There are exceptions

The intro is a little unfair. Some architects are pushing the boundary and creating ultra-low energy ‘forever’ homes, SAP assessors and BCO’s can be flexible and supportive, and there are Environment Conscious builders out there. They’re all a pleasure to deal with when they’re on the same team. But it’s easy to be flippant when you’ve worked with as many sceptics as we have. That’s why we aim to win clients over first, the people with the most to gain, and then work on their design and build teams.

Great building fabric is what makes a low energy building, low energy. Poor building fabric is what ruins it, sometimes literally. So if this is your project always think ‘fabric first’, and let everyone else know you understand Thermal Bypass, Cold Bridging, Vapour Diffusion, and the benefits of Airtightness.

Let them know you’re planning a Thermal Imaging survey at the end of the project too. It normally helps to focus attention.

Spray Foam Insulation

There are two kinds of ‘mass fill’ expanding foam insulation; open cell and closed cell. And there are two ways to apply them, either spray-on or injection-fill. Both options are quick, easy, comparatively eco-friendly, very effective, and offer a number of other benefits depending on the application. But be warned, it’s still a relatively new industry in the UK and it’s largely unregulated. Anyone can buy the equipment and become a ‘sprayer’ overnight. And new sprayers always make mistakes.

When it comes to spray foam insulation providers, experience and accountability are everything. You should expect a little resistance from people who’ve never tried it, and you will need to overcome the fear factor (see ‘Mortgages’ below), but apply the right product in the right way and there’s simply no more effective way to insulate.

The mortgage question

Let’s deal with this potential showstopper first.

In November 2021 the Residential Property Surveyors Association (RPSA) issued technical guidance claiming their investigation had failed to identify any circumstances where a roof with spray foam could be given a “clean bill of health”. This provided ammunition to the sceptics who prefer business as usual because it’s easier, and created a spiral of negativity that led warranty providers and financial institutions to issue a ‘zero tolerance’ policy. The problem is, their technical guidance was inaccurate and they’ve admitted as much.

RPSA chairman Alan Milstein said: “We have agreed to retract our current guidance pending a full review. We know there are both good and bad installations but until now there has been no documented way to accurately differentiate one from another”. To rectify that little industry-killing snag a Technical Working Group comprising surveyor organisations (RPSA, PCA & RICS), industry bodies, central government, lenders and asset managers, are working on new guidance they expect to launch in the Spring. Nationwide opened the first meeting with the statement: “We are not averse to lending on spray foam, both mortgages and equity release”.

Read the RPSA press release issued 25th October 2022   

NOTE: It’s likely the new guidance will require approved installers to apply only Agrément certified products (BBA/Kiwa) to BS5250. Every application will need accurate U-value calcs with Condensation Risk Analysis, a long manufacturers product warranty, and an insurance-backed workmanship guarantee. Fortunately, we already do all that because it’s what every insulation company should provide.

Open Cell Foam

Flexible, airtight, soundproofing foam for timber

Open-cell is a lightweight, flexible, and very breathable polyurethane foam ideal for timber frame structures, particularly pitched roofs. It’s applied directly to the breathable underlay, sarking boards, or OSB without needing a ventilation gap. It can be used on flat roofs too, to reduce (not eliminate) the insulation required above the deck. But there are places you shouldn’t use it.

Modern 2-part open cell PU foam was developed in Canada in the ‘80’s as a soundproofing product. It therefore provides excellent acoustic attenuation and is particularly good at stopping flanking sound transfer from busy roads or overhead aircraft. Insulating the roof with open cell foam is like putting ear defenders on your property.

Spray-applied open cell foam expands around 100 times its original volume within five seconds, meaning every crevice, crack, and exterior penetration is effortlessly sealed to stop uncontrolled air leakage, aka Thermal Bypass. Below the ridge, soffit boxes and other awkward details can all be packed to create a breathable airtight ‘lid’ for the whole building, and you won’t need eaves vents, ridge vents, or even Vapour Control Layers that cost time and money.

