Summary
- Part D is the shortest Approved Document: it controls fumes from urea formaldehyde (UF) foam cavity insulation, requiring a continuous barrier between the insulation and occupied space and suitability of the wall.
- It matters most on retrofit cavity fill and on older properties where historic UF foam is discovered during works.
- Compliance is simple to evidence: record the insulation product, its certification (BBA or equivalent) and the installer's declaration in your requirements tracker — a two-line entry that closes the part cleanly.
What the Building Safety Act requires for Part D on non-HRB domestic jobs
Requirement D1 states that if insulating material is inserted into a cavity wall, reasonable precautions must be taken to prevent permeation of toxic fumes into occupied parts of the building. In modern practice this targets urea formaldehyde foam: the inner leaf must provide a continuous barrier and the wall must be assessed as suitable before filling.
For the Principal Designer (BSA) on a domestic refurbishment or extension the duty is proportionate: confirm what insulation is being injected or installed, check it holds third-party certification for the wall construction in question, and log it. Where your thermal upgrade strategy under Part L involves retrofit cavity fill, Part D is the companion check that stops it becoming an indoor air quality problem.
Where existing UF foam is found during opening-up works, record its condition and any advice given — disturbed historic foam is an occasional source of formaldehyde odour complaints.
Key design risks and evidence needed (RIBA Stages 2–4)
- Stage 2: note whether the thermal strategy involves cavity fill; flag older or exposed walls where suitability assessment is needed.
- Stage 3: coordinate cavity fill with new openings, cavity trays and ventilation paths so barriers stay continuous.
- Stage 4: specify the insulation system by name with its BBA (or equivalent) certificate number and require an installer's compliance declaration as a handover document.
Standards and references you should be citing
- BS 5617:1985 — specification for urea formaldehyde foam systems for cavity walls.
- BS 5618:1985 — code of practice for thermal insulation of cavity walls using UF foam, including wall suitability assessment.
- BBA/KIWA third-party certification for any injected insulation system — cite the certificate number in the specification.
- CIGA (Cavity Insulation Guarantee Agency) guarantee documentation for retrofit fill.
Part D at a glance
| Scenario | Part D action | Evidence to log |
|---|---|---|
| New cavity walls with built-in insulation | Standard products — no UF issue | Product spec entry; mark Part D 'considered, low risk' |
| Retrofit injected cavity fill | Suitability assessment + certified system | BBA certificate no., installer declaration, CIGA guarantee |
| Historic UF foam discovered | Assess condition, advise client | Site note and advice record |
Common pitfalls that catch out sole practitioners
- Marking Part D 'not applicable' with no note — a one-line 'considered: no cavity insulation being injected' entry is stronger evidence of a managed process.
- Retrofit fill specified generically ('cavity wall insulation by specialist'), losing the certification trail Part D relies on.
- Assuming Part D covers all toxic materials — asbestos and contaminated land sit under CDM 2015 and Part C respectively, not Part D.
Frequently asked questions
Is Part D still relevant if nobody uses urea formaldehyde foam any more?
Yes, in two ways: it still applies where any insulating material is inserted into a cavity (so your specification and certification trail is the compliance evidence), and UF foam does survive in the existing housing stock, where it can be disturbed by alteration works. A short, dated entry in your requirements tracker closes it either way.
Does Part D cover asbestos found during a refurbishment?
No. Asbestos is managed under the Control of Asbestos Regulations 2012 and your CDM 2015 duties (pre-construction information, surveys). Part D is limited to fumes from cavity insulation. Record asbestos findings in your CDM documentation instead.
Track this natively instead of in Word templates
PD Pathway gives small practices an automated Parts A–T relevant-requirements tracker, stage-by-stage Principal Designer checklists and one-click compliance documents — built strictly for non-HRB projects. First project free, then from £10 per project or £99/yr unlimited.