Airtightness in detail
Airtightness is absolutely key to energy efficient buildings!
We've written a little about the principals before here but we thought we should go a little deeper as it's such an important factor in our design work.
Airtightness detail and blower door test
Airtightness is measured in two ways – building regs use a q50 test which is measured in meters cubed per hour per m2 of floor area (m3/h/m2). PassivHaus uses a slightly different measurement – n50, which is in air changes per hour (ACH). The two amounts can often be similar in numerical terms – eg 1 ACH might equal 1.15m3/h/m2 – but they are different so it is important to understand the two. Either way, the smaller the number the better!
What is required for building regs now depends on the SAP/EPC calculation, which takes into account u-values and energy sources, but the accepted value can be as high as 5 m3/h/m2. PassivHaus by contrast requires a maximum of 0.6 ACH, so you can see there is a huge difference in performance.
Achieving great airtightness can be difficult on new build homes and it is even more challenging on retrofit/refurb projects! The same issues come up on both types of developments – junctions at eaves, intermediate floors, and partition walls, and windows/doors for example. These can all be critical! Typically we (as the designer/contractor/client team) have less choice or freedom on retrofits - access to certain areas might not be possible at all. We've built up some very valuable experience and have discovered a few useful tips along the way. As a result, our projects achieve great scores (often far better than required).
Retrofit Difficulties
Our best result has come back recently for a new-build house. We are very chuffed with 0.35 m3/h/m2 and huge praise must go to Dodden, the contractor's and energy efficiency specialists who delivered this on site. They have also given us assistance in the design stage; specifying products that are great for retrofit projects. We'd certainly recommend reaching out to experts at a very early stage in your projects. Other companies we've found helpful are suppliers of these products like Mike Wye or Back To Earth
Vapour control details
Our 5 top tips:
Design issues out – it's easier to solve issues on the drawing board than on site!
Aim for zero – attention to detail is key!
Appoint an Airtightness Champion on site and train all subcontractors!
Test before the project is complete – so that you have a chance to access holes!
Avoid membranes wherever possible – hard surfaces (like parge coats or air-tight OSB) are easier to deal with!