Common objections, old wives tales and misconceptions about polyurethane spray foam

1) It causes the roof timbers to rot when timber is encapsulated in foam.

Answer: Firstly, foam in itself does not rot timber or affect it but damp and water does. The cell structure of ‘roof grade’ polyurethane foam is 95% closed cell and 5% open cell. The closed cells afford the foam its water resistance properties whilst the open cells provide a means for the foam to ‘breathe’. In essence circa 5% open cell foam allows the substrate (tile, slate, timber etc) to ‘breathe’ and act as a semi permeable membrane.  The cellular structure of the foam will act to dry out excess damp and work like slow blotting paper whilst the foam remains water resistant.

If the correct type of foam is used rotting of roof timbers cannot occur as damp will be evaporated off via the open cells whilst affording resistance to water penetration and condensation. If foam is used against timbers it must not be marine grade foam that is designed to be effectively 100% closed cell structure but rather a foam that has circa 5% open cell structure.

2) Slates or tiles once bonded with foam cannot be removed and it is almost impossible to repair a broken tile or slate once spray foam sealed and bonded.

Answer: Spray bonded tiles and slates can be removed with a few tricks of the trade. A quick and easy way to remove foam bonded tile or slate is to use a old saw and from the outside roof saw the foam bond away by pushing the saw up between the tiles or slates and gently sawing. Foam can also be removed by mechanical abrasion, a carpenter's wide chisel works well on foam as the material is easy to shape and cut. If a tile or slate has cracked then the back of a brick layers hammer can be used to gently chip away the tile so as to insert a replacement tile or slate.

3) Spray foam seal will stop a roof leaking without making any roof repairs first.

Answer: Spray foam is water resistant rather than water proof and will generally stop water from penetrating. However, if the foam has prolonged exposure to water the open cells will act to transmit the water and drip from one side to the other. This is a relatively slow process particularly if low percentage open cell foam is used. In general use, foam will prevent the ingress of wind blown rain but it is not itself designed to be a final roof covering material. It is quite satisfactory to use as a quick touching material but broken and badly chipped tiles or slates must be replaced first. Also, it is not always possible to ensure that the foam has got to every area underside of a roof; a cracked tile will still potentially let in water despite foam being installed. Don't rely on foam alone to make an already leaky roof leak proof, do the repairs first, replace any cracked or damaged (e.g. shaled, frost bitten, chipped) slates or tiles and also repair any leaking flashings and valleys. Then foam bond to stabilise and insulate.

4) Spray foam is very expensive.

Answer: Spray foam is a superior product with superior benefits and comes into its own when the following are required in one hit: roof stabilisation, low loading, excellent thermal and sound insulation, curing nail fatigue, fast time to install, minimal disruptions to other works and very easy retro fit creating a warm roof. Because spray foam creates a composite and sealed product (the foam and the substrate) there is no requirement for top ventilation on the ‘cold’ side so another cost can be saved that of top venting the insulation so as to avoid condensation.

There is no method and no product that can compete to achieve this as cost effectively as spray foam. Spray foam is remarkable cost effective for the applications it solves best and will save many consumers money over say the cost a new roof or a reroof, perhaps up to 70%. As always, cost has to be judged against investment value for money and spray foam should be judged as such like everything else competing for your wallet's attention.

Iso-Spray foam can also be provided in low cost professional kits so contractors do not necessarily have to be employed. Do it yourself and save money.

5) Spray foam is toxic and releases toxic gases over time (‘off gases’).

Answer: This is true of formaldehyde based foams but not true of polyurethane foams which are inert and can actually be eaten without toxic effect but not recommended! But did you know that the binding agent used in mineral glass/wool insulation uses a formaldehyde base? Iso-Spray only use polyurethane foams.

6) Spray foam attracts vermin that will eat it.

Answer: Spray foam polyurethane is not only inert it also has zero calorific value so vermin do not attack it for its food value. Spray foam when cured is relatively dense (though light weight) and hard so will help prevent vermin from gaining easy access.

7) Spray foam is too heavy and will put excess loading on the roof timbers causing them to buckle.

Answer: Where this one came from we do not know but it is rubbish. Ever held a piece of polyurethane foam? It is as light weight as can be, even the ‘high density’ foams.

8) Spray foam is very flammable and pose a major fire hazard.

Answer: Roof grade polyurethane foam meets British Standard Class 1 fire rating and is therefore resistant to burning and ignition. Roofing felt poses a greater fire risk. But true, the stuff you buy at the local DIY store will be very flammable, do not use on a roof as a cheap substitute, not only is it a fire hazard but its largely open cell structure makes it useless as a water sealer.

British Standard Class 0 can also be achieved with a spray foam system by a two coat process.

Do not be tempted to use a contractor unless he is willing to specifically show you a fire test on a sample of spray foam before installation. It is not unknown for a rogue cowboy contractor to tell you that they are using Class 1 foam and then install something completely different because it is cheaper to supply and makes the contractor even more profit. A none Class 1 foam is just a fire bomb waiting to go ignite, would you want that for your family?!  Not to mention that insurance companies are unlikely to insure the property if they knew and the difficulty in selling the property later on. Class 1 is also the legal minimum that can be accepted on all commercial and public buildings.

9) The Building Regulations rule out the use of polyurethane spray foam

Answer: No one has yet been able to cite which particular Regulation rules out the use of spray foam because no such Regulation exists! This objection is often used by arm chair critics who like to sound more knowledgeable that they actually are. The Building Regulations are in essence the minimum legal construction standards in the UK, they do not define what products can and can’t be used only what standard a product or design must meet as a minimum.

10) Building Regulations on Ventilation Rule out the use Spray Foam on roofs.

Answer: Another myth usually propagated by armchair know-it-alls on various websites. There are indeed Building Regulations for roof ventilation. The 2006 Building Regulations in Part F Ventilation essentially require that the building must have adequate ventilation and provision to prevent condensation occurring in the roof structure. Use of spray foam helps that requirement because it provides a ‘warm roof’ which by definition decreases the likelihood condensation.

Ventilation is required in areas where relatively warm and moist air can meet cold surfaces and the potential for condensation will exist. Spray foam acts to minimise condensation by providing a ‘warm’ surface that resists condensation so they requirement for ventilation is reduced. Also, spray foam does not have to be ‘top vented’ by a continuous air gap, as is the case with mineral wool and friction fit insulation products, as it forms a composite product where there is simply no air gap for condensation to occur in.

The Building Regulations on Ventilation actually make spray foam in many situations the only viable way of complying particularly in retro fit conversion projects like loft conversion and barn conversions where head room needs to be maintained and not sacrificed for bulky non polyurethane foam insulation.