Relearning timber frame

Robin Lancashire of frameCHECK, part of TRADA Technology's Construction Services team, looks at the issues addressed in the latest edition of the industry 'bible' Timber Frame Construction and their 2009 training course programme.

With more emphasis on modern methods of construction, house builders and developers have been taking a fresh look at platform timber frame construction. Even wellestablished players in the industry will say they never stop learning - new Building Regulations and codes, plus changing client expectations and specifications, mean that the industry can never stand still. There is a strong demand for up-to-date, accurate information on this modern method of construction, as platform timber frame construction continues to build on an already substantial share of the construction industry.

In the seven years since Timber Frame Construction was last published by TRADA, there have been many changes in Building Regulations, client expectations and specifications. Environmental considerations, cost and time saving methods, plus thermal and acoustic performance have all played their part. This has helped timber frame increase its market share and prominence, as it already delivers many of the solutions needed for the future.

In the new fourth edition of the publication, many benefits of this form of construction have
been highlighted and developed, with details incorporated to reflect current best and future
practice. Highly regarded as the 'bible' of timber frame construction in the UK, the book is an important tool for anyone using this method of building.

Under current Building Regulations, air tightness testing of new buildings is mandatory in England & Wales, unless a high default value is adopted. The current requirement of 10m3/m2 @ 50Pa is, in reality, quite a poor performance target. In our view, future regulations are bound to make air tightness performance criteria much tighter, therefore placing greater emphasis on detailing and workmanship. In the past, details for junctions with walls and floors did not consider the importance of achieving an airtight seal. New details have been developed to create more airtight buildings. Many of these are discussed in our training courses, with TRADA Technology best practice shared.

Now, with mandatory air tightness testing, consider the effect that even a small horizontal
gap repeated around much of the building perimeter could add up to - and the effect on the overall result. The new book shows a detail to create an airtight barrier at the junction of the ground floor and external wall by using a wider damp proof course.

This is installed flush with the bottom edge of the sole plate externally, passed under the sole plate, with the remainder lapped up the inside of the wall panel. The damp proof membrane installed as part of the floor construction is also brought a short length up the inside of the panel to lap next to it. Finally when the vapour control layer is installed to the inside face of the external walls, it covers both the dpc and dpm lap. So, when the plasterboard lining is fixed in place, the three membranes are trapped together against the inside edge of the sole plate and bottom rail forming an airtight seal. For additional assurance and protection, a bead of mastic could also be applied at the membrane lap location.

Another area that can be prone to considerable air leakage is the upper floor junction with an external wall. A new detail in the 4th edition of the Timber frame construction book shows how the upper and lower externalwall air barriers can be linked through the floor zone in an efficient and effective manner. The method involves using a length of breathable air barrier membrane which the erection crew install during the build process.

They begin by fixing one end of the membrane over the top of the external wall, lapped over the inside face of the top rail, with the remainder passing over the top of the wall panel, hanging down the outside of it. The head binder is then fixed, holding the membrane in place. The floor is then installed and the next sole plate laid. The membrane is now pulled up the outside edge of the floor zone, laid over the sole plate onto the floor deck and pulled tight. Once the wall panels are installed, the membrane is folded up over the inside face of the bottom rail, with any excess cut away. By taking the air barrier to the outside face of the timber frame, this avoids complicated installation and sealing around joists and services within the floor zone. However, to avoid interstitial condensation, it is important to remember that an air barrier used externally must be breathable.

Further changes to thermal performance requirements are just around the corner. With the zero carbon requirements of 2016 looming, attention is turning to how timber frame will meet the challenges of very low U-values and better air tightness. Typically, deeper external wall stud voids are being constructed, with 140mm studs for external walls used, with thermal insulation installed between them and a reflective breather membrane externally. The illustrations have been redrawn to indicate 140mm studs for external walls. I joists for studs can also be used, with more insulation installed between, and different types of insulation products can also be selected. The book offers information and a comparison table for consideration, showing typical U-values for popular wall configurations. The training courses offer an opportunity to explore the pros and cons of these different options in practice, to get across the message that no one method will meet all situations.

In roofs, the raised heel truss is shown as a method of reducing thermal bridging at the
wall/roof junction, thereby improving thermal performance. This truss type raises the top
chord at the end bearing point without requiring deep top chords. Although this may vary the roof pitch, projection and eaves level, its use allows full depth insulation to be installed across the full width of a ceiling and over the external wall in a cold roof situation.

Acoustic requirements have come under the microscope in recent months, partly because
tighter thermal performance has a positive knock–on effect on acoustic performance, and because some developers will be looking to gain points under the Code for Sustainable Homes,
where up to 4.7 percentage points are available for considerably improved acoustic performance.

With regard to timber frame, acoustic performance requirements now mean that many voids between internal wall studs and floor joists are filled with insulation to absorb sound. Resilient bars and acoustic battens are now familiar products used in separating elements to reduce hard contact and acoustic transfer between dwellings. A wide variety of these products are used in differing combinations to achieve and exceed the statutory requirements. In the fourth edition, drawings, text and tables have been updated to provide information and solutions around these changes. A better understanding of detailing and consistency is required to make sure that the highest levels of performance can be repeatedly met.

There is also a strong correlation between good detailing for acoustic performance, air tightness and even fire resistance in timber frame construction. As regulations become
tighter, we believe that the different areas of industry must work together in detailing constructions. A detail which works well for, say, thermal performance must also deliver
good fire resistance, acoustic performance, structural integrity, buildability and overall
longevity. Throughout the training courses, these relationships are highlighted. We encourage delegates to ask specific questions and discussion usually underlines that to achieve best practice, there is plenty of room for improvement!

When using timber in structural applications, it is vital that detailing and site construction
practices acknowledge that fact and protect the timber where necessary. Those against the
use of timber in structural applications have often failed to understand key principles when using this material. Long-term, the timber frame industry relies on the buildings constructed
today still remaining fit for purpose in decades to come.

Explaining the whole complex subject of timber frame construction in just one day, of course, can be a bit of a gallop. We all have a responsibility to our industry to learn and then educate our clients and colleagues about appropriate detailing when using timber. This is an underlying theme throughout the courses and, as independent experts in timber construction, it is something which the timber frame consultants who deliver these courses are passionate about.

As a Canadian who has worked with timber frame for decades, Huel Twist, co-author of
the latest book, is passionate about building accurately and efficiently. Working as a member
of the frameCHECK team, Huel was keen to impart many of the tried and tested techniques
used in his home country to the UK industry. He has brought many practical ideas and details
to the new book, enhancing its value as an essential guide for those involved in timber
frame at any stage, from design through to construction on site.

Many main contractors approaching timber frame for the first time are challenged by it. The
speed of construction, order of work and different trade disciplines often call for some fast relearning to enable efficient management and delivery of a quality building to their client. TRADA guidance has a firm place in meeting this need.