are industry standard terms, but just in case:
- Extract air: The air sucked from the house, before it transits the HRV/ERV
- Exhaust air: The extract air that is blown outside, after it transits the HRV/ERV
- Outside air: The fresh air, sucked from outside, before it transits the HRV/ERV
- Supply air: The fresh air, blown inside, after it transits the HRV/ERV
Based on their other drawings, I'm guessing the configuration involves pulling air both from the extract air and the supply air, dehumidifying it, and returning it to the supply air. This dehumidifies both incoming fresh air and increases the cfm of the extract registers to dehumidify and recirculate some indoor air. The Ultra-Aire has an intake for recirc and for fresh, and one output for dehumidified - so, this lines up nicely. The catch to this is that it seems impossible to keep the HRV/ERV balanced properly with the dehumidifier adding roughly 150-200CFM of flow to the existing registers (since Zehnder uses flow restricters at each register to balance the system). I suppose we could put the balometer over the exterior intake/exhaust for the hrv/erv and adjust internal registers restricters until the exterior CFMs match, but it is messy.
Dehumidification is the primary goal. I believe that indoor comfort will be achieved, even on 85F outdoor days, once the indoor humidity is lower. If humidity is under control, and we decide we want it cooler, that is where we will start looking at adding a split chiller to our existing radiant flooring system.
Thanks for the pointer to the Chiltrix and HTP systems. I'll dig in to it, but I'm guessing it would be a pretty large undertaking (removing lots of drywall and re-routing radiant trunk lines) as the trunk lines are fairly centralized right now. Also, getting them from the floor to the ceiling would require modifications to the manifolds. Not impossible, but likely to end up with my wife trying to suffocate me in my sleep.
Sounds like you want a DOAS system, so reading this might be helpful:
wikipedia: Dedicated_outdoor_air_system
Consider using portable dehumidifiers instead of whole house models. Lower purchase cost and no pressure balance problems. Distribution may or may not be adequate.
If you need cooling, take a good look at mini-splits instead of radiant.
Seasonal average outdoor dew points are low enough in SoCal that indoor humidity can be controlled MOST of the time by managing the ventilation rates, even in summer. The 67.2F outdoor dew point mentioned in the original post is above the mid-summer average for say Los Angeles, where the mid-summer dew points are below 65% more than 45% of the time, even during the peak humidity weeks of half-past July to half-past August, and only above 65% about 12 hours during the most humid week of the year:
https://weatherspark.com/m/1705/7/Average-Weather-in-July-in-Los-Angeles-California-United-States#Sections-Humidity
Even in more-humid San Diego the outdoor dew points are above 65% fewer than 40 hours on the most humid week of the year.
https://weatherspark.com/m/1816/8/Average-Weather-in-August-in-San-Diego-California-United-States#Sections-Humidity
Spending money on whole house dehumidification or swapping out an HRV core for an ERV core in a climate that dry isn't necessarily a good investment unless very high ventilation rates are mandatory. Most weeks of the year (including the summer) dehumidification of the conditioned space could be achieved by putting the ventilation under dehumidistat control.
Ertwer,
I'm late to this party, but here's my 2 cents:
1. My first reaction was, "Four HRVs? Sounds like an overventilation problem, along with failure to address construction moisture (usually done with portable dehumidifiers)."
2. I agree with Charlie Sullivan -- the solution has nothing to do with swapping the HRV cores for ERV cores. Start with the basics:
(a) If you want lower indoor humidity, keep the windows and door closed unless the outdoor RH is quite low.
(b) Install and operate a few portable dehumidifiers (or make sure that your air conditioner is operating) to address construction moisture. The need for a dehumidifier should be temporary.
(c) Limit operation of your HRVs until the situation is under control.