Achieving a continuous and efficient wind tightness and airtight layer is only half the battle. Thermal bypass (commonly known as wind washing) can have a detrimental effect on a buildings heating efficiency.
Why invest in Wind Tightness? Penetration of air into and around the insulating fabric from outside, reduces the performance of the insulation. Thermal bypass causes the air circulation within the space, or air passing through the space from inside to outside the thermal envelope. Thermal bypass can reduce performance of an insulated building element by 40-70%.
Insulation is not fully effective unless installed without gaps, voids and compression. We should align it with a continuous and durable air barrier.
Generally, a designated wind tight layer insulates buildings externally, to prevent thermal bypass. The wind tightness 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. It also improves thermal performance of your insulation by keeping air out, reducing wind washing.
Wind washing is common in pitched roofs where ventilation is required and wind tightness not always achieved. 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, the vapour pressure differential, or gradient, for most of the year will be from inside to. Building materials should be sequenced to be more vapour permeable from inside to out. Passive house systems provide a variety of external tapes and sealants that are specifically designed in achieving effective airtightness layer.
The insulation protects against the elements on the outside by the external breather membranes and on the inside by the airtight seal (e.g. vapour check) in this case the PHS membrane. The construction is wind-tight externally and air-tight internally. That keeps the insulation nice and cosy, giving it every opportunity to work effectively.