Using Window Insulation Tapes and achieving a continuous and efficient airtight layer is only half the battle.
Thermal bypass (commonly known as wind-washing) can have a detrimental effect on a buildings heating efficiency. Penetration of air into and around the insulating fabric from outside, reduces the performance of the insulation. Air passing through the space from inside to outside of the thermal envelope causes wind-washing. Thermal bypass can reduce performance of an insulated building element by 40-70%. That’s the main reason why window insulation tapes are so important.
Insulation is not fully effective unless installed without gaps, compressions and alignement with a continuous and durable air barrier.
Generally, a designated wind-tight layer used with externally insulated buildings prevents thermal bypass. The wind-tight layer is often a breather membrane with a suitable external tape at the membrane overlaps, such as Gerband 587 or 386. This keeps the building warm similarly to a protective wind-breaker, improving thermal performance of your insulation by keeping air out.
Wind-washing – very common in pitched roofs where required ventilation and wind-tightness not always achieved. Wind-washing will most likely compromise the performance of fibrous insulation products, such as fiberglass and mineral wool. The cold air can move freely through the fibers, cooling the material with very significant negative affect on the heat retention of the roof’s insulation.
This wind may never affect the airtightness but can cause a detrimental effect to the performance of the insulation. Wind tightness and airtightness work in tandem to harness the best performance from the building fabric and heating system.
The wind-tightness layer is also critical in preventing the occurrence of interstitial condensation. In cool, temperate climates such as Ireland, the vapour pressure differential, or gradient, for most of the year will be from inside to outside. Building materials should be sequenced to be more vapour permeable from inside to out.