Q&A / 

Spray Foam Insulation Condensation

Basil Pallone lives just south of me in Sudbury, MA. But he's got a problem with his older home he's trying to improve. Here's what he wrote to me:

"I recently installed spray foam insulation in the attic of my two story colonial home, built in 1968.   Since I installed the spray foam insulation, the windows on the second floor are experiencing excessive window condensation.   I checked the humidity levels throughout the house, and the humidity levels are fine.   I am at loss to explain the cause of the situation and have no idea for a solution.

My HVAC contractor suggests replacing the bathroom fans.  I was thinking that I might need some sort of air exchange unit, but a consultant that I hired thought that might make the problem worse.   The insulation contractor has no clue.

Any suggestions on the source of the problem or remedy?

Thank you in advance."

Well, Basil, I'm willing to bet dollars to doughnuts I know the problem. I've got an entire category here at my AsktheBuilder.com website about Condensation. CLICK HERE to see all my condensation columns.

Water vapor is in every house, unless you live in the Atacama Desert.

In the winter, this water vapor usually travels right through regular insulation, plaster, wood, etc. and makes its way into your attic. If you have great ventilation, then in gets outside into the air where it belongs.

Water vapor can and does turn into liquid water when it contacts cool or cold surfaces that are at or below the water vapor's (air in your house) dew point. You see that happening on your windows.

Older homes have a tendency to be more drafty and this is not a bad thing when it comes to water vapor. The incoming cold air has much less water vapor in it and it helps to absorb some of your interior water vapor as this incoming air is heated.

In addition, leaky houses have plenty of places for the humid air to escape to the outside air.

If your foam insulation is closed cell, it's a very effective vapor BARRIER. It's blocking the escape of the water vapor. This allows the relative humidity to climb inside the house and with that up goes the dew point.

As the dew point goes UP, condensation gets worse because the vapor turns to liquid faster on the cold window surfaces.

You need to LOWER your relative humidity in the house by finding out all of the things that are contributing to the addition of water into the air in your house.

It could be from excessive boiling of water, aquariums, hanging laundry indoors, lots of indoor plants, lots of showers with no one using the exhaust fans, etc.

The big danger is what's happening inside the walls where you can't see?

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25 Responses to Spray Foam Insulation Condensation

  1. The answer seems somewhat inconsistent with the writer's statement that "I checked the humidity levels throughout the house, and the humidity levels are fine." Maybe Mr. Pallone needs to be more specific as to the basis for his statement.

  2. My guess is that by reducing the ability of air to move up and out it is now trying to equalize pressure by going out around the windows through the rough openings. These are often skipped when it comes to installing insulation. A smoke stick or cheap candle might help confirm this. State of the art might have someone take a look at the windows on the outside with an IR camera. A noticeably warmer parameter would almost certainly be warm air escaping. A result would be a more focused/concentrated stream of air, no more moist than in the past, at the windows. IMHO

  3. See answer above . You say your humidity is OK .
    Please give us some measurements and what the outside temp is .
    What you think is OK does not seem ok to me . thanks .. Roy.
    I good automatic Dehumidifier with a hose draining to a sink . Not one that you need to empty when full

  4. A nice 45% relative humidity at 68F can still spell disaster when that same air contacts cold surfaces, whether windows (as Mr. Pallone has indicated), or deep inside the wall where it can condense on the sheathing (as alluded to by Mr. Carter). I am not a building science expert, but I do read Joseph Lstiburek's work. I hypothesize that the condensation on the sheathing will not occur due to the fact the wall assembly in such an old house can freely breath, either inward or outward. It will dry long before damage occurs, so long as the 'reasonable humidity levels' are in fact reasonable, as directed by ASHRAE. Congratulations on sealing up your house, and reducing the chimney effect. Time to install some better windows on the second floor! All that said, I'm not volunteering my house to test the hypothesis before reading more on the subject!

  5. My bet is that the insulation mechanic covered over access to the vents that were in the attic.
    Either covered over the access or created a ridge or barrier that is preventing the natural flow of the moist air for it would exit
    so this might be the ridge vent or could be access to soffit vents. It would not surprise me if there was some type of insulation that ran up the walls in the attic the hat loud moist air to creep up and exit, this could either be through natural creeping of the moist air up through fiber insulation or flowing of the air through cavities behind or in front of rigid insulation that was in the knee walls and that terminated in some sort of an exit either to the outside or that was just sore by the exposed wood in the attic. People forget that wood comes from trees and that the cellular structure of wood is to conduct moisture up. I would have that insulation man come back and scrape the foam off any timbers and only leave it in the spaces in between. I would also carefully examine any place where one would expect to see light or feel air exchange and remove it from there using a razor blade or saw of some type. I would also check carefully and make sure that something that did not exhaust the bathroom fans right into the attic space. Or that insulation did not creep into the exhaust ducts. Finally I would want to be very careful to make sure that there is no place that water is coming in from the outside that either wasn't noticed prior to the installation of the foam because it was able to evaporate or it simply gotten more volume or came into existence coincidental to the application of the insulation. The only other consideration would be whether there has been some change outside such as the removal of trees or another building that had served as a windbreak protecting the windows on the outside from some degree of cold ass. If there has been a change outside then these windows maybe getting colder than they did before and that might explain the problem. Tim I am no expert but I am an avid reader of your very informative Colin in the Toledo Blade as well as online so if you want to give me a grade on my answer I would be honored.

