Understanding spray foam insulation

The UK government’s £2bn plan to offer home insulation of up to £5,000 per household in England, made available at the end of September under the Green Homes Grant, brings a welcome boost to the renovation industry, as well as offering homeowners an opportunity to improve energy efficiency via accredited local suppliers.

Previous schemes have been plagued with quality control problems, so it is essential that works are carried out to a consistently high standard – and to ensure that they do not contribute to other problems such as condensation.

One so-called improvement that is often guaranteed to send a shiver through any self-respecting conservationist is sprayed foam insulation; I would hazard a guess that most surveyors take a cautious view of this technique, and probably advise their clients to pass it by. But is that prejudice fair or is it based on assumptions and half-truths?

To begin with, sprayed materials were often formulations of urea formaldehyde foam, but these have given way to improved materials using isocyanate and polyol resin that when mixed together expand around 60 times, and in some cases up to 100 times. The material is usually sprayed on under pressure to a thickness of up to 300mm.

Open-celled products are reasonably vapour-permeable, and often used for thermal insulation purposes. Closed-cell products are used in cases where a manufacturer perceives a competitive advantage by using it to extend the life of a roof with defective coverings, corroded fixings or leaks. Closed-cell products can also be used to improve airtightness – and because the insulation line and the air line can be combined, there are arguments that the system is more efficient than insulating at ceiling level.

Reliable performance data on spray foams is hard to come by. One of the frequent comments is that interstitial condensation could occur on the cold side of the insulation and cause decay in the tiling or slating battens or the rafters themselves. It’s fair to say that the quality of application is crucial, but in theory at least if the insulation is efficient at forming an air barrier then it follows that any water vapour finding its way into the roof void is less likely to be vented away, leading to a gradual increase in vapour pressure and therefore an enhanced risk of condensation.

Depending on the underlay type, a vapour-control layer may still be needed on the warm side of the insulation. Gaps in the insulation, if poorly applied, could then permit air leakage and consequent condensation on the underside of the roof covering. If you are following the principles of breathability, then spray foam may not be an option.

A conventionally insulated roof void could also suffer from acute condensation; a lack of ventilation coupled with high levels of insulation and poor vapour control can have very harmful effects, so it would be wrong to criticise sprayed foam simply on the basis of perceived risk. It is more a question of recognising the physics and designing accordingly.

If decay were a major problem with spray foam, I would have expected to have found more research that confirms this. However, there does not appear to be much in the way of reliable scientific evidence. Most information is in fact authored by suppliers, and for that reason may be less reliable.

For instance, although there is an association between thermal insulation and reduced carbon dioxide emissions, suppliers’ claims of the material being environmentally friendly are questionable. Scrap foam arising from manufacture can sometimes be recycled, but sprayed foam presents more of a challenge owing to the fact that it is bonded to other materials, which could restrict the opportunities for recovery and the granulation often required in the recycling process. The alternatives would be landfill or incineration – both unsatisfactory options.

Older formaldehyde foams have been known to degenerate over time, sometimes breaking down or crumbling and in doing so releasing an irritant dust that can be harmful to susceptible individuals. The crumbling and shrinkage of spray foam is often associated with problems in the mix proportions or formulation of spray, possibly due to inexpert application.

The chemicals used in the manufacture of foam are quite noxious and can cause problems as a result of off-gassing. This was a known problem with formaldehyde, although the effects were usually transient and the gas dispersed after a few days. Modern products such as H2FoamLite are sprayed using a water carrier and do not contain the blowing agents or polybrominated diphenyl ethers that are linked to several health issues.

There have been some recent reports of banks refusing to lend on properties containing spray foam insulation. The precise reasons for this are uncertain, but could be due to some of the performance concerns expressed earlier. Such attitudes will undoubtedly affect property values if they persist.

Like many things, the results of poor application can bring a system into disrepute, and with a number of DIY spray foam kits now becoming available, one could rightly understand a cautious view of the results of inexpert application.

From a practical point of view, the mere existence of foam does not spell disaster. The photo accompanying this article shows 1970s foam insulation sprayed to the underside of a Cornish slated roof property purchased in 1995, incorporating what is thought to be a Renotherm system installed by the previous owner in the 1970s.

Major extension work provided the opportunity for intrusive inspection but revealed no evidence of rot to the rafters or battens at the time. The roofing had evidently been in good repair when it was sprayed and the durable, 90-year-old roofing timbers had performed well. However, spray foam on a historic building would I think, be an extremely unwise course of action – even if the conservation authorities could be persuaded to accept it.

In conclusion, concerns over spray foam might not be fully supported by examination of the evidence, and failures could be due to a combination of other causes rather than the foam itself – for example, high humidity, poor ventilation or the presumption that a roof covering in poor condition can somehow be made sound by a sprayed coating.

Related competencies include: Building pathology, Design and Specification, Inspection