New build: only the best will do

New build: only the best will do

BBA Certified products were the only choice for this project

When it comes to specifying materials for a high end development, choosing quality BBA-certified products is a must. The houses in this particular small development in Hertfordshire incorporate many energy-saving features, are airtightness-tested and built to the highest standards – and this extends to the choice of roof insulation.

Nice deep rafters? Lots of insulation then…

The roofs have 200mm deep rafters - designed as part of a Fabric First approach, to allow sufficient room to accommodate the insulation required to achieve the low U-values that a SAP calculation may require. However this also made possible the use of mineral wool, which, post-Grenfell, is increasingly favoured by purchasers for its fire resistance, as well as offering a cheaper solution and better soundproofing than rigid board.

insulating breather membrane new buildDeep rafters? Also no increase in roof height

TLX Gold can be draped between the rafters and the tile battens fitted directly over. The choice of 140mm of Knauf Earthwool Flexible Slab (λ = 0.035 W/mK) between the rafters allowed for an unventilated heat-reflecting air gap underneath the shiny underside of the TLX Gold, and this contributes to the thermal performance. Being flexible, the mineral wool was able to conform to the exact dimensions of the rafters without any gaps, and with rigid board below – possibly in the form of insulated plasterboard – the desired U-value could be achieved. For example, the mineral wool plus (40+12.5) mm insulated plasterboard achieves 0.15 W/m2K, and (15+12.5) mm would be required for 0.18 W/m2K.

How was condensation risk managed?BBA Approved insulating breather membrane buildup

Interstitial condensation is a particular concern with new builds, as so much drying out occurs over the first year or so. The use of a vapour barrier should largely prevent this, but if it is breached (for example, through down lights being installed) or there is inadequate jointing, then even with high humidity conditions there is no risk of interstitial condensation occurring when TLX Gold is used over the rafters, as it alters the temperature profile such that a dewpoint does not occur.


The problem: the best choice of build up for high-value houses that will achieve the specified low U-values and minimise condensation risk.

The solution: TLX Gold combined with mineral wool between the rafters and rigid board below offers a cost-effective solution with no condensation worries.

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