Spray Foam Insulation may save energy, but at what cost?

Over the last few years we have built really good relationships with many professional organisations. The article this week about our continued work with RICS on the surveyors’ guidance on Japanese knotweed and our ongoing work with British Standards on three separate standards, is another.

It was good to find a message from the Residential Property Surveying Association (RPSA) this week. Alan Milstein has overseen the rapid growth of this highly credible and accessible organisation, who are committed to the support and development of surveyors operating in domestic properties.

Alan, who is the long standing chairman of the organisation, has attended and spoken at PCA events in the past and we are very pleased to have had the opportunity to speak at RPSA conferences in years gone by too. The subject of this conversation related to spray foam insulation products. Alan is aware that the PCA has expertise when it comes to understanding the causes and effects of moisture in buildings and assumed correctly that we might have an opinion.

Spray Foam Insulation

So, what do I think about adhesive foam sprayed onto the underside of roofs?

Now it would be easy to just respond with a string of expletives but that wouldn’t explain my visceral loathing of the stuff, so off the back of my chat with Alan here are my considered, if not slightly animated, thoughts.

Rain falls out of the sky. This rain falls onto our roofs. The roof is usually constructed of slates, tiles or stones that are built over a wooden structure. It is an irrefutable fact that roofs leak sometimes.

In normal conditions with a small leak, this will lead to bit of localised wetting to the roof timbers and a wet patch on the ceiling. Easily spotted, easily fixed. The timbers dry out with no great dramas.

But what happens if the roof leaks, but the water is held within or behind a layer of foam? Well the water stays close to the timber, it doesn’t wet the ceiling, so the early tell tale sign is eliminated and the timbers don’t dry out.

Insect attack and fungal decay

Wet timbers are susceptible to both insect attack and fungal decay. The water that gets in can (depending on the type of foam) move about within the structure of the foam, through the wood behind the foam or in voids behind it. The structures can and do get wet, and stay wet. Best of all, this happens in a way that makes early observation of defects difficult or impossible without pulling the foam off to look.

The result is calamitous defects with the roof structure. There are potentially very dangerous consequences, but even when there is no risk to the occupants from collapse, cost of repairs are massively increased.

A bit of woodworm is easy and relatively inexpensive to treat in an exposed roof. Early diagnosis is easy and if insulation can be removed and timbers cleaned down, preservation treatments are safe and effective. Add spray foam and diagnosis is impeded and chemical treatment is made impossible – and all that in timbers that are possibly slightly wetter and so more conducive for wood destroying bugs.

Atmospheric moisture inside the roof space

Then consider what we do and don’t know about the effects of atmospheric moisture inside the roof space after spray foam insulation is applied to the roof…!

Some companies who apply the foam say that it reduces air exchange and because it is a “closed cell” you don’t even need to consider a vapour barrier. Well unfortunately they seem to be missing the point.

We don’t use vapour barriers against the roof coverings of a traditional building. We encourage vapour exchange. Even when high performance underlays are used they are sold on their vapour permeability. Besides, when installed they are loose laid. As a result, roofs by their nature are not good at holding air or moisture vapour inside the voids.

However this is good news…

That is good, because a little the moisture generated by occupation in the rooms below the roof space can and does get into the roof void (when there is no vapour check in the ceiling). This is usually dealt with by the fact that the vapour can then get out pretty easily. Unfortunately, if you cover the roof in a closed cell insulation it can’t get out. It has to go somewhere! So where will it end up?

In a perfect world the insulation would be continuous. I would eliminate bits of structure that might be uninsulated or cold. In practice this is almost impossible. Add to the problem of poor continuity the fact that the timber that forms the roof is hygroscopic (able to absorb atmospheric moisture) and so the building and the roof timbers are working in an area of increased humidity and are then at risk of wetting up.

The effects of condensation

The only solution I see suggested for condensation in the roof void is to ventilate the roof space. Well it will have an effect, but doesn’t this then totally negate the value of the insulation designed to prevent warm air getting out and cold air getting in?

Those that then claim to use permeable insulation would argue that the problems highlighted above are negated as the vapour can pass through the insulation. The roof can breathe – yeh right!

Let’s have a think. Any surveyor that has looked at a decent number of homes in the winter will have seen water on the underside of roofing felt. This is common on tiles, slates, bituminous felts and vapour permeable sarking membranes. Condensation happens on anything that is at or below dew point, it doesn’t matter how permeable something is, if it’s cold enough and there is water in the air, it will wet up.

Is this a problem?

This can be a problem, but often the effects of roof condensation are relatively short lived. As the roof warms in even the weakest winter sun, the problems diminish and the roof dries quite quickly. I would speculate too that these wetting events happen quite frequently but seldom result in significant decay to structural roof timbers. A bit of mould on rafters, spar feet and purlins is a common site in winter and an indication of imbalance that should not be ignored, but in our experience, roof timbers rarely rot out as a direct result of roof condensation.

So instead of allowing a roof to get wet and dry out quickly, think about the potential effects of interstitial condensation within open cell insulation. As long as dew point is achieved water will be deposited as condensate. If this is trapped in the insulation against the wooden structure it wont dry out quickly. Any water that makes it into the wood may not be able to evaporate out rapidly. This is in part because the insulation reduces the rate of warming, and all the good things that come from that, as well as the rate of cooling.

Spray foam insulation may save energy, but at what cost?

Spray foam insulation may save energy, but at what cost? A roof that holds water ingress against the vulnerable timbers, a roof that loses its ability to give early warnings of a leak, a space where moisture imbalance is masked rather than corrected. A roof where there is a significant possibility of pushing wood into the risk zone for insect attack and fungal decay. A roof that is not easy to repair when it fails and can’t be treated if insects decide to eat it. The question for me is, why would you?

No roof covering has ever claimed to last forever. Spray foam doesn’t increase the life of any roof covering. In fact, it has the potential to reduce it.

PCA members are experts in timber decay and wood destroying insect control. Spray foam insulation will do nothing to reduce opportunities for our members to trade successfully long into the future. Do I blame lenders for running shy of this stuff – not at all. Would I buy a house with spray foam insulation in the roof – of course I would…but I would factor in the cost of a new roof when I made the offer.

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