Pitched-roof rafter insulation: only with a roof design | Homecode

What is known — and what is not

When this improvement may fit

• There is a reason to bring the roof volume inside the insulated envelope. Evidence needed: Use and options appraisal against ceiling-level insulation. [9]
• Roof covering, underlay, rafters and moisture condition are known and sound. Evidence needed: Independent roof survey. [9]
• Insulation, vapour/air control, ventilation and every junction form a buildable continuous system. Evidence needed: Hygrothermal and construction design. [9][3]

When it may not fit

• A cold loft would meet the need more simply at ceiling level. Decision: Prefer the simpler boundary unless storage, services or habitable use justify rafter-level work. [9]
• Spray foam would hide the roof structure or be applied directly to tiles or unknown underlay without property-specific design. Decision: Do not proceed; obtain independent advice and consider inspectable alternatives. [9]
• Active damp, water ingress, condensation, mould, decay or a fabric defect has not been diagnosed and resolved. Decision: Do not cover the symptom with insulation. Repair the cause, verify drying and then reassess the measure. [9]
• The post-retrofit ventilation and moisture strategy is unknown, or the work would block an airbrick, trickle vent, eaves route, sub-floor path, flue or other designed opening. Decision: Pause for a ventilation and moisture assessment; a warmer or tighter element is not a reason to remove required airflow. [9][3]
• The work would disturb unknown material in a home or part refurbished before 2000, or suspect board, coating, lagging, loose fill, tile or adhesive is present. Decision: Do not disturb it. The asbestos survey scope must match the planned work before drilling, cutting, lifting, stripping or removal. [9][8]
• The proposal affects listed fabric, a protected elevation, a conservation-area character feature, a leasehold/common part or another owner's structure without the required advice or consent. Decision: This is a consent and compatibility stop, not proof the measure can never work. Obtain written decisions before specifying the installation. [9]

Assessment before spending

• Diagnosis first: Determine why rafter-level insulation is needed and establish the complete roof assembly and moisture condition. [9]
• Why is ceiling-level insulation not the better boundary? [9]
• What covering, battens, underlay and ventilation exist? [9][3]
• Can rafters safely take fixings and remain inspectable? [9]
• What happens at eaves, ridge, gables, party walls, dormers and rooflights? [9][3]
• Will existing ceiling insulation create an unintended intermediate cold zone? [9]
• What are the property and refurbishment dates, and does the planned work area have asbestos information appropriate to the intrusive scope? [9][8]
• Are there active leaks, damp, mould, salts, rot, corrosion or defective rainwater details, and has the source been diagnosed rather than labelled from appearance? [9]
• Which background, extract, sub-floor, roof-space or combustion ventilation paths could the measure change? [9][3]
• Is the home listed, in a conservation or other designated area, leasehold, or subject to freeholder, lender, insurer or planning restrictions? [9]
• What does the current EPC actually record, and has the construction changed since that assessment? [9][1]
• Independent roof survey [9]
• Roof build-up and underlay identification [9]
• Moisture/condensation design [9]
• Junction and service drawings [9]
• Product/system and lender-facing documentation where spray foam is considered [9]
• Decision output: Require a written suitable / suitable with conditions / not suitable decision, with reasons, excluded areas and dependencies. [9]
• Likely mechanism: Rafter-level insulation reduces heat flow through the sloping roof and brings some or all of the roof space into the insulated volume. [9]
• Lower heat loss through roof slopes [9]
• A warmer usable roof volume [9]
• Potentially improved comfort where sloping ceilings bound occupied rooms [9]
• Uncertainty: Moisture risk depends on the exact roof assembly and climate. Gaps, thermal bridges, hidden timber and incompatible layers can undermine the result. [9]
• Not guaranteed: No bill saving, EPC change, comfort result, payback, mortgage acceptance, sale price or rent increase is guaranteed. [9][1][4]

Safe observations only

• Collect roof-covering, underlay, loft conversion and insulation records. Boundary: Do not expose layers yourself. [9]
• Record staining, odour and seasonal condensation from safe occupied areas. Boundary: Do not enter unsafe voids. [9]
• Ask an independent surveyor to compare ceiling and rafter boundaries. Boundary: Do not accept spray foam as the default answer. [9]

