We are building a church in upstate New York. The building is a ‘state of the art’ structure made of Arxx blocks with a radiant heated floor. The attic insulation is approx. 10″ of open cell spray foam sprayed directly to the underside of the roof sheathing. We have a very tightly sealed building. When opening up the roof sheathing to install a vent we discovered a lot of moisture underneath the sheathing but above the insulation. The insulating company inquired as to if we had any ventilation running (which we have not) since the installation of the foam ( which has a sealer coat sprayed on below the actual insulation). The relative humidity level is at about 52%. We have purchased and are running two dehumidifiers which had initial success but seem to have evened out. Our HRV system is now functioning but the contractor is hesitant to turn it on as he thinks it will bring in more moisture. I believe just moving the air will start removing more moisture than it will bring in. Please any advice will be beneficial as I really have no expertise in this area.
Thanks
Jay
Replies
During winter, the actual moisture content of the outside air will be quite low, even though the RH might be high. When that outside air gets heated to the indoor temperature, the RH will low. For example, when the outdoor air at a temperature of 30 degrees has RH of 70%, that same air heated to 70 degrees will have RH of less than 20%.
This means that running the HRV will help dry the building and freshen the air for the occupants. (It might even help more of them stay awake.)
Thanks for your posts! When I received your post this morning I copied it and sent it off to my contractor. After reading it he called me within 15 minutes and through some discussion he agreed we should turn the HRV on. This is after I had suggested it at least one week ago. Others also brought it up and he wouldn't budge. I had told him the warm air holds more moisture than the cold outside air but he never put the argument together. It was your succinct reply that made him stop and think about it. So thanks for your message.
I'll let my wife (alias 'the reverend') know that it'll also help her to command more attention during her sermons!
Thanks for everyone's replies.
Jay
Thanks for your reply. After receiving your post this morning I copied and sent it over to my contractor. The end result is we now have the HRV up and running and hopefully we'll see some results bythe time our new pews arrive on Monday.
At least it should improve by the time of our first service Easter Sunday.
Thanks to everyone for your replies. By the way my wife (alias 'the reverend') is excited by the fact that maybe she won't be losing half of the congregation during her sermons!
Jay
Concrete releases moisture for quite a while as it cures. It might just be that.
Yep, definitely run the HRV, unless the temp outside is above 40 or so and it's actually raining outside.
(Hint: From the indoor temp and humidity, figure out the dewpoint. Any time the outdoor dewpoint (available from the weather service, et al) is below the indoor dewpoint you come out ahead with the HRV.)
Understanding water vapor/condensation/relative humidity
When you write. "The relative humidity level is about 52%."
This level is unusually low and would not be expected to create
condensation.
But,' relative humidity' doesn't actually mean anything unless it
has a temperature attached to it.
A relative humidity of 50% merely means that the air is half full
of water vapor or half dry, if you prefer.
If you have air that is at 30C and it has 100% relative humidity
it holds 2 and a half teaspoons of water per 35 cubic feet. Not much
you may think, but it is hot and uncomfortable.
At 10C and 100% relative humidity the air is normal and it holds
one teaspoonful, and it feel cold and normal.
At zero centigrade and 100% relative humidity the air is dry, you
will have a problem finding any water vapor and it feels cold and
dry. The water vapor has dropped out as frost.
The relative humidity is changing all the time as the temperature
rises and falls - the amount of water vapor changes in a room
depending on the number of people who are there, even when the
temperature holds steady.
When you turn the heating down or off, when the people leave
the temperature drops, the relative humidity rises and if a
surface is below dew point - you may get condensation. Unless the
wood is untreated when the water vapor will be absorbed into
the wood without trace.
There are many absorbent surfaces where water vapor merely
soaks in - you only see condensation on cold water vapor proof
surfaces, like windows, mirrors, metal, plastic.
Thanksn for the info - Thr RH numbers at this time are the only quantifiable readings we have to go by. Since the building is so tight and temperature remains constant we assume other relative conditions stay the same. The foam patches have been resealed so knowing the RH in the building at 70 degrees was 62% and now after running the HRV is at 47% we assume it's helping.
Actually, 52% humidity is pretty high -- too high for these parts during the winter. We generally try to hold the RH below 40 during the winter, and knock it down to 20 when the temps drop below zero.
That's why we are trying to get it down by running dehumidifiers 24/7 and now the HRV. There is so much moisture in a tightly closed building of 7,500 sf that it is going to take some time I am afraid.
Humidity
What do you acheive in reducing relative humidity to 20%?
At what cost?
What is the costed benefit?
He never said he was reducing the humidity to 20%.
I said we let the humidity drop to 20% or so (by not running the humidifier so much) when the outdoor temp dips below zero. This is to minimize condensation. There is no "cost", actually a miniscule savings in water and electricity.
The original post explains the problem we are dealing with and what we are trying to achieve.
Condensation on sheathing.
Where you have a water vapor proof surface, like glass, metal, plastic you can see condensation when the surface temperature is below dew point. To ascertain dew point, you need to read the surface temperature and measure the relative humidity at that point. Then compare with the general room temperature and humidity.
Soft furnishings, clothes untreated wood, leather all absorbe water vapor, there is nothing to see as the molecules of water vapor move inside the object, except that as condensation continues the items feel damp.
In most instances the windows are the coldest point in a room and condensation takes place there.
You have a situation where condensation is forming on what would appear to be a water vapor proof surface, that is cold and below dew point. As with mirrors and windows, the condensation may be irritating but it does no harm and does not need remediation.
Isn't the sealer coat on the underside and the insulation designed to stop the transmission of the moisture laden warm air?
That depends
What IS the intended purpose of the "sealer" coat? Is it a vapor barrier? If so, what is the perm rating, and the perm rating of the entire foam+sealer assembly? Is it an ignition barrier?
I agree with others here that you should continue dehumidifying and ventilating. What model dehu's are you running? I typically use Drizair 110 and they remove a lot of moisture at first, then slow down. There is often a lot of moisture in new construction, and you need to remove it.
Open cell spray foam is vapor permeable and depending on your location and heating degree days a vapor retarder may need to be installed.