We are considering a large extension to the rear of our property and the builder is pushing us towards a timber frame structure as he says it’s easier to get an airtight seal to prevent draughts. He says this is a big issue with concrete houses. I’m unsure what advice I should take as my architect specified concrete but won’t guide me in terms of what option to take. What are the disadvantages/advantages with timber frame?
There is a long-running debate as to which construction method is better - timber frame or masonry. There are, understandably, marked differences between the two methods, especially when you consider that timber frame is usually made in a controlled environment (factory) and delivered to site, while masonry is constructed in an uncontrolled environment (on site) with all the required materials delivered and built.
Airtightness is one aspect that should be taken into account when deciding on a construction method but there are others such as insulation, ventilation, energy source etc. For a building to be constructed correctly we need to start with good design. With every design there should be a design strategy and for your question specifically, there should be a clear airtightness strategy. If not, challenge the designers; remember you have engaged the services of a professional to provide you with a design that should not only achieve your brief/requirements but also comply with building regulations.
Assuming there is an airtightness strategy, what rating (Air Permeability m3/(h.m2) at 50 Pascals) is the design going to achieve and how would your design compare with a timber frame building?
Airtightness simply means cutting out unwanted draughts, creating an internal seal from the external environment. Draughts can be so slight as to be unnoticeable, but even slight draughts increase heat loss and affect the performance of the ventilation system.
The way to achieve good airtightness is by constructing a continuous air-resistant layer all around the inside of the building. This includes under and around the ground floor (radon membrane/DPM), across the external walls and under the roof to seal the inside from the outside.
Let’s review this based on elements of the build, starting with the ground floor. Ultimately this is the same construction method for both timber and masonry construction. Moving on to the external envelope: with timber frame, the internal structural walls are constructed off site and erected on site – assuming no damage to the membrane – the internal seal is complete with the exception of taping along perimeter joints. When this is viewed against blockwork construction, there is a longer process but a high level of airtightness can be achieved using correct detailing and construction skill. Finally, the roof covering, and this is ultimately the same in both construction methods.
Good airtightness is achievable in both masonry and timber frame. However, current construction details and workmanship skill, in my view, makes it more achievable in timber frame construction.
Both construction methods have their pros and cons, and it’s not so much about which is better, as which is better for you.
Andrew Ramsey is a chartered building and project management surveyor and member of the Society of Chartered Surveyors Ireland (SCSI)
