Approved Document D (Toxic Substances): What Homeowners Need to Know

The Building Regulations requirement under Part D is simple: adequate precautions must be taken to prevent substances used in the construction of a building from causing harm to the occupants. In practice, this almost exclusively applies to insulation materials injected into cavity walls.

The primary concern is urea-formaldehyde (UF) foam, which can off-gas formaldehyde — a known irritant and potential carcinogen — into the living space if the product is not certified to an appropriate standard or if it is incorrectly installed.

Other insulation materials (such as mineral wool, EPS bead, or injected polyurethane foam) are not specifically excluded by Part D, but the compliance regime is overwhelmingly focused on UF foam products.

Part D is most likely to be relevant to your project in the following situations:

• Installing cavity wall insulation — whether in an existing house or as part of new construction with filled cavities
• Using spray foam insulation — particularly products injected into roof voids or wall cavities
• A new extension with insulated masonry cavity walls — if injected insulation is specified

Part D does not apply to:

• External rigid insulation boards (such as PIR or phenolic foam used on the outside of a building)
• Internal dry-lining insulation
• Standard structural and finish materials that do not off-gas toxic substances

Compliance with Part D for UF foam cavity wall insulation is normally demonstrated by using a product that holds a current British Board of Agrement (BBA) certificate — or an equivalent European Technical Assessment (ETA) — covering its use in the specific cavity type and thickness you have.

The BBA tests products against BS 5617 (specification for UF foam systems) and BS 5618 (code of practice for installation). A valid certificate confirms that:

• The formaldehyde emission levels are within acceptable limits
• The installer is working to a defined, tested specification
• The product is appropriate for the cavity width and wall construction type

You should always ask your installer for evidence of BBA certification for any injected insulation product. If a contractor cannot produce this, the work is unlikely to comply with Part D.

Spray polyurethane (SPF) and UF foam applied inside roof voids have become controversial in recent years — not solely because of Part D, but because of a combination of concerns:

• Mortgage refusals — many lenders refuse to mortgage properties with spray foam in the roof space because it can mask structural defects and make future repairs difficult
• Vapour control issues — incorrectly applied spray foam can trap moisture and accelerate timber decay
• Part D compliance — only products with a valid BBA or equivalent certificate meet the Approved Document D requirement

If you are buying a property with spray foam insulation, or if a contractor proposes using it, take independent advice before proceeding. Removal can be costly, and the impact on saleability is real.

For most standard householder projects, Part D is not a primary concern:

• Rear extensions — if your extension uses rigid insulation board or mineral wool batts in a conventional masonry or timber frame build, Part D does not impose additional requirements beyond choosing suitable, non-toxic materials
• Loft conversions — if you are using rigid PIR boards or mineral wool quilt between rafters, Part D does not specifically apply. The concern arises if a contractor suggests injecting foam into existing cavity sections during the conversion
• New builds — Part D applies across the board, but for standard construction using conventional insulation products, compliance is straightforward

The practical rule is: if nobody is injecting foam into a cavity, Part D is unlikely to be a live issue on your project.

Your building control body (BCB) — whether the local authority or an approved inspector (registered building control approver) — will check for Part D compliance where it is relevant. In practice, this usually means:

• Asking to see the BBA certificate for any injected insulation product
• Checking that the product is being installed by a registered installer (the BBA certificate will specify the installer requirements)
• Confirming that the cavity width and wall type match the certificate scope

Building control is unlikely to inspect every stage of insulation installation, which is why it is important to use reputable, certificated installers rather than relying solely on a post-installation sign-off.

If your project involves any injected or spray-applied insulation, work through this checklist before allowing work to proceed:

• Ask for the full BBA or ETA certificate number and check it on the BBA website
• Confirm the certificate is current (not expired)
• Confirm the cavity width and wall construction type is within the certificate scope
• Ask for the installer registration number under the relevant scheme (for example, the National Insulation Association or CIGA for cavity wall insulation)
• Keep copies of all certificates — you will need them if you sell or remortgage the property
• If spray foam is proposed for a roof void, seek independent structural advice before agreeing