Does spray-foam insulation in your attic cause mold and rot? Explanation of the problems found in English attics

If you have an older home in a northern climate, invariably you’ve thought, how can I make my home warmer?  Insulation is the first thing that comes to mind.  Yes, insulation can help, but without air and vapor barriers, it is like stuffing a cotton ball into a crack in the wall without sealing it up first: cold air and water vapor will still come in through the cotton ball.  This is why insulation, or “thermal control” is the last layer you need to consider on your home; it falls below water control, air control, and vapor control (see our article here). 

Similarly, homeowners in England want to be comfy, too.  We kind of knew that England tends to be wet, but did you know that London is on a similar latitude as Vancouver and Newfoundland?  This means cold.  Cold and damp require homeowners and builders to be very careful about these “control layers”, because if you add one (insulation) without considering another (water and vapor), you get big problems.   The other consideration is age of the home: the average age of homes in England is 50-70 years, while 20% of homes are over 100 years old.  When they were built, there was little consideration about air and vapor control.  So these old, leaky homes worked fine with a roaring fire throwing heat out.  But today’s homeowners, especially in the UK, are discouraged from using combustion heat sources, and want to save money on their heating as well.  In 2021 and 2022, homes in England that had spray foam installed were beginning to have mold and rot issues, and some insurers proclaimed that they would not insure them.  What a nightmare for the homeowners–after all, most mortgage lenders require insurance on the mortgaged home.  There was an uproar, not least of all because the English government had sponsored programs to fund the installation of spray foam in such homes.  Our questions are, why did the mold and rot happen, and could this problem affect homes in the US?

Expert builder Matt Risinger investigated this problem and made a video on his conclusion: Major Lenders Reject Homes with Spray Foam Insulation - WHAT HAPPENED?) First, he noted that spray foam was installed in existing homes’ roof rafters to prevent heat from escaping.  For most homes, there was open-cell spray foam in the rafter area, and rafters (and even underside of roof sheathing) were becoming wet with condensation, causing mold and rot.  He next looked at the weather conditions that might cause the condensation.

In the winter in the UK, the roof deck could be in the 30’s (degrees F).  In the now-insulated attic, it could be 69-70 degrees F with 35-40 % relative humidity (inside the home envelope).  He calculated the dew point (the temperature at which condensation will form) for the attic side and presented the following two scenarios:  (Remember, if the cold side temperature is colder than the dewpoint, condensation will form.  If it’s warmer than the dewpoint, condensation will NOT form)

Since the temperature of the roof deck is in the low 30’s, the cold side is colder than the dewpoint, so condensation will form.  The greater the relative humidity, the more moisture in the atmosphere of the attic can be condensed on the underside of the roof.  In one part of his video, he shows how a man reached his hand into the insulation and touched the rafter and roof deck, and it was soaking wet.  

This highlights three problems: first of all, the spray foam job in that case was not great–there were some gaps in insulation, exposing the cold roof deck.  He didn’t even have to pry away any insulation to stick his hand in.  

Secondly, open cell spray foam does not act as a vapor barrier.  Building experts say that it is not recommended to use open cell spray foam in a cold climate because of the lack of vapor barrier.   If there’s any surface that’s going to get cold, vapor can transmit through that open cell spray foam (like a kitchen sponge).  On the other hand, closed cell foam does act as a vapor barrier.  They probably should have used closed cell foam and encapsulated the rafters. 

Thirdly, there’s not enough foam– it looks like the foam is not thick enough  For example, in the southern US, the proper depth for spray foam would be 10.8” open cell or 6.3” of closed cell foam.  If you’re in climate 4, 5, 6, you need 8.1” of closed cell.  In climate zone 7 and 8, 10” of closed cell.  Climate zones 6-8 are likely to be similar to England…but this is where you really need to have an experienced contractor review your plans and give the best insulation solution, as they are able to read the codes and apply them to your location and home.  For example, in America, in IECC Climate Zones 5 and higher only high density closed cell spray foam should be used.  (Residential Foam Guide).  

