Our VCL (vapor control layer) is on the inside face of external walls, provided by a foil-back plasterboard. The cavity between the plasterboard and external airtight board is then filled with super-high density cellulose fibre insulation. Some clients opt for a membrane (Intello) approach instead of foil back plasterboard, most commonly when a service cavity is required. We can add a service cavity if required – OSB applied to inside face of external walls, though many clients don’t feel the need.
“Breathing wall” concept
Putting the vapour barrier on the inside surface is standard practice for breathable walls in the UK. The idea behind this is that it makes it difficult for the water vapour to get into the wall from inside. Any moisture that does get into the wall can travel through the wall to outside because the exterior board is vapour permeable, and the cellulose insulation provides moisture transport – a ‘vapour gradient’ from inside to outside.
External airtight board
We use a rigid 15mm thick board on the exterior surface to provide our airtight layer – an approach that is gaining ground in the UK after being more common in European and US passivhaus homes. It has many advantages over a standard internal air-tight layer formed by a membrane:
- Follow-on trades (electrician/plumbers) can work a lot quicker since they have minimal interaction with the airtight layer (unlike with an internal airtight layer). This saves significant labour costs.
- Eliminates the need for a service cavity, so saving a significant amount of money and time in the build process.
- It offers a more reliable air-tight solution as the whole of the exterior of the building is covered with the airtight board, and our detailing makes junctions between roof and wall simple for airtightness, whereas the traditional system involves tricky detailing when wrapped around beams attaching to the wall or roof. We usually achieve 0.3ach – much better than passivhaus
- Any changes to the internal face of the exterior wall does not affect the airtight layer. The services are run within in the walls and so later DIY modifications will not damage the air-tightness
Proof of build resilience
Touchwood Homes has commissioned dynamic seasonal moisture calculations using WUFI, which show there are no issues with moisture in the walls in even the wettest climates in the UK.
On our first building to be Passivhaus certified, the UK assessor was unfamiliar with this build-up method, but we were able to address his concerns, with back-up from the PassivHaus Institute in Germany, who were familiar with this the build-up, and so the certification was completed.
The WUFI calculations were performed by well-regarded Joseph Little architects (http://www.josephlittlearchitects.com/architecture/wufi) in consultation with the Fraunhofer Institute for Building Physics to confirm specific details/modelling assumptions. They show our build method to be acceptable. The conclusion section states “In discussions we had with Fraunhofer IBP staff they informed us that they had also seen successful testing or simulation of this kind of build-up where vapour resistance are well graded but the air barrier is on the outside.” Finally, quite an extensive set of scenarios were examined. Starting with a number of base scenarios considering multiple climate zones continuing to a number of stress cases including high internal moisture, rain penetration in fibreboard, low ventilation in cavity, high indoor air infiltration (i.e. very leaky plasterboard). None of the scenarios result in sufficient moisture build-up to cause damage to the wall/roof. Details of the WUFI calculation can be provided
1. 2013 Ph.D thesis ‘Feasibility of Exterior Air Barriers in Timber Frame Construction’:
There is no condensation risk if the exterior board is airtight, and the insulation buffers moisture and prevents convention currents. This will always occur with densely installed cellulose fibre insulation, but not loose fill rockwool –which is one of the reasons we only use cellulose fibre insulation in our walls. Additionally, the systems is relatively immune to small holes in the internal VCL
2. 2014 Nordic Low Energy building conference presentation
The presentation examined the risk with an exterior air tight layer in the Belgian climate with cellulose fibre and rockwool insulation. In conclusion there is no risk when cellulose fibre insulation is used in the 12 European climates tested. Using rockwool though does create a risk (we always use cellulose fibre insulation).
3. 2010 paper from same group re external airtight layers
External airtight layers can work very well, and achieve passivhaus levels of airtightness (as we have proved). Air leakage can occur between walls and roof though – a detail we have solved in our design, along with the potential air leak between TG board connections (we seal with mastic to overcome this).