New Build Roof Insulation — Braunton, North Devon
From failing roof to finished system
The Challenge
Open web joists solve a structural problem elegantly. Timber top and bottom chords with steel web bracing between them span further than solid timber, weigh less, and let services run straight through without drilling anything.
For insulation they present the opposite of a clean rectangle. The bay is deep. Steel webs cross it diagonally every few hundred millimetres. Structural steel beams interrupt the runs, and hangers and straps appear wherever the design needed them.
Rigid board can be made to work, but every board must be cut to the bay, every joint taped, and every awkward junction packed and sealed by hand. The performance of the finished roof then depends on how well several hundred cut edges were fitted — and any gap left behind a web or against a beam is both a thermal bridge and an air leakage path.
Project file
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The Solution — step by step
Board or spray foam?
Both can reach the target; they fail in different ways. Rigid board is cut to size, so accuracy depends on every cut, air tightness relies on tape and sealant at every joint, and thermal bridging risk sits at every gap and junction. Spray-applied foam expands into the actual shape of the bay, flows around and seals against every obstruction, and cures as one continuous mass with no joints to fail.
On a simple roof with regular rectangular bays the two are close. On this roof — deep bays, steel webs, exposed structural steel and a large vaulted volume — the number of hand-sealed junctions in a boarded solution was exactly the risk the specification was written to remove.
The system
Enverge Sucraseal open-cell spray foam, applied between the rafters in a single operation. Applied as a liquid that expands in place, it takes the exact shape of the space it is sprayed into — around steel webs, against beams, into corners — with no cut edges and no taped joints anywhere in it.
Open cell was chosen deliberately. Unlike closed-cell foam, open-cell has a low resistance to water vapour, so in a warm roof build-up moisture can move through the insulation rather than being trapped behind it. That is a design decision, not a default, which is why it was supported by a static condensation risk analysis rather than assumed.
Evidence before installation
Spray foam in a new build roof attracts scrutiny — from Building Control at the time, and from surveyors and lenders later. The answer to that scrutiny is documentation produced up front.
The roof was surveyed and measured on site at 141 m² on 16 June 2026 before any materials were ordered. The static condensation risk analysis and supporting information were issued to the client for Building Control on 24 June, before installation, so the build-up was agreed on paper before anyone sprayed anything. Installation followed on 2 July in a single visit, and the achieved performance was confirmed afterwards by U-value calculation against the average installed depth.
The Outcome
It is natural to judge a sprayed roof by how deep the foam looks. It is the wrong measure, and on a joisted roof it is misleading.
Foam expands to fill a bay, so the finished surface is never perfectly flat: it sits proud in the middle of a bay and closer to the chord at the edges, and the visible timber face gives the eye a reference line that has nothing to do with thermal performance. A roof can look uneven and be comfortably compliant, or look tidy and miss its target.
What matters is the average installed depth across the roof, measured against the U-value the build-up has to achieve. That is the number Building Control assesses and the number the calculation is run on. Here that number was 294 mm, and the calculated performance was 0.11 W/m²K.
Where a roof is to be fully filled and cut back flush that is a different specification and is priced as such — worth agreeing which of the two is wanted before the rig arrives on site.
141 m² of open web joist roof insulated in a single visit to an average installed depth of 294 mm, achieving the design U-value of 0.11 W/m²K confirmed by calculation.
The insulation is continuous across the whole roof plane, sealed around every steel web, hanger and structural beam, with none of the hand-sealed junctions a boarded solution would have relied on. The 25-year material warranty and 10-year workmanship warranty were issued on 9 July 2026.
On the roof — the full picture
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