Airtightness and natural building materials – Part 2.

With thanks to Es Tresidder who provided invaluable comments, and has also written some insightful stuff on thermal comfort. Check him out at Highland Passive.

In the previous post, we discussed why natural builders should pay attention to airtightness. To recap; air contains water vapour. If warm air from our building is leaking out through the building fabric, then as the air cools down, water will condense in the building fabric. This can potentially cause materials to rot, and give rise to the growth of toxic moulds that can impact on health of the occupants.  Consequently, our optimum solution is a building where we can control air movement as we would want to, independently of prevailing winds or outside temperatures. This means a construction that is as airtight as reasonably practicable, along with some kind of ventilation system. At its simplest, a ventilation system might be extractor fans in the bathroom and kitchen (where most moisture is generated), with openable windows in other rooms. In more complex buildings, or those built to Passivhaus standards, mechanical ventilation with heat recovery is often used; fresh air is supplied by a network of ventilation ducts, with stale air removed by a separate ducting system. The exiting air is passed over a heat exchanger in order to warm the fresh air. Occupants still open and close windows as they wish, but the system can produce a comfortable internal environment without. And since as humans we’re not actually that good at sensing air quality, we might argue that it’s a good thing to have a degree of automation to keep air quality within reasonable limits as opposed to relying on our imperfect senses.

How airtight do we need our buildings to be?

If our primary concern is climate change, then we should be pursuing energy efficient buildings that are easy to heat. For the vast majority of new buildings the energy used over the lifespan of a building is far greater than the energy required to make the construction materials. It’s only when we build to extremely high energy efficiency standards (e.g. Passivhaus, which is less than 0.5% of the current market), that the annual energy use is so low and the embodied carbon is a significant proportion of the (much smaller) total life cycle carbon. Our embodied carbon versus in use cycle carbon is also highly dependent on the assumed lifespan of the building; if we discount our embodied energy over a 100 year lifespan we get a much better looking result than if we discount it over 50 years.

Airtightness is often expressed in air changes per hour; it’s obvious that if we have a lot of air changes per hour, then we are potentially wasting a lot of energy heating that air only for it to escape through cracks and gaps. Proponents of Passivhaus aim for a standard of 0.6 air changes or lower. At the other end of the scale we have people who don’t pay particular attention to airtightness and don’t design it into their buildings.

Many eco-builders see Passivhaus as complicated and restrictive, reject the use of mechanical ventilation systems and have a different design philosophy. We can disagree about whether this is reasonable or not, but even if this is your philosophy, I will stress again – if you are building structures that are designed to be vapour permeable and are using biodegradable materials then airtightness is a crucial part of the process if you want to prevent your building from biodegrading! There is no inherent conflict between airtightness and natural materials (read about the strawbale houses built to Passivhaus standards if you don’t believe me here, and here) and you don’t have to subscribe to the ideals of Passivhaus if you don’t want to!

Why do buildings leak air? Joints and junctions

Lots of building materials that an eco-builder will use are themselves airtight, but we still end up with buildings with poor airtightness. The key point of failure is generally the junctions between materials. For example, we might plaster a strawbale wall, and the plaster itself is airtight. But if we simply plaster down to floor level and cover the junction with a skirting board, it isn’t airtight. Continuing with the plaster theme, since plaster will shrink, plastering up to the edge of a timber post will result in a gap. Junctions between window frames and walls are another key area which often suffer from poor airtightness. The points at which pipes and cables go through materials are always vulnerable, whether it is toilet and bathroom waste pipes through floors, or electrical cables and ducts in walls. Specific products make airtightness at these points easier to achieve (scroll down for examples).

How do we improve airtightness in practice?

So if you’re an eco-builder simply trying to make your next building more airtight than your last, there are three key steps:

  1. Decide where your airtight layer is going to be in your construction, and make sure it is included on plans and drawings.
  2. Design the building details that form this airtight layer and make them as simple as possible.
  3. Put measures in place to ensure that these building details are installed to the required standard.

This last point is generally harder than it sounds, and depending on the building site it can include nominating individuals as airtightness champions, training courses, regular pressure testing and chocolate based reward systems. For further detail, Aldas have produced a 12 step guide to achieving airtightness. It’s well worth a read. For even more detail, the Passivhaus Trust run training courses.

Architects drawings should specify where the airtight layer is going to be, and then you should be provided with a set of detail drawings for each individual junction.

A key error that we see time and time again on eco-building projects is a lack of detailed design drawings. The client picks up the drawings that were used for the planning application, and then assumes that’s sufficient to give to the construction team. Even with a highly skilled and experienced team on site, it is very difficult to design on the job and then implement. Lack of detailed design is a fundamental reason for poor build quality. To achieve airtightness we need detail drawings with the airtight layer highlighted, instructions on how to deal with each junction and the products to be used, and then tradespeople that understand the importance of getting it right. As we’ve seen above, this doesn’t have to be complicated; there are standard details available that can be adapted for most situations and plenty of products available to ensure that our buildings are dry and comfortable.

Products that will help achieve airtightness at key junctions

Siga Fentrim tape; airtightness tape with a fuzzy surface and holes in it to allow adherence to very rough surfaces such as straw. Pro clima do an equivalent tape (Contega PV) would do the same job and is more like conventional plaster mesh. If your plaster was your airtight layer in a strawbale building, you could use this tape at every service penetration and junction with timber to provide continuity in your airtight layer. Described in use on a strawbale building here.
When boxing in, use tapes over the OSB/ply to ensure airgtightness. From here.
Airtightness tapes to be used on corner details generally have multiple strips protecting the adhesive backing. This makes application much easier.
Tape installation at the junction between a window frame and the wall. From here.
Airtight backbox for electrical sockets. Electrical sockets and light fitting penetrations add up to substantial breaches in airtightness layers unless properly installed with backboxes and cable gromits.

Final thoughts

I could blather on for ages on this topic, but I’m not sure it would have much impact on anyone that had the ability to change things. So all I will say is this: if you’re involved in the construction industry, try and get yourself along to be present when a building gets tested for airtightness. It will change the way you think.

3 thoughts on “Airtightness and natural building materials – Part 2.”

  1. I’m really enjoying this series of yours – something I’m wondering: how long does a Passivhaus-standard building’s airtight envelope last, and what can cause it to degrade? Is it customary to test for airtightness of a building after it’s in operation? It’s not something I’d thought about at all till recently (I’d just assumed the airtight envelope would last as long as the building).

    Like

    1. Recently did a re-test on the UK’s first passivhaus, which is 10 years old. Still passive, and almost as good as it was. Germans have much longer experience of PH, so there is data on the question (I think their first one was 1980’s).
      I think in practice it’s more likely that the airtight envelope is punctured in discrete places (by new services being installed, or a new kitchen cupboard being fixed into a wall for example) than degrades significantly.
      Building products (including airtightness ones) do get tested for longevity using accelerated wear tests, although how well those tests mimic real conditions is debatable (but we can’t really wait 100 years to find the answer, so we live with them).

      Liked by 1 person

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