Spraying a standard timber frame roof is like creating one big SIP panel on site instead of in the factory, only it’s quicker, cheaper, more effective, more eco-friendly, and potentially means you can hide services inside it rather than below it.

NOTE: Open cell foam is not suitable for application to steel, masonry, non-breathable underlay in a room-in-roof scenario, or below a flat roof with no insulation above the deck. The product is not dense enough to stop warm air condensing on these exceptionally cold surfaces. If you need to spray open cell foam near to a steel frame, coating the steel with closed cell first will alleviate the risk.

Closed Cell Foam

Rigid, high-performance, structural foam for steel & masonry

Closed-cell is a high density, rigid polyurethane foam insulation that’s spray-applied or injected into cavities. Same product, just one ‘rises’ faster than the other. Think of it as spray-on rigid PUR board insulation, the same you get from the merchants, only it’s applied ten times faster and leaves no gaps. Physically the ‘set’ foam is almost identical, as is the Lambda or K-value, so if your spec says you need 150mm PUR board insulation, it should be a straight-swap for 150mm of closed cell instead.

In terms of density and R-value performance, 150mm of closed cell is roughly equivalent to 300mm of open cell. That means it can be useful in vaulted ceilings where head height is restricted, or in standard 140mm timber frame walls. Both applications needing PUR board insulation below or inside the frame too, to make the Condensation Risk Analysis work. The only drawback is the reduced attenuation and the loss of flexibility.

Closed cell foam is ideal for insulating masonry cavities, internal solid walls, flat roofs, and tight spaces. It’s good below the screed on an excavated ground floor too, as the compressive strength exceeds most PUR boards. It just need skimming level before the underfloor heating pipes go down.

NOTE: Closed cell foam should not be sprayed directly to the underside of roof tiles or used for fixing leaks, even though there’s a BBA to support this application. It’ll just glue everything together. Closed cell foam should not fully encapsulate timber either because the timber will sweat and then rot. And closed cell foam is not a substitute for tanking where there’s penetrating damp, although it can spray on to the tanking itself.


When Solarcrest insulates something we always start with U-value calcs and Condensation Risk Analysis. Accurate calcs that model repeating thermal bridges like rafters, timber frames, and cavity wall ties, not generic off-the-shelf calcs that won’t stand up to scrutiny. Because without those calcs we can’t offer insurance-backed workmanship guarantees. You should be asking for these calcs regardless of how you insulate.

All our products have BBA or Kiwa Agrément certification, which means an independent body has tested them for Building Regulations compliance. There is no legitimate reason why architects, builders, Building Control, warranty providers, or lenders should reject our products, but that doesn’t mean they won’t try so it’s worth checking first. Innovating means more work, so it’s easier to find problems than solutions. But you don’t get Modern Methods of Construction without pushing the boundary and that’s exactly what we’re doing. “There is clearly something wrong with current UK construction methods” according to the BBA’s own report.

Some of our industry friends spray bits and pieces for people trying to save time, but you can’t be airtight unless you’re airtight all over, so we aim to spray the whole building. Every kind of floor, wall, and roof, ideally after the windows are in. When we seal the lot we’ll throw in an intermediate air pressure test too, before you start second fix, to check you’re airtight before any remaining leaks are hidden forever.

Over the last decade Solarcrest have insulated everything from Social Housing R&D projects funded by the NEA, ERDF, DECC, and local government; to football premiership palaces with underground pools and spas. High risk, high scrutiny jobs. We’ve satisfied hundreds of architects, BCO’s, asset managers, Passivhaus designers, principal contractors, and warranty providers including the LABC. Whatever you’ve got, and whoever you’re working with, chances are we can help.

Tell us about your project
Or get in touch on 01625 423020.
Ten years ago I overcame the fear of the unknown, overruled the builder, and challenged Building Control who wanted to block something I knew was compliant. It's the best move I ever made in terms of comfort. I was so impressed I bought the company who sprayed my home, just like Victor Kiam.
Eliot Warrington
Early adopter
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