The short answer
Spray foam can save energy, because it insulates well and seals air leaks in one step, so less heat escapes through the roof. How much you save depends on what you had before, the thickness applied and how airtight the rest of the house is. A loft with thin insulation will gain more than one with a deep, well-laid quilt, and any saving is wasted if damp damages the roof.
Where the savings come from
Heat escapes through a roof in two ways: through the material and through gaps. Spray foam addresses both. The foam itself has good insulating properties, particularly closed-cell foam, which is denser and resists heat flow well for its thickness. And because it is sprayed on as a liquid that expands and sets, it fills awkward corners, seals round pipes and wiring, and closes the small gaps at the eaves that let warm air out. A loft with a thin layer of old mineral wool and plenty of draughts can feel noticeably warmer after foaming, and the heating works less hard.
That is the best case. The result depends on the starting point, and we do not publish a typical percentage, because honest figures vary with the house. Savings are largest in a house with little or no insulation, and smaller where the loft already has a deep, well-fitted layer. In that case, topping it up with mineral wool or sheep's wool costs less and does nearly as much. See spray foam alternatives for the other options.
The scale of the roof matters. Heat loss through the roof is only part of the picture, because walls, windows and floors lose heat too, and in many homes the walls lose more than the roof. If the loft is already reasonably insulated, the money may do more good spent on draught-proofing, glazing or wall insulation, so a sensible plan looks at the whole building before choosing a product.
Open cell and closed cell
The two foam types perform differently. Open-cell foam is softer, lighter and cheaper, and it needs more thickness to give the same insulation. Closed-cell foam is denser, insulates better per millimetre and adds some rigidity. Typical UK costs are reported at around £20 to £35 per square metre for open cell and £40 to £60 for closed cell, and a fixed written quote comes after the survey. Our comparison of open-cell and closed-cell spray foam goes into the trade-offs.
Cost matters when you judge the saving. The money you spend on installation has to be set against how much your heating bill falls, and for many houses the payback period is long. Our page on spray foam insulation cost sets out what drives the price.
What can undo the saving
An energy saving that costs you the roof is no saving at all. Foam can trap moisture against timbers, and the RICS consumer guide warns that it can make the roof hard to inspect. If condensation or a leak damages the rafters, repair costs can wipe out years of lower bills. Some lenders have declined mortgages on homes with spray foam too, positions that vary and change, so the effect on selling is a cost to weigh. The saving is best judged along with those risks, not in isolation.
Thickness and application also matter. Foam applied unevenly, too thin in places or over damp timber, loses much of the benefit. And a warm roof only works when the rest of the house is reasonably airtight, since leakage elsewhere carries the heat out regardless.
Comfort differs from energy use, too. Many people notice a warmer top floor in winter and a less stuffy one in summer after foaming, because the air barrier stops draughts and the roof no longer leaks warm air. That feeling is real, but it is not the same as a measured drop in the gas bill, and it does not offset a hidden moisture problem. A fair test is to ask a surveyor what the existing roof timbers look like, then decide whether the comfort gain is worth the risk.
How to decide
Ask what is in your loft now and how well it is laid. If you have bare joists or a thin, patchy layer, almost any proper insulation will make a visible difference, and foam is one option among several. If you already have deep insulation, foam adds little. If you are thinking of having foam removed because of worries about damp or a lender, ask how the roof will be insulated afterwards, since the aim is a dry roof that also keeps heat in.
We survey roofs, not heating systems, so we cannot calculate your bills. What we can do is inspect the foam and the timber, photograph what we find and tell you whether it can stay or should go. The service is described on spray foam work across Basingstoke, and any price is fixed in writing after the survey.
Every roof is different. A free survey with photos settles it for yours.
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