Risinger also noted that in England it’s quite common to have tile roofs installed on “skip sheathing” which is not a solid surface.  It consists of something like a 1x6 lumber, gap, 1x6, etc., covered with tar paper, making the roofs more permeable to moisture than a full layer of wood.  That kind of sheathing is ok for a vented attic, which is how they were originally built, but when enclosing the attic with sprayfoam, the sprayfoam slows down the drying potential of the roof. 

Risinger thinks that these mold and rot problems are not going to be widespread in the U.S.  There are still risks, but we build differently with roof and deck materials.  Another  important thing to note is to make sure the attic is sealed (no airflow in) and the roof will not leak. 

Another expert, Mike with Spray Jones, has over 20 years as a spray foam insulation contractor in Saskatchewan, Canada.  He dived into the whys of this problem in this video.   He concluded that the problems stemmed from two main reasons:

• The wrong product was used for the application (open vs. closed cell foam)

• In some situations, compromised roofs were not repaired before foam was installed, allowing moisture to come in and be harder to detect under the foam. 

In a thoughtful review of the UK webinar “New Guidance on Sprayed Foam Insulation Inspections”, Mike made some observations on the second point: how the homes should have been inspected and and repaired before proceeding with a detailed plan for spray foam installation.  He brought up these points:

• There was inconsistency in lenders’ policies (they need consistent lending requirements).

• There was a lack of accountability from installers (not surprising because there are always unscrupulous people), but credible product and credible installation should remove and mitigate risk.

• Inspectors did not know what they were looking at/for.  

• There was no mechanism to detect and deal with moisture risk (roofs were already wet and compromised, but installation companies were still selling).

• Quality assurance of the spray foam material and installation was lacking (right foam, right surface, right thickness, did foam bond well to surfaces, were there gaps, etc).  The industry needed additional training and knowledge.

• Most of the existing roofs should have been evaluated thoroughly (including with ladders and drones on exterior) to determine where moisture was coming in, pre-existing workmanship issues like penetrations that are failing, etc.  Homeowners should have received a detailed inspection report to repair existing issues, as well as product specifications,  before contracting for foam.  Then there should be inspection and followup after installation.

It seems that spray foam was made out to be a “waterproof” layer, when that is not its function.  Yes, closed cell foam is water resistant, but the ideal would be to waterproof the outside with multiple layers of protection, then choose the best insulation to back it up and take care of the thermal envelope internally (open or closed cell).  Installers must deal with vapor drive through open-cell foam in damp climates (like the UK): they should be installing vapor barriers inside when using open-cell foam. This type of foam has a reservoir effect (it actually stores moisture!). 

When these types of precautions and requirements are in place to get quality information in a professional manner, lending can continue.  Then risks can be known, using a verifiable product that will last.  All products require care and oversight, but spray foam should not be used as primary waterproofing (it’s easy to blame the product for something it’s not doing, when it’s not designed to do that!). 

We at HypoAir also want to add that since the attics were now being included in the home envelope, and since humid air rises (it’s less dense than dry air), it’s likely that moisture from human activity in the homes caused the air in the attic to be higher than the 35% estimated above.  Insufficient dehumidification/ventilation in the home can easily cause that attic humidity to go above 50%, which would make the dew point 49 degrees F or higher, way above the assumed roof deck temperature (in the low 30’s).  

Looking at evidence from both reports, we agree that using spray foam alone in cold, damp climates like the UK, in insufficient thickness, and on structurally damaged roofs, were probably the main causes of condensation, mold and rot in attics.  It’s also easy to see how unscrupulous installers sell to people without doing proper inspections and repairs beforehand.  You can’t make up for planning, planning, planning, especially in the tricky area of air barriers, water vapor retarders, and insulation.  

Here’s some more information if you don’t quite understand dew point, and check out our favorite quotes about planning and how to understand where to place air barriers and vapor retarders in our article here.

Photo by Matthew Harris on Unsplash