Stop conditions and professional route

• Do not apply spray foam directly to roof tiles or unknown underlay. [9]
• Do not sandwich a cold, unventilated zone between ceiling and rafter insulation. [9]
• Do not cover wet timber, cables, lights or fire-stopping. [9]
• Do not treat insulation as a cure for an undiagnosed damp or mould symptom. [9]
• Do not block or reduce designed ventilation, drainage, flues, airbricks or fire-stopping. [9][3]
• Do not drill, cut, lift or strip unknown pre-2000 fabric without asbestos information that covers the exact work. [9][8]
• Do not rely on a product brochure, an EPC recommendation or a grant offer as a property-specific suitability survey. [9][1]
• Next assessor: An independent building surveyor or retrofit roof designer with hygrothermal and pitched-roof expertise. [9]
• Installer: A competent roofing/insulation contractor working to the complete designed system, with qualified electrical trades where needed. [9]
• Relevant pitched-roof and moisture-design experience [9]
• System-specific training and evidence for the exact build-up [9]
• Independent rather than sales-only roof condition advice [9]
• For government-funded work, verify current TrustMark registration and PAS 2030 certification for the exact measure, with PAS 2035 roles where the scheme requires them. [9][5][7]
• For privately funded work, verify relevant experience, insurance, references, product-system authorisation, written scope, exclusions, guarantees and a complaints route; TrustMark is a quality option, not a universal legal licence. [9][5][7]
• Check Building Regulations for roof, fire, thermal and ventilation work [9][3][13][12]
• Planning/listed-building consent may be needed where roof height, covering or appearance changes [9]
• Check lender and insurer evidence requirements before spray foam [9]
• Survey, design and moisture/ventilation assumptions [9][3]
• Building control or competent-person evidence where applicable [9][13][12]
• Product and system specification, photographs of concealed details and commissioning checks [9]
• Guarantee, maintenance instructions, invoice and complaint route [9]

EPC, grant and property evidence

• Relationship: An EPC roof recommendation does not choose between ceiling and rafter level or establish underlay, timber condition, ventilation and lender acceptance. [1]
• Limit: An EPC recommendation is modelled decision support, not a condition survey, moisture design, quotation or assurance that this measure fits the property. [1]
• Post work: Keep installation evidence and consider an updated EPC where a current certificate or letting decision needs to reflect completed work. [1]
• ECO4 was current and scheduled to run to 31 December 2026 as reviewed on 2026-08-13, but household eligibility, measure packaging, contributions and delivery remain case-specific. Treat this page as measure guidance, not an ECO4 approval; recheck Ofgem and the obligated supplier before relying on the route. [5]
• Condition and sale: Inspectability and records matter. Undocumented spray foam or concealed timbers can complicate survey and lending even though spray foam is not automatically a defect in every case. [9]
• Letting: Landlords must coordinate safe access, room loss, fire/escape and ventilation, and retain complete roof records. Tenants should not enter voids or authorise roof alterations. [9][3][4]
• Pre-work survey and photographs [9]
• Permissions, notices and building-control route [9]
• Design, quotation, variations and installer credentials [9][7]
• Concealed-work photographs, completion records and guarantee [9]
• Maintenance and any post-work moisture or ventilation checks [9][3]
• Value statement: Treat any market effect as property-specific and evidence-dependent; this page makes no claim that the measure increases value or rent. [9]
• Why not insulate at ceiling level? For a normal cold loft, ceiling level is often simpler. Rafter level is for a roof volume that needs to be within the insulated envelope. [9]
• Does spray foam automatically make a home unmortgageable? No universal outcome can be promised. Lenders assess evidence and roof condition differently, so obtain independent and lender-specific advice before installation. [9]
• Can existing roof ventilation simply be blocked? No. The designer must establish the correct moisture-control strategy for the exact underlay and roof build-up. [9][3]

Stay off roofs, ladders and guttering

• HSE's work-at-height hierarchy starts by avoiding work at height and doing as much as possible from the ground, for example with extendable tools. [14]
• HSE says you are working at height if you are on a ladder or a flat roof, or could fall through a fragile surface. If this task needs roof access, a high ladder, guttering access or improvised access, stop and use a competent contractor. [14]
• Do not go onto a roof, lean a ladder on guttering, or use furniture or another improvised platform. This page does not teach ladder setup or roof walking. [14]