Air Tightness for Timber Frame Structures

Posted on June 29, 2012 by davidb There have been 1 comment(s)

We all know how uncomfortable it can be to sit in a draft. Besides being cold and miserable it can cause joint ache, stiff necks and lead to other maladies.

Think then of how many older homes might feel if they were sentient beings. Not only would the building creak and groan, but it would have palpitations at the amount of money that was literally flying out of the rooms and up the chimneys as hot air rushed to get out.

Now it's easy to see the cracks through which it escapes around door and window frames, and we can all recognise the big holes running up through the roof that form chimneys. But it's not so easy to see the hidden gaps, under skirting, between joists, around pipe and electrical fittings, etc., never mind the near invisible seepage that can occur through poorly sealed walls, floors, ceilings and roofs themselves. In fact in many older houses it was deliberate construction detailing to put air bricks into outside walls to get air circulation into otherwise closed spaces, including the rooms themselves.

The diagram below gives an idea of the main 'air leakage points'

large 1. Loft hatch

2. Skirting

3. Flues

4. Eaves

5. Ceilings

6. Ducting

7. Light fittings

8. Waste outlets

9. Electric fittings

10. Windows

11. Doors

12. Plasterboard joints

& Other penetrations

Until quite recently, ’Wet Build’ houses would commonly be subject to around 15 air changes an hour as a result of these drafts and seepages. Many houses built before about 1960 could even see this as high as 20 air changes an hour. Now, because of the importance attached to making homes much more energy efficient, the new Building Regulations require homes to be built with a maximum of 10 air changes and hour, with a target of 5. However, the new Code for Sustainable Homes, introduced in April 2007 sets even tighter standards. We can expect to see all new houses being built with 6 air changes an hour as a maximum.This is a level accepted as commonly achieved with panelised timber-frame structures. Such homes built to Canadian standards generally achieve as low as 1 or 2 air changes an hour, with some results being below 1, which is the current long term UK target, although targets from other countries, e.g. Sweden, Germany and Canada, are even lower than this.

The ‘target’ air tightness set by the designer is used in calculating the energy and carbon ‘SAP’ (Standard Assessment Procedure) ratings of the building.These have to be attached to applications for Building Regulation approval and form an important part of the submission.Completed buildings are now subject to rigorous air-tightness testing and, by law, must achieve these target levels before they can be occupied. Such a test must be carried out by registered contractors using pressure or evacuation testing equipment. Such tests can cost up to £1,000 per test.

This greater airtightness is achieved through careful design and scrupulous attention to on site trade detail.By reference to the diagram above the areas requiring special attention are easily recognised. While it is possible to reach present requirements using all masonry construction, it should be noted that, for example, block-makers have had to devise a ‘thin joint’ system to make this more reachable. Since panelised systems are generally more impermeable than masonry, it follows that it is generally easier for them to attain lower rates of air change.However, even these are subject to possible on-site failures. They need to be properly sealed and caulked to eliminate leakage at all weak points and, like all other systems, can be weakened by careless puncturing of the fabric, e.g. by follow on trades inserting electrical wiring, plumbing and waste pipes.

Today our understanding of buildings and how they work is rather different. In the 50 or 60 years since the 'revolution' that started to bring central heating into more and more people's homes in the middle of the last century, we've learned a lot about how our homes perform under different conditions and how they need to be 'cared for'. Many people will be familiar with condensation and mould problems resulting from cold wall surfaces in unventilated rooms. Higher standards of insulation have helped prevent much of this, but sometimes at the expense of creating other problems, like 'interstitial' condensation - condensation that occurs between the inner and outer wall surfaces.

But that is another subject, for another day.

This post was posted in Articles

1 Response to Air Tightness for Timber Frame Structures

  • nice article, really informative, thanks for sharing…

    Posted on March 31, 2015 at 11:42 am

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