Hello energy pros.

I need your combined wisdom before I proceed to help a client of mine address what appears to be mold growth and draftiness after I completed attic air sealing and other miscellaneous measures on her home.

The HO owns a split level in Northern VA. She, her husband, and three children have lived in this house for three years.

The house has existing exterior wall insulation, and some attic insulation. No rim joist air sealing or insulation and no foundation wall insulation. 

We sealed the entire attic above the two-story section of their split level (above the bedrooms), and replaced the existing blown-in fiberglass. We also insulated a section of exterior wall in their laundry room that was driving discomfort in their house with closed-cell spray foam. Finally, we insulated the floor between the 1st and 2nd story (between bedrooms) for sound attenuation purposes.

About one month after completing the work the HO e-mailed me to say that surface mold had begun to form on the exterior wall of the 1st story bedroom that we insulated the ceiling above and adjacent to the wall we spray foamed. This is the main concern at this point.

Before I run over there later this week and try to determine what is going on, I'd like to tap the knowledge that you all bring to the table.

My sense is that there is cold air rushing into this exterior wall through the rim joists in the basement. The cold air is mixing with warm conditioned air that is leaking into the walls through outlets and baseboards, and the air sealing of the top plates is keeping the air in the walls, but also allowing cold air to mix with warm, introducing condensation within the wall.

The HO also mentioned condensation on some of the glazing. One final detail I want to mention is that they have a gas insert in the 2-story section of the house (on the 1st floor).

Any ideas on what may be taking place, or tips on how best to proceed? 

Thanks in advance!

Views: 815

Reply to This

Replies to This Discussion

Hi Patrick,

A few questions and comments.

You suspect: <My sense is that there is cold air rushing into this exterior wall through the rim joists in the basement. The cold air is mixing with warm conditioned air that is leaking into the walls through outlets and baseboards, and the air sealing of the top plates is keeping the air in the walls, but also allowing cold air to mix with warm, introducing condensation within the wall.>

IMO, low probability as an accumulation of moisture would usually involve the warm air leaking into a cold area.  Adding cold air would be reducing the potential for condensation.

The air sealing has reduced the air exchange in the house increasing the RH, a normal occurrence.  The area near the laundry room could be an additional source of moisture.  This area could have been borderline prior to your work and now with an increase in humidity it has become a problem.


Could the foam itself also be a source of moisture?

How old is the house?

Explain the draftiness?

Is there an exhaust fan in that laundry room?

You replaced the existing blown in with what? and the floor/ceiling with what?

What is the RH in the house and in the problem area?

Type of heating?



I have been thinking about what you said with regard to the foam itself being a source of moisture. I use a product by Dow. It is a 2-part disposable kit called the Froth Pak. I haven't had any issues in the past, but I am not an expert on this matter.

The house was built in 1966.

I am waiting to meet with the HO before I can explain the draftiness.

No exhaust fan in the laundry room.

I only temporarily removed the blown-in FG to seal the attic floor plane, and then replaced it. No new insulation.

I will be logging RH in the house and problem room this week.

Thanks Bud.

Bud is correct

Because of the air sealing & insulation the RH in the house has increased, and now the 1 wall & some windows are cold enough that condensation happens, and mold grows.

It's important to understand that air leakage (in the winter in most climates) drys the house. Cold air can't hold much moisture, and when it comes into the house it's RH drops (as it is warmed), drying the house. Also heat loss tends to drive moisture out of the house, and you added insulating, reducing heat loss.

I've solved mold problems by air sealing & insulating houses. Insulating raises the surface temperatures increasing the RH that condensation occurs at. While reducing air leakage reduces the drying effect. I added bath & kitchen exhaust that was missing, and covered the ground with a vapor barrier.

So you need to look at the sources of moisture; bathrooms, kitchen, dryer, basement. If there is no exhaust fans, they need to be added. The basement likely needs to be air sealed too. And basement moisture needs to be dealt with (drainage, water proofing, vapor barrier, de-humidification, etc.), perfect place for a heat pump water heater.

Remember; The House IS A System

I also wanted to mention, that gas insert, if it isn't venting properly it could be a source of additional moisture, plus other concerns.


Is there a sure-fire way to tell if a gas insert is vented properly?

You can turn it on and see if it gets very hot and humid in the room!


I would think your biggest concern would be if it's a 'vent free' fireplace with no chimney or vent to the outdoors. These usually have a hood-shaped piece of metal at the top and put out a lot of hot air.



As Doug said, a ventless would be the biggest problem.  For a vented, the additional question becomes zone pressure, aka CAZ.  If other exhaust appliances can backdraft this gas fireplace, you might only see an increase in humidity with, say, the dryer running.  You said the fireplace was on the first floor of the two story section.  Vented directly out or up through a chimney, inside or outside??



The DC area had a damp, cool-but-not-cold fall and we're seeing mildew in lots of houses that don't normally experience it.

