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Is spray foam the best way to insulate our attic?

Property Clinic: What are the pros and cons of this method compared with other solutions?

There has been a lot of talk about attic insulation to reduce heat loss and, in certain cases, SEAI grants available to get it done. I have explored several options and am attracted to the idea of Icynene lining, which would appear to be a simple matter of spraying the inside of the roof and allowing the substance to expand and solidify, thus sealing the roof from inside. Can you offer any advice as to the pros and cons of this, including environmental considerations, compared with other solutions?

Most people seek to live in a healthy, warm and efficient house, but when they start researching how to create such a home, either in a new build or through the refurbishment of an older building, the options, advertising and conflicting advice available is often confusing.

It’s generally assumed you start by installing the latest and most efficient devices whether that’s solar panels, heat pumps and/or heat-recovery systems. And while that should not be ruled out, as you’ll discover quite quickly, the installation of such technologies comes with a financial cost.

Perhaps the best place to start is by examining how you can improve the energy efficiency of the fabric of the building that you are exploring.


An understanding of the underlying building physics that either solve or create issues is often overlooked so it’s important to ensure that any product chosen carries the appropriate documentation such as a National Standards Authority of Ireland (NSAI) certificate that explains which product is appropriate for your circumstances and the risks and requirements involved to install it without detrimental or unforeseen outcomes – simple might not always be the best.

Regular commentary online suggests that interstitial condensation occurs on the cold side of the insulation causing decay in the tiling or slating battens or the rafters themselves

Sprayed insulation foams have been available for more than 30 years with two basic variants, closed cell and open cell, and each have their pros and cons.

Earlier versions were often formulations of non-breathable urea formaldehyde foam, these have given way to improved materials using isocyanate and polyol resin that when mixed can expand about 60 or 100 times in volume up the slope of and between roof timbers. These materials are usually sprayed on under pressure up to a thickness of 300mm to provide a high level of thermal insulation and create a “warm roof” and useful attic space.

Closed-cell products are sometimes used to extend the life of a roof with defective coverings, corroded fixings or leaks. Closed-cell products can also be used to improve airtightness which might be thought more efficient than insulating at ceiling level albeit the sloped area is greater than the flat ceiling.

Reliable performance data on spray foams is thin on the ground with people often left to rely on information from the various manufacturers. Regular commentary online suggests that interstitial condensation occurs on the cold side of the insulation causing decay in the tiling or slating battens or the rafters themselves. Therefore, the material choice and quality of application is crucial. In theory, 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 the enhanced risks suggested. However, no empirical information has been published on this topic.

The slate underlay type, if it exists, is also important so a vapour-control layer may still be needed on the warm side of the insulation and the important air gap for venting and protecting the roof timbers must be maintained. Gaps in such insulation, if poorly applied, can also permit air leakage and consequent localised condensation within the roof structure so if you are following the principles of breathability, then spray foam may not be an option for you.

Lack of ventilation coupled with high levels of insulation and poor vapour control can have harmful effects to a roof, 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 adopting an appropriate course of action. It’s also important to remember that issues with condensation and ventilation can occur with other types of insulation too.

As a rule, we don’t discuss specific brands in this column. Suffice to say the NSAI is the national certification authority for CE marking and it certifies spray foam products. Spray foam is usually installed by approved contractors who should be able to advise you of the circumstances of your home and whether it’s appropriate for use in accordance with its certification requirements. You might also ask them to carry out a WUFI analysis to prove the effectiveness without condensation risk of the thickness proposed. (WUFI is an industry-wide tool for hydro and thermal analysis of building elements.)

There is general agreement that human exposure to isocyanates should be minimised because of potential health hazards linked to volatile organic compounds (VOCs)

The main chemicals in spray polyurethane foam insulation (SPF) and other similar types of foam insulation are isocyanates, which have been widely documented as posing some health risks to people exposed to them. This was reported on several years ago in a court case where compensation was awarded to a family who the judge ruled were not properly advised by the installing company of the risks of remaining in the property while the spray foam was being applied. To my mind this is the most significant factor if selecting this insulation method, particularly as there is general agreement that human exposure to isocyanates should be minimised because of potential health hazards linked to volatile organic compounds (VOCs).

Some manufacturers claim their products have a low flame-spread rate, do not contribute any fuel to a fire and will not sustain flame upon removal of the flame source. They also claim they will not drip or melt and will leave a charcoal residue. This is reassuring from a fire-safety perspective. Against that, however, in the event of a fire, fire service personnel may have difficulty in locating the source or spread of the fire using thermal cameras as they may be blinded by the insulation if it is adhered tightly to the rafters. This could lead to greater fire damage to the structure due to the extra time involved.

It is also advisable to inform your insurer regarding your house cover before proceeding to use spray foam insulation.

If you are still in doubt, you should contact a local chartered building surveyor to assess if the insulation proposed is suitable for your home.

Fergus Merriman is a chartered building surveyor and a member of the Society of Chartered Surveyors Ireland

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