Internal wall insulation in a damp solid-brick house
You are the retrofit assessor for a Victorian end-terrace house with 220 mm solid brick walls, cement render externally and lime plaster internally. The EPC is F (score 28). The owner has a grant for internal wall insulation and an air-source heat pump and wants work to start in four weeks. You find damp staining at the base of the rear wall, blocked air bricks, 100 mm loft insulation and single-glazed sash windows. A builder proposes 100 mm PIR insulation bonded directly to the internal walls, sealed with a vapour barrier, to achieve a target U-value of 0.30 W/m²K. The owner asks whether this is suitable and whether the grant can be used before any damp work.
1. Evaluate the builder's proposed internal wall insulation for this house.
Read the worked answer
The proposal is high risk in its current form. A 220 mm solid brick wall with cement render externally and lime plaster internally is moisture-sensitive; bonding 100 mm PIR directly to the internal face and sealing it with a vapour barrier can trap moisture and create interstitial condensation, especially where joists, lintels and wall plates are embedded. The target U-value of 0.30 W/m²K may be achievable, but the U-value alone is not the test: the detail must manage moisture and thermal bridging.
The existing damp staining, blocked air bricks and high likelihood of rain penetration through the cement render must be addressed first. A hygrothermal assessment should be carried out, and the insulation should be breathable and well detailed, with a continuous vapour control layer, service void and ventilation strategy appropriate to the building. The base of the wall, floor junctions and window reveals need particular attention. I would not approve direct-bonded PIR as a default solution without a whole-dwelling assessment and a moisture-risk assessment.
2. What sequencing and conditions would you require before the grant-funded works start?
Read the worked answer
Before IWI or the heat pump is installed, the damp must be diagnosed and corrected: clear and repair the blocked air bricks, check external ground levels, rainwater goods and render defects, and allow the wall to dry. The loft insulation should be upgraded and the ventilation strategy agreed; the single-glazed sash windows may need repair or secondary glazing rather than wholesale replacement. The heat pump should be sized only after the fabric improvements are decided, not before, because the heating load will change.
Under a PAS 2035-style process, the work should be led by a retrofit coordinator with a whole-house assessment, a medium-term improvement plan and a clear roles-and-responsibilities matrix. The grant should not be used to install insulation over an unresolved damp problem, because that would risk the building and may breach the funding conditions. I would require a written moisture-risk assessment, a detailed specification, competent installers and a post-installation monitoring and commissioning plan.
3. How would you explain the risks and alternatives to the owner in plain language?
Read the worked answer
I would explain that the house is built to breathe, and that trapping damp behind a sealed insulation board is a common cause of hidden condensation and timber decay. The grant is an opportunity, but it should not drive the order of work: fixing the damp and ventilation first is part of the retrofit, not a separate optional extra. I would describe the possible alternatives: external wall insulation, which may be more suitable for some elevations but changes the appearance and needs planning consideration; breathable internal insulation with a careful vapour control layer; or a phased approach starting with loft insulation, draught-proofing and ventilation improvements.
I would be clear that the cheapest headline U-value is not necessarily the best value if it creates damp risk. I would give the owner a written summary of the options, the indicative costs and the sequencing, and record that the decision rests with them. If the owner insisted on proceeding without the damp work, I would decline to sign off the design as suitable and would advise them to obtain independent specialist advice.