A single-storey extension in Millom had persistent condensation dripping from the flat roof deck despite having insulation above. Our vapour barrier team identified the missing VCL as the root cause and retrofitted a continuous vapour control layer from below, permanently solving the condensation problem without disturbing the existing EPDM roof above.
This kitchen-diner extension in Millom had been built with a warm flat roof construction — EPDM membrane over insulation over a structural OSB deck. The EPDM was performing perfectly and the roof was not leaking. However, the builder had omitted the vapour control layer from beneath the insulation, and the warm, moisture-laden air from the kitchen was passing through the OSB deck and condensing on the cold underside of the insulation boards above. This condensation was dripping back down through the OSB joints onto the plasterboard ceiling below, creating water stains that the homeowner initially mistook for a roof leak. A roofing surveyor correctly diagnosed the problem as interstitial condensation caused by the missing VCL, rather than a waterproofing failure.
Rather than stripping the perfectly functional EPDM roof to install a VCL from above, we retrofitted the vapour barrier from below. The existing ceiling plasterboard was carefully removed, exposing the underside of the OSB deck and the ceiling joists. All surfaces were dried using dehumidifiers and treated with a fungicidal wash. A high-performance aluminium-foil-faced VCL membrane was then stapled to the underside of the ceiling joists, with all overlaps and perimeter edges sealed with aluminium tape. Every penetration — downlights, extractor fan, electrical cables — was individually sealed. New plasterboard was then installed over the VCL and the ceiling was re-skimmed and decorated. The foil-faced VCL also acts as a radiant barrier, reflecting heat back into the room and improving the extension's thermal performance.
The condensation stopped immediately. Without moisture vapour reaching the cold zone above the deck, there is no condensation to drip back down. A six-month inspection confirmed completely dry conditions throughout the ceiling void. The foil-faced VCL has also noticeably improved the extension's warmth — the homeowner reported reduced heating requirements in the kitchen-diner. We provided a 10-year guarantee on the VCL installation and ceiling work.
The vapour control membrane comprises a reinforced polythene core with aluminium foil laminated to the warm-side face. The foil provides an extremely high vapour resistance (approximately 600 MN·s/g) — far exceeding standard polythene VCLs — virtually eliminating moisture vapour transmission through the ceiling plane. The foil face also acts as a radiant barrier, reflecting infrared heat back into the room and improving the extension's thermal efficiency.
All overlaps and seals used a pressure-sensitive aluminium foil tape that bonds permanently to the foil face of the VCL. This tape maintains its adhesion at temperatures from -30°C to +80°C, ensuring the vapour barrier remains airtight through all seasonal conditions.
The VCL perimeter was sealed to the walls using a permanently flexible acoustic sealant that maintains an airtight bond while accommodating the slight structural movement between the ceiling and walls. This flexible seal prevents moisture-laden air from bypassing the VCL at the edges.
The replacement ceiling was installed using moisture-resistant plasterboard (green board), providing additional protection in this kitchen environment where humidity levels are naturally higher. The board's moisture-resistant core prevents the swelling and deterioration that can affect standard plasterboard in humid conditions.
Condensation causing ceiling damage in your extension? Our vapour barrier team serves Millom and all of South West Cumbria with expert VCL installation and condensation solutions. Contact us for a free diagnostic survey.
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