There are some situations where your insulation project might not go exactly to plan, which may mean that you end up with a spray foam thickness that doesn’t measure up.

What do you do in this situation? Are there ways to improve the installation? How will you know whether the manufacturer or installation contractor is proposing a solution that is guaranteed to work? There is sometimes too much contradictory information.

Look no further. We have carefully investigated all the issues and are confident that you are in the right place to have many of your spray foam insulation questions answered.

Read our guide to spraying foam over existing spray foam.

Can You Spray Foam Over Existing Spray Foam At All?

The short answer to this question is yes you can. As long as the spray foam is in good condition, dry and clean, then applying an additional layer to augment a shallow area, or repair damage, should not be a problem.

If you can identify the manufacturer then it would also be a good idea to source the same product. Spray foam is being developed and is being improved by manufacturers all the time, however, the basic chemical composition should be compatible.

When Covering Spray Foam With Spray Foam is a Good Idea? 

It makes particular sense to respray foam when, following an installation that should have delivered a prescribed depth, the spray foam comes up short. To achieve an R-value of 7 a closed cell spray foam should reach a depth of at least one inch.

In this case it would be sensible for either you or the contractor responsible to return and respray the affected area to ensure the depth required is achieved, especially if the issue is identified as being widespread. 

The sooner a problem like this is identified and rectified the better, any delay could impact your program of works on a new construction, or cause access issues on a renovation. After all, you will have already had to vacate the building for a 24 hour period to mitigate the effects of off-gassing while the product cures from the original installation.

Do You Want to Cover Old Spray Foam?

The general rule with spray foam is that you should not respray an area until the original application has completely cured. If you are considering augmenting an existing layer of spray foam that has been installed for some time, then this should not be a problem, but will it be worth doing at all?

The thing to think about with installing additional layers of spray foam insulation is whether it will improve the performance of the thermal envelope of the building or not? Usually, a 2-inch depth of closed cell spray foam is recommended as the ideal level of insulation when considering other factors such as the depth of the rafters or wall studs.

In the more Northern US states 3” is seen as the standard for wall insulation. Going deeper will not guarantee a higher R-value across the entire thermal envelope of the building once the limited R-value of the wall studs has also been factored in.

There is a point at which there are diminishing returns to be had spraying between the wall studs or rafters and so in this case perhaps it would be better to consider using an alternative insulation product to over-clad the entire wall or roof space, taking in the wall studs or rafters to increase the R-value of the total area?

Has Your Original Spray Foam Installation Failed?

Should your spray foam insulation be delaminating, i.e. it is peeling away from the wall studs or substrate, then this points to an issue with either water ingress or contamination with grease or other substance such as WD40 lubricating oil.

Cut away all the loose insulation and clean the substrate or timber wall studs before drying out and respraying the area. If you overspray the existing foam that has remained secure this will not be a problem. 

It is important to ensure that any water or other substance leak is properly dealt with prior to respraying the foam and closing up the wall, or considering the attic good to go. 

Do You Have Holes in Your Spray Foam from Rodents? 

It’s never pleasant finding out that critters have been chewing their way into your home. It puts in mind all sorts of risks to health, to electrical services or pipes, and sometimes the structure depending upon the size and invasiveness of the rodent.

Recommended reading: Is Spray Foam Insulation Rodent Proof?

Spray foam is not a robust barrier to the teeth and claws of some creatures, and if they do get through the siding, soffits, or shingles there are a few ways they can be thwarted. Tomcat makes a canned spray foam that is designed specifically to deter mice, which is great, but to absolutely make sure they are put off a top tip is to stuff the hole with wire wool before applying the spray foam.

Again, before filling any gap make sure that the existing spray foam is clean and dry. Cut out any damage and make sure any mess left behind by the rodents has been cleaned up. It’s not a good idea to trap mold or animal waste inside the spray foam as it may lead to further problems later.

When It’s a BAD Idea? 

My spray foam smells, so covering it up with another layer should stop the smell from escaping, right?

Sorry, no. This is not the right way to deal with a faulty installation that has led to an extended off-gassing problem. There are no situations where either removal of the existing problem spray foam, or mechanically ventilating a space, will not be a better way to deal with this.

Spray foam is mixed at the nozzle, and this issue usually arises when the installer has not applied the product correctly, or is using out of date material. The smell may indicate a failure of the product to completely cure, so it is unlikely that the spray foam will be able to take another layer anyway.

My spray foam is moldy, so covering it up will kill it and solve the problem, right?

If you have mold growing on your spray foam insulation then you need to look at other reasons why it is there. Mold needs moisture, food, and oxygen to thrive, so chances are that your problem is probably one of ventilation. Check that your vents are clear and operating properly if the issue is in your crawl space or attic.

Covering up the mold will make it disappear the day it is sprayed, but the spray foam will not stick to damp or dirty substrates, which means it will likely not stay put and your mold issue will not have gone away.

Over-spraying fiberglass insulation will mean my attic is better insulated, right?

Should your fiberglass insulation be sprayed over? Is this even possible? Technically, practically, it can be done, but in reality it is much better to take out any existing fiberglass, mineral wool or cellulose insulation if you are going to spray between the ceiling beams in your attic.

Spray foam always works best when it is applied to a clean, dry, stable substrate that will not flex, harbor moisture, dirt, or animal waste. It can create a very competent vapor barrier and spraying over fiberglass, etc, could end up trapping moisture between it and the drywall, leading to mold and associated problems.

Recommended reading: 11 Spray Foam Insulation Tips for DIY Homeowners


Justin's been in construction for over 20 years in both new build and renovation. With experience in both commercial and residential construction, he specializes in healthier and more energy-efficient homes.

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