A holiday let property in Arnside was suffering severe condensation in the loft space caused by inadequate vapour control in the ceiling construction. Our vapour barrier specialists installed a continuous vapour control layer (VCL) across the entire ceiling plane, combined with improved ventilation, eliminating the condensation that was causing timber decay and mould growth.
This popular holiday let in Arnside overlooking Morecambe Bay accommodates up to 8 guests, generating significant moisture from cooking, bathing, and breathing — particularly during winter lets when windows remain closed. The property had been upgraded with thick loft insulation to improve energy ratings, but no vapour control layer was installed beneath the insulation. Warm, moist air from the living spaces was passing freely through the ceiling into the cold loft space where it condensed on the cold roof timbers and the underside of the breathable membrane. Black mould was growing extensively across the rafters and purlins, the insulation was damp and compacted, and water was dripping from the most affected areas back through the ceiling, causing staining in the bedrooms. A structural surveyor had warned that the timber decay, if left untreated, would compromise the roof structure within 5 years.
We accessed the loft space and temporarily rolled back all existing insulation. The timber surfaces were treated with a fungicidal wash and allowed to dry thoroughly using temporary dehumidifiers over 5 days. A continuous vapour control layer (VCL) of 500-gauge polythene was then installed across the entire ceiling plane. Every sheet overlap was sealed with specialist VCL tape, and every penetration — light fittings, cables, pipes, loft hatch — was individually sealed with grommets and tape to create an airtight barrier. The loft hatch itself was upgraded with a draught-sealed, insulated unit. The mineral wool insulation was replaced with new 300mm material laid in two layers at right angles for consistent coverage. We also installed additional roof ventilation — tile vents at low level and ridge ventilation at high level — to ensure that any residual moisture in the loft space is carried away by airflow rather than condensing on cold surfaces.
The loft space has remained completely dry since the VCL installation, even during fully occupied winter lets. The combination of effective vapour control and improved ventilation has eliminated the condensation cycle entirely. A follow-up inspection six months later confirmed that the treated timbers had dried completely, the mould had not returned, and the new insulation was performing at full efficiency. The property's EPC rating improved from D to C following the insulation upgrade. The property owner was particularly relieved to have avoided the costly structural roof repairs that the surveyor had warned about. We provided a 10-year guarantee on the VCL installation.
The vapour control layer is a 125-micron (500-gauge) low-density polythene sheet that provides a vapour resistance of approximately 250 MN·s/g. This is sufficient to reduce moisture transmission through the ceiling by over 95%, preventing the vast majority of warm, moist air from reaching the cold loft space above. The material is robust enough to withstand installation handling without puncturing.
All sheet overlaps and penetration seals used a specialist single-sided butyl rubber tape with a polyethylene carrier. This tape provides permanent adhesion to the polythene VCL, maintaining an airtight seal even under the temperature and humidity fluctuations experienced in a loft space. Standard adhesive tapes are not suitable as they lose adhesion over time.
The 300mm replacement insulation achieves a U-value of approximately 0.13 W/m²K — significantly exceeding current Building Regulation requirements. Laid in two 150mm layers at right angles, the cross-laid pattern eliminates gaps at joist locations that reduce the performance of single-layer installations. The insulation sits above the VCL, with the vapour barrier on the warm side as required by building physics.
New ventilation tile vents at low level and improved ridge ventilation at high level create a continuous airflow path through the loft space. This cross-ventilation carries away any residual moisture that penetrates the VCL, preventing it from accumulating and condensing on cold surfaces.
Condensation causing mould and timber damage? Our vapour barrier specialists serve Arnside, Grange, Kendal and all surrounding areas. We install VCL systems, improve ventilation, and solve loft condensation permanently. Contact us for a free assessment.
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