Airtightness and natural building materials

With thanks to Es Tresidder of Highland Passive for help with moisture load calculations and useful critique. Airtightness testing was undertaken by Paul Jennings.

[first of 2 parts, second part available here].

It was still dark when we arrived. It had been a cold night, not quite a frost, but not that far off. The householders had cranked the heating up to the max as requested and it was toasty warm inside their brand new eco-house. Our equipment consisted of a fan that fits into a door opening, we got it set up fairly quickly, switched it on and then attempted to suck all the hot air out of the house. Paul did clever things with pressure measurements whilst I did the colourful bit; walking round the building with an infrared camera looking at where air leakage was occurring. When I went back indoors I was shocked, inside the building was almost as cold as outside. Undeterred, I carried on taking pictures.

Air tightness testing in action; a fan is placed in a doorway to the building and is used to either suck air out of the building or push more air in. The amount of air needed to maintain a fixed pressure difference is used to calculate how leaky the building is. As well as giving an overall value, the pressure difference makes air leaks much easier to locate, either with a smoke gun or an IR camera.

The idea of air tightness testing is to create an air pressure difference between indoors and outdoors by putting a fan in a doorway and sucking the air out of the building. The amount of air required to maintain the pressure difference is measured, and when combined with data on the volume of the building, the number of air changes per hour can be calculated. This is effectively a measure of how draughty a building is; a building with fewer air changes per hour will be easier to keep warm in winter. Sometimes it gets expressed as the surface area that is exposed to the outside – understanding that the all the cracks and gaps add up to the equivalent of leaving a window open helps concentrate the mind!

Thermal imaging can be used to image building details to look at heat loss. Blue areas are cold, red/white areas are hot. In this image we see the junction between a wall and a pitched roof in a timber frame building. The positioning of the horizontal beam and vertical post has created an area that is difficult to insulate and we can see the resulting cold spots. Cold spots are prone to condensation (and therefore mould).

As the name suggests, infrared cameras detect infrared radiation. All objects above -273 degrees centigrade emit infrared radiation, and the amount they emit is proportional to their temperature. These cameras therefore allow us to take pictures of how hot or cold objects are, and so if we take pictures of buildings when the heating is on and it is cold outside, we can highlight features such as a lack of insulation behind a wall panel. They are particularly powerful tools when used in combination with air tightness testing; accelerating air movement makes it much easier to find draughts.

A striking aspect of the thermography at this property was the differences between the pictures taken indoors compared to outdoors. Whilst there were a few exceptions (for example around an ill-fitting window, or a failure in the sealing round an extract fan), the vast majority of cold spots visible indoors did not correspond directly to a visible point of heat loss outside. Cold air was entering a room via a gap between plaster and timber, but there was no equivalent gap on the outside of the wall. Effectively, this means that the pathway the air is taking between outdoors and indoors is convoluted rather than passing straight through a short pathway. So what you might ask? Air movement means heat loss, and we could just crank up the central heating and put up with it. Since the air leaks in most buildings are undetectably small unless there’s a howling gale outside, this is what most of us do.

The relationship between air movement and water vapour

We’ve cranked up the central heating, to compensate for our air leakage. However, there is an additional problem to worry about. Air also contains water vapour, and when the temperature of the air changes, so does the amount of water vapour it can hold. So in this wonderful new ecohouse, warm, moisture laden air is leaving the room via an electrical socket (for example), travelling along an un-determined pathway in the wall, and exiting the building via the junction between the wall and the roof. As the temperature of this air will drop during its journey out of the building, water can condense out of the air into the building fabric. If this happens in part of the structure that is built out of a natural material (timber frame, straw, recycled newspaper, sheeps wool etc) then there is a risk of the material rotting.

How much water vapour moves through the building fabric in less airtight buildings?

We can calculate the amount of water that is contained in the air that leaves the building to help us understand this problem better. For a wall in a small single floor dwelling, with an airtightness of the minimum required by UK Building Regulations, we can expect between 500ml and a litre of water to move through the wall over a 24 hour period in winter. Assuming this isn’t evenly distributed (i.e. it’s via small cracks and gaps in the building structure), we can see that we are at risk of having significant moisture condensing out into the building fabric in localised areas. Contrast this with the same building built to Passivhaus standards, which would have just 27mls of water moving through the same wall. These are worst case assumptions so it might well be less water than this, but assumptions are the same for both building types and simply to illustrate how much riskier non-airtight buildings are.

Note on assumptions: 2.4m high, 5 m long wall. Indoor temperature 20C, outdoor temperature 5C. Air permeability @ 50Pa of 10m3/hr/m2. Gives a total condensing potential for the wall of 741ml in a 24 hour period. For the same built form built to the minimum airtightness required by PH standards (air permeability of 0.37), the condensing potential is 27mls.

Why don’t natural builders like airtight buildings?

Many natural builders reject the idea that airtight buildings are good. “I don’t want to live in a hermetically sealed box” they will say, or “I want natural ventilation”. But it is clear from the calculations above that by not building to high standards of airtightness, we are running significant risks of moisture condensing in the structure, and with it both the risk of degradation of the building fabric and the growth of toxic moulds. Given that many in the eco-building movement want to use natural materials because of their perceived health benefits, this is somewhat ironic.

The attachment to natural ventilation also poses a risk. If you are a proponent of these systems it is worth bearing in mind that we are much more sensitive to thermal comfort than we are to air quality. As such, our first instinct will be to conserve heat by not opening windows, and blocking up vents, and this will undoubtedly increase the risk of poor air quality. It seems likely that by 2025, changes in Building Regulations will mean that natural ventilation systems will no longer be acceptable in new dwellings because of the accumulating evidence regarding the associated risks of poor indoor air quality.

There is really no excuse for building houses with high air leakage rates; it is generally a consequence of poor design of details, and/or poor building technique. If you’re an eco-builder who has never been in a building during air tightness testing, I would recommend that you tried it; it illuminates mistakes and will force you to think differently about construction details. And we cannot defy the physics of moisture movement. If your building isn’t airtight then you are increasing the risk of condensation occurring in the building fabric, and if you have also chosen to use natural materials, then you will certainly be decreasing the lifespan of your building and risking the health of the occupants.

In a future blog post we will look at a few strategies that can drastically improve the airtightness of eco-buildings and so decrease risk. Part 2 now available here.

Judith Thornton works with support from BEACON. BEACON is funded through the European Regional Development Fund (ERDF) by the Welsh European Funding Office (WEFO).


5 thoughts on “Airtightness and natural building materials”

  1. Yes! Will be sharing this blog widely – very nicely summed up and demonstrated. Airtightness (leak-freeness) is hugely important, regardless of what materials you build with. I still struggle getting people to accept this…


    1. Thanks John. Indeed, perhaps I should have emphasised the ‘regardless of what materials you build with’. It isn’t intended as a dig at people using natural materials, it’s more trying to remind people that natural materials and airtightness are not conflicting concepts, and the consequences of lack of airtightness are rotting buildings. Obviously a ‘conventional materials’ building risks moisture condensing in the same way, and one could argue that if this condensation occurs at the junction of a material with low moisture buffering capacity (like rigid foam insulation) and a timber frame, it’s actually just as bad if not worse than if it occurs in a building with materials that might dissipate the moisture better. This is something Es suggested, and he is probably right. Nevertheless, we need to build airtight buildings!


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