Self Building: Airtight Design Explained (1/2)

A guide to achieving a fully airtight design when constructing your home. Part 1/2.

air

The advice of energy efficiency experts has for over a decade been to produce an airtight design with proper ventilation. However, only recently has the construction industry taken notice of this advice.

For many people in the UK, the idea of an airtight design has seemed odd, and therefore undesirable. A common opposition to the idea is that airtightness means staleness, smelliness, and a home full of condensation. For many years we have lived in houses such as those which the Victorians built and which were designed to leak as much air as possible to lose the smoke which emitted from coal-burning appliances. However, times and habits have changed, and continue to change, and with growing awareness of concepts such as PassivHaus and zero carbon homes, the population is beginning to realise just how important airtight design really is in achieving low-energy living.

Airtight houses do not rely on draughts to maintain air quality, and ventilation is of major importance. Air quality becomes a case of careful planning, with home utilising whole-house systems to regulate the air inside. The low-energy use home works on a principle of three key elements:

- A lot of insulation;

- Airtightness, and;

- Whole-house ventilation systems.

These systems cannot work properly individually, but combine all three and you are left with a low-energy, comfortable home, with good indoor air quality.

Testing an Airtight Design

The airtightness of a home is a measurable quality. In order to carry out a test, the front door of the home is removed to be temporarily replaced by a blower door. The blower door is used to pump air into the house, putting it under a minor pressure of 50 Pascals. The blower system measures how much air must be pumped into the building in order to keep the house at a constant 50 Pascals.

Air leakage measurements are scored as cubic metres of air moving through a square metre of the building per hour under 50 Pascals of pressure, or q50. The reading doesn’t relate directly to how a house will leak under normal circumstances and pressures, but the figure is a good way of comparing the leakiness of two or more houses. The figure also shows how well a house was designed and has been built, and can be seen as a confirmation of the building’s quality.

The current Building Regulations in England and Wales states that an airtightness pass score is just 10 q50. This is not a high score, and a house with such a score could by no means be described as an airtight design. It is possible for sites with just one or two homes built upon them to forgo the air pressure test, which costs around £300, but the score must then be assumed to be 15 q50. This will impact upon the Dwelling Emission Rate calculations and ultimately require that better insulation and other energy efficiency measures are used to combat heat loss. You may find it is cheaper just to have the test carried out, and for anyone aiming to build a fully airtight house, the test is essential. Under the rare circumstance that the test is failed, remedial steps can usually be taken at the point of failure.

A score of 10 q50 is not high, and can be very easily reached by using careful draught-proofing measures and by applying mastic where it is needed. Scores below 3 q50 are considered good, and it is at this point that ventilation becomes necessary. The gold standard of low-energy house construction – PassivHaus – requires that airtightness is just 0.6 q50.

Desiging Airtightness into your Home

Airtight design should really be considered from the outset. In order to achieve a good score, the air barrier should be designed early on, and then properly installed and not tampered with over the construction period. It is important that details such as service connections are planned well in advance of any of the other works being carried out. This ensures, for example, that the plumbers do not drill holes wherever they think the holes should be drilled, thus preventing any potential damage to the air barrier by unplanned drilling.

Airtight Design: Tapes and Membranes

A simple method of reaching high levels of airtightness in your home is to use airtight tapes and membranes. These materials are highly flexible and very adhesive. They can be used for sealing any circular pipework or ductwork penetrations, as well as the corners of window fixtures and door frames, and also the abutments between OSB or wood-fibre insulation boards or even vapour barrier sheets. Companies such as NBT offer tapes such as those in their Siga range which are toxin free, solvent free, and plasticiser and formaldehyde free too. They will also work very well to maintain their absolute best performance for well over 60 years, whilst allowing for the natural movement of the building over that time.

 Self Building airtight design explained 1 500x516 Self Building: Airtight Design Explained (1/2)

Continue to Part Two

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