The ultimate roof for an energy efficient building is airtight, breathable, vapour impermeable, very resistant to heat transfer (in both directions) and obviously water tight. Breathable open-cell foam gives you all this and more, quickly and easily. If you want vaulted ceilings our simple roof insulation system will give you ample space for services, including MVHR ducting, with no cold bridging whatsoever. If you're not sure what this means in terms of comfort and performance ask your architect. Until now it's been very difficult to achieve this result without a great deal of time and expense.

Open-cell spray foam insulation has been tried and tested for decades in countries far hotter and colder than ours. It can be applied directly to the underside of breathable and non-breathable roof membranes, felt or sarking boards, making it perfect for both retrofit applications and new roofs looking for the best possible long-term performance.

With a Global Warming Potential of only '1' water-blown foam is the ideal product for those building to the Code for Sustainable Homes. It'll give you Passivhaus performance without the Passivhaus price tag.

Click here to download the 'Airtight Roof' schematic above
Click here to download the U-Value calcs and condensation risk analysis

A wall filled with insulation boards, loose beads or wool might appear to be well insulated, but is unlikely to be airtight. Warm and often damp air escapes through the gaps before it meets the cold surface of the outer wall or skin. Condensation forms when warm air meets a cold surface. Condensation leads to damp, which makes the insulation heavier, which causes it to slump over time. Dense, damp insulation at the bottom of the wall acts as a thermal bridge that creates a cold strip along the bottom of the wall. You also get another cold strip at the top of the wall where the insulation used to be. Both cold strips then become a magnate for damp and then mould, especially in a poorly ventilated house.

Blowing in mineral fibres is the cheapest way to hit the target U-value in a cavity wall, which is why most insulation firms use it. The problems might not become noticeable for years, but when they do they're not easy or cheap to fix, if they can be fixed at all. Of course by that time the installer has long gone.

In a new cavity wall insulation 'bats' are slotted in during the build, typically mineral or PUR boards. Even the most meticulous builder will struggle to create an airtight seal inside the wall. Many will simply slip insulation in where they can to satisfy the building inspector when he calls. In theory the wall hits the target u-value because of the combined thermal resistance of the brick, the block and the insulation board. In practice any gaps in the insulation board or between the board and the inner/outer brick/block will lead to convection inside the cavity.

Convection occurs when warm air trapped between the inner block wall and the insulation bat, and cold air is trapped between the outer brick wall and the bat. As warm air rises and cool air falls the circular movement of air wicks the heat away despite the insulation in the wall cavity. Sadly the accepted U-value calculation is based on test results carried our inside a vacuum where there's no draughts, damp, dust or debris. No credit is given for the unique airtight qualities of expanding cavity foam.

Pour-fill cavity wall foam lasts for the life of the building. Because it expands after it's injected into the cavity it tightly packs every square millimetre within the void space, adhering to both inner and outer wall surfaces and tightly wrapping around pipes, cables and anything else that penetrates the wall. Originally developed as a wall stabilising system to counter failed wall ties, pour-fill foam perminently bonds the inner and outer wall together to massively improive structural integrity to prevent hairline cracks caused by settlement over time..

Pour-fill cavity wall foam might appear to cost more initially, but if you're planning to stay in the property for the foreseeable future you'll soon recover your investment. In the meantime you'll feel a lot warmer and more comfortable. If mineral fibre insulation is like a wooly jumper for your house, pour-fill foam is like a duck down lined Goretex jacket.

Hard-to-treat cavities

Spray foam insulation is the ideal product for treating old solid masonry walls internally, or new timber frame walls. Applied to a masonry wall the foam provides an airtight vapour barrier and insulation layer in one. It'll also help to stabilise crumbly or uneven walls making it ideal for agricultural or commercial building conversions. For masonry or steel surfaces Solarcrest recommend the market leading 'closed cell' BASF Walltite product which carries the BBA certification. For timber frame walls where breathability and flexibility are considerations Solarcrest normally recommend an 'open cell' soft foam.