I have a couple of spots in my own house where mildew has formed on exterior walls. Also we have had a lot of window condensation.

Now that it's actually cold and the outside absolute humidity has plummeted, I believe we'll see the indoor humidity levels drop in most  houses as dry outdoor air flows through. By 'most houses', I mean those with sufficient air exchange to the outdoors. In my experience around here, that would include almost any older home, even with significant air sealing, and from your description this house probably still has a significant amount of leakage.(Do you know ACHnat for this house? If you're near or above the BAS I would bet it's enough.)


You can certainly learn a lot in half an hour with a humidity meter and a walkaround checkup looking for unusual moisture sources, such as disconnected dryer, uncovered dirt crawlspace floor, bathroom with broken or no fan that is used frequently for showers, HVAC humidifier with broken control (or control set on 50%), even roof leaks...no harm fixing those...but if you don't find them, just monitor the humidity level. Should drop below 35 this week or soon.


Again, we've seen mold in houses where no new work has been done, I believe due to high humidity and low heat demand this fall. There may be no connection to your work.

Thanks a lot Doug!

I will be monitoring the humidity levels in this house after I take a look around it to see where the excess moisture may be coming from.

ACH50 is 7.99 and ACHn is 0.52.

The most recent blower door was 2,850 CFM50, with the BAS at roughly 1,900 CFM50.

That's not all that tight. I would not be worried about 'too tight' issues where you need air exchange and have to worry about air conditioner performance.


The previous comments have addressed a lot of good possibilities and areas to look at, but I would like to expound a little further. I hope it helps. As mentioned in several of the other comments sealing a building can cause the inside RH to increase because it is not being reduced by the colder - lower humidity - air from infiltration  (As an aside this also causes indoor air pollutants to become more concentrated because of the lack of dilution air.) It may seem counter intuitive to some but when a building is sealed tightly we must introduce outside air. The best way to do this in this application is through the addition of an ERV (Energy Recovery Ventilator). An ERV will provide dilution air from the outside while tempering this air by extracting heat from the exhaust (indoor) air. It will also help balance the building pressure. If a house is tightly sealed exhaust fans will fail to work properly because at some point they will be unable to overcome the negative pressure that they (and other appliances) may be creating.

This raises other questions; primarily about the heating appliances, the water heater and the fireplace(s). If they are fossil fuel and not isolated combustion (drawing combustion air from outside the structure - typically 90%+ AFUE) you have possible additional - more serious problems. A natural draft appliance in a tightly sealed structure will typically fail to vent properly. Consequently, you get by-products of combustion introduced into the space. Which include moisture and (if there is not enough oxygen or if you have incomplete combustion for any other reason), Carbon Monoxide. Even if they are heating with a heat pump, or an isolated combustion (Category IV) furnace and using an electric water heater, the fireplace insert you mentioned is still a concern for the same reasons. You did not mention whether this insert is vented, or ventless. If ventless, it is adding moisture to the space and will eventually deplete oxygen in the space (and likely produce CO). If it is vented, and the house is tight, it is probably not venting adequately and thereby creating the issues mentioned earlier.

In addition to a psychrometer (to check humidity) you may also want to take a CO detector.

And finally: by adding insulation and sealing air leaks you have decreased the house cooling load, so you can anticipate the moisture/mold problems being worse in the summer (unless changes have been made to the HVAC system).


Latest Activity

Dara Bowser replied to Don Fugler's discussion joining ducts in the group HVAC
"Good to hear you have clean dryer ducts Don. The dryer duct issues that we run into are…"
3 hours ago
Don Fugler replied to Don Fugler's discussion joining ducts in the group HVAC
"David, If the IMC mandates metal dryer duct and prohibits metal screws, they must be accepting…"
5 hours ago
David Butler replied to Don Fugler's discussion joining ducts in the group HVAC
"I never checked that before. Checking now... Section M1502.4.2 of the International Residential…"
5 hours ago
Joseph Frasier is now a member of Home Energy Pros Forum
5 hours ago
Don Fugler replied to Don Fugler's discussion joining ducts in the group HVAC
"Brendan, Thanks for that response. Because of your warning, I just went down to the basement and…"
9 hours ago
Brendan Reid replied to Don Fugler's discussion joining ducts in the group HVAC
"Hi Don, Summary: DRYER VENT: screws bad idea, possibly prohibited by code. METAL DUCT: must be…"
9 hours ago
Profile IconBrendan Reid and David Butler joined Allison A. Bailes III's group


HVAC design, Manuals J, S, T, & D, Duct leakage, Air flow, ENERGY STAR new home requirements,…See More
9 hours ago
Profile IconSean Wiens and David Butler joined Chris Laumer-Giddens's group

Energy Efficient Design

Energy efficiency starts at the drafting table...How architects and designers can help buildings…See More
18 hours ago


  • Add Photos
  • View All

© 2017   Created by Home Performance Coalition (HPC)   Powered by

Badges  |  Report an Issue  |  Terms of Service