  6. Another question would be did he apply the foam to the underside of the roofdeck (correct place) or on top of the attic floor. And if closed cell it is a problem. I am in Virginia and having great results with my foamed system, bugs the roofer and a air quality guy, both wanted to open up the end gable vents. But air quality guy said the air in attic was dryer then in the house and he liked the air quality alot.

  7. Mr. Pallone did not say how the insulation was installed. Spray insulation should have been installed to the underside of the roof and not on the attic joists. The attic still must have good ventilation. Make sure the spray insulation did not cover the soffit air vents and the gable or roof vents.

    Insulating an attic is a great idea, but making it air tight is not.

    • If you spray the underside of the roof, you don't want "good ventilation" in the attic because it becomes part of the conditioned space inside the building envelope.
      Mr. Pallone's problem stems from tightening up the house and his humidity overload can't escape like it did in the past.

      • Exactly... If you don't want good ventilation in the attic then you should expect problems else ware. Attics are not supposed to be sealed. If you are going to seal the attic then just leave all the windows open and problem solved.

  8. This is an interesting problem since it's common practice to humidify a house in the winter because the air otherwise becomes so dry. Especially an older home. I wonder if the windows are also part of the problem. We also need to know the numerical humidity level. I believe it should be in the 40 percent range ideally.

  9. It would be interesting to know if A blower door test was taken before the new foam was installed and what the result's were. Perhaps A blower door test could shed some light on the window situation the op has now.
    I would like to know what type of attic vent's the house has and the sq. footage of these vent's. IMO he does not have enough venting and probably should have A continuous ridge vent coupled with A continuous soffet vent.
    What is the temperature difference between inside the attic and outside temperature? They should be nearly the same.

  10. I see no mention that the bathroom ventilation fans are actually ducted outside. Being an older house, it is quite common to find the exhaust fans dumping just into the attic, which is now within the building envelope.

  11. Condensation on windows can indicate a cracked or rusted-out heat exchanger in a gasthe or propane furnace. The moisture created by the spent fuel normally goes up the chimmey

    Also from gas log fireplaces or other "ventless" heaters, gas heaters or appliances, including gas kitchen ranges.
    Strongly suggest testing the carbon monoxide levels if you use gas.
    Best of luck!
    -Brad

  12. I am a 2nd generation residential contractor and one of the biggest problems I see in homes is improper attic ventilation. Even in homes without spray foam most roofers in our area just put one or two whirlybirds on and think its sufficient with no regard to the soffit (intake) vents. As far as closed cell systems go I'm not sold yet. The proper way to install closed cell spray foam is to close off soffit by sealing the top plate of exterior walls to the roof sheathing and then applying foam to underside of roof deck. This eliminates all attic ventilation but is supposed to be way more efficient with higher R-values. If you had a proper dehumidifier connected to a drain system this might work but this still doesn't address roof warranty issues. The brand of shingles I use has addressed this and states in their warranty that any shingles (even the 50 year heavyweights) are limited to a 10 year warranty (5 years pro-rated) when installed over a non-ventilated insulated deck. This includes spray foam and SIP panels. While I'm complaining about ventilation/condensation issues another big one I see often is bathroom vents vented into the attic instead of out the roof! Some people vent them to the soffit but I avoid that since those vents are intake vents and can pull some moisture back in. Sorry for the long post! Hope it helps somebody or some house!

  13. We have to be really careful about this sort of thing in the south. It’s always so hot and humid, so if we mess up the insulation it could really come back to bite us. Thanks for all the info about this! I’m hoping to build a really eco-friendly house in the future and I think this will help dramatically.

  14. I'm having this same issue with water condensing on all my first floor windows which are all brand new Harvey ones. Great company not home depot ones. I have spray foam in all the first floor walls only. R21 to R28. This is my first winter after the remodel. Humidity level went up from low 30s from last winter before spray foam to mid 40s to 50s now. Bath fan is brand new works great. Exhausted outside. No kitchen exhaust yet. I disagree with a lot of you and don't think just cause of water on the windows means issues in the studs.

    • The condensation on the glass is caused by too much humidity in your home as it relates to the CURRENT outdoor temperature.

      It's a MOVING TARGET. As the temperature outdoors gets colder, so does the temperature of the inner piece of glass - even though they're great windows!

      Once the temperature of the glass reaches the DEW POINT of the air inside your home, the condensation starts.

      Re-read my column above as well as all my other condensation columns.

      The cause of water vapor in your home could be many things.

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