Please note; applying spray foam insulation to small areas (typically below 50m2) is not generally cost effective given the set up costs, plant and manpower required to manufacture a small amount of foam on site. Applying spray foam to the internal walls of an occupied and furnished property is not generally advisable.. Spray foam insulation on interior walls is best suited to complete building refurbishments or as part of a wider home improvement project.

Click here to download the 'Airtight Wall' schematic above

Click here to download the BASF Walltite product datasheet

Generally speaking the roof and the walls of your property have some level of insulation. It might not be the best available but at least it's offering some protection between indoor and outdoors. But what about your floor? What's between your feet and the space below the suspended timber floor, which is normally the same temperature as your garden? An inch of timber full of gaps, half an inch of underlay, carpet and the sole of your slippers. If your walls were that thin you'd be living in a garden shed. If your house were a box, the floor would be the largest surface area after the roof, so if it can be insulated it should be.

If you're lucky enough to have a crawl space below your floor spray foam insulation is the ultimate under floor solution. It'll stop all draughts completely, it'll flex as the floor moves, it'll insulate the central heating pipes that normally run below the boards (often without any lagging), and it'll allow the timbers to breathe to avoid rot. All you need is room for a man to work safely below the floor, ideally 1m.

If walking on your floor is like walking on a sheet of ice in the winter, a coat of spray foam insulation below will feel like you just installed under floor heating. Sadly you do need at least a reasonable crawl space in which to work and solid floors are not suitable.

The slightly higher Lambda value of the foam means that you need to apply a little more of it to achieve the same U-value as a high-density PUR insulation board. To achieve U:0.18 on a roof with a breathable foam you may need 230mm depending on the timbers, to achieve U:0.3 on a wall you'll need around 75mm of a closed cell foam to match the theoretical benefits of PUR insulation. Before you dismiss spray foam on this basis, bear in mind the standard U-value calculation only measures one of three types of heat transfer and it doesn't give any credit to the airtightness you'll achieve with spray foam insulation.

• The myth of the U-value
• How air leakage effects the U-value
• The importance of airtightness
• U-value calculation - pitched roof

Like all new technologies people fear what they don't understand, particularly council building control officers and more traditional architects. Not that spray foam insulation is new, it's been the market leader in Canada for over 25 years. People who ultimately take responsibility for the products specified are understandably cautious, potentially too cautious. It's far easier to sign off a traditional system consisting of boards or wool with recognised (theoretical) U-values than it is to approve something new. The "ain't broke don't fix it" approach to building is fine when energy costs are low, but as heat and power costs start to exceed mortgage repayments something clearly is broken. Keeping a leaky house warm ultimately become unsustainable as energy costs continue to rise. Fortunately the 2014 changes to building regulations are starting to address this, but those changes will only make a difference to new houses that aren't even on the drawing board yet. Existing and even many new houses will continue to leak until they're fixed.

First impressions can be misleading. Where 150mm of spray foam might cost £25/m2, a budget insulated panel that gives the same theoretical performance might only cost £10/m2. But think about what needs to happen to that panel. It needs to be delivered or collected from the merchant, stored on site undercover, cut to size as accurately as possible (always wear a mask), carried up and down ladders until it fits perfectly, supported in place, taped and/or foamed in place. The considerable waste needs sweeping and bagging up then removing from site prior to paying gate fees at the local landfill. The material cost might appear to be less but the lifecycle cost is a great deal more. Spray foam costs include supply and fit, materials delivered, stored and waste removed. The whole job takes around 20% of the time it takes to fit boards, meaning the whole project is that much quicker. There are far fewer road miles and far less disruption overall.

In truth it's probably not any cheaper to use spray foam instead of PUR boards or wool, but you'll certainly get a far better result in less time and with less mess. You get piece of mind safe in the knowledge the whole structure is sealed with no hidden air gaps above the plasterboard, and you get lower heating bills for the life of the property.