Air leakage expert Paul Jennings looks at how best to achieve that all important airtightness when using particular construction methods.
Interview with Paul Jennings
Paul Jennings is an air leakage expert with over 25 years experience. In our previous interview with him he explained how to achieve airtightness in low energy buildings. He also appeared in our documentary “The Future of Housing – and How Airtightness Can Help”, taking a look at housing construction quality and building performance to see how they could be improved.
For this podcast, he looks at some of the issues with achieving airtightness in different construction types.
Cross Laminated Timber (CLT)
Paul recently tested a bungalow that was built using Cross Laminated Timber. The airtightness is essentially in the timber, and in this bungalow there were up to 8 laminations in each of the panels. The roof was a sandwich panel with layers of lamination, insulation, and more laminations on top. With the panels being larger than standard OSB boards, there is less taping involved. Each panel is also very strong, and strengthened further when screwed together. The taping goes where the panels butt up to each other, around windows, ventilation ducts, sun pipes etc. Once you have taped to the timber you have airtightness.
Paul is starting to think that there is no need for a membrane on the outside. The CLT doesn't meet insulation values but he thinks it's perhaps easiest to insulate it externally and make it airtight on the inside.
Structural Insulated Panel Systems (SIPS)
Paul has experienced very good results with SIPS panels, which he feels have the potential to be very airtight. So far he hasn't seen anyone using the airtight OSB as one of the faces of a SIPS panel system, which he thinks could improve it further.
He explains there is a difference in results depending on whether foam is injected into the pre-made box, or whether a piece of foam is cut and put into a section of wood which then has sides added. The latter won't be as airtight.
SIPS panels can be a problem on site if they get saturated with water as they take such a long time to dry out, and the airtightness tapes do not adhere properly.
Masonry construction ticks the box of being a system that normal builders are used to building, where most materials can be sourced from a standard builders' merchant. Paul offers Denby Dale Passivhaus as a great example of this.
Paul explains that any construction with a cavity in, is a weakness for airtightness. If air can move laterally it can get around things and leak where you wouldn't expect it to. He prefers a masonry build that has a masonry wall which is ideally wet plastered for airtightness on the inside, with insulation and render on the outside.
“Because at that point, you have to have a continuous hole from the inside to the outside. You can’t have a hole through the inner leaf, then air moving in which way it likes in a cavity and then coming out somewhere else. It’s more robust for airtightness.”
Paul points out that almost every masonry build has a timber frame roof, so as soon as you've opted for masonry construction you've doubled the number the number of build forms in your house anyway.
Paul emphasises how the more complicated a build is, the harder it will be to achieve the airtightness. There is greater scope for people on site getting confused, doing the wrong thing or not checking details properly.
This isn't a widely used construction method in the UK but there are increasing numbers of companies trying to develop it. The benefits of this method are that production has less of an issue with weather, shortage of trades people etc, than for on-site construction. The ones Paul has seen have been generally at a prototype stage with initial positive signs and great potential. He believes they will have to be built to be incredibly robust, as otherwise they wouldn't survive being craned on to a lorry, driven to the site and craned off again.
One of the big advantages of modular construction is there is less time spent on site, which means less financial impact from weather delays, and less overheads on things like site supervision, site cabins and toilets.
Airtight OSB is gaining in popularity
The trend that Paul is seeing at the moment is that membranes are losing out to the airtight Oriented Strand Board (OSB) and, for refurbishment particularly, to blowerproof paint. The most popular he currently sees in the market, and which he thinks are proving to be very successful, are the Durelis and the Propassiv.
“At the same time, the refurbishment market is a bigger market than the new build market. We need to tackle refurbishment and we need to tackle it better. And we’re not. We’re learning lessons but really we’re taking our time over getting it and I don’t think we can afford that time.”
Choose the right tape for the job
Choose the right manufacturer, and the right tape. For example, don't try and use 50mm tape to cover a gap of 60mm. Choose one that is wide enough for the gap.
Plan early and get advice from the experts
“You really want to get your designing done early, get your planning done early and then don’t mess with it. Because every time you change it, there will be unforeseen consequences. Usually those will be things that make your airtightness worse or harder at least.”
Paul encourages self builders to take advantage of the Passivhaus Open Days and talking to the people who have built them. There are also the AECB and UK Passivhaus conferences that people can attend and find out more about the network of experts.
“There are networks of people and a lot of them are very committed people. They give their time, they give their energy and they often don’t charge as much as they should for it, because they believe in it.”
Find out more
Visit the website of Design Buro
Download the transcript of the interview with Paul Jennings.