Timber Frame vs SIPs Comparison

20 July 2018 Potton News

How Do I Compare Timber Frame Build Systems?


There is a lot of design choices for self builders wanting to use a timber frame system for their new home. The reason that so many seem to thrive is that self builders choose timber frames for different reasons, and the different systems on offer cater for the wide variety of priorities that people have. If you’re new to the idea of self build, it’s quite difficult to weigh up the pros and cons of the various systems - which is why you need to be clear on what’s important to you.

Each of the major timber framing companies have their own versions of the timber frame walling construction. Some, including Potton, have more than one. These build systems show how the wall is to be constructed, where the insulation fits and what thickness it should be, and often shows the detailing as the wall meets the floor and roof structures, and so on.

Instantly, you’re not just buying a piece of timber - you’re buying a walling system.

Timber Frame SIPs Comparison Key Elements

Let's take a look at the key considerations for choosing the right build system for your self build. When deciding upon a timber build system, keep in mind these 6 important aspects:
  1. U-Values
  2. Wall Thickness
  3. Construction Speed
  4. Cost
  5. Timber Frame Construction
  6. Comparing Suppliers
U-values are a way of measuring heat loss, in this case through a wall system. The lower the U-value, the better the heat retention, and greater the energy efficiency.

The building regulations requirements for U-values have become very complicated in recent years and can vary depending on other factors in the construction, but generally, walls need to enjoy a U-value of below 0.2 to meet the most recent regulations for energy efficiency.

What is immediately noticeable is that all of the materials and systems on offer claim to provide U-values of around 0.15. There is some variance - some get down to 0.11, others up to 0.2. To put that in context, the U-value requirement for walls of homes built in 2006 was 0.3; if you’re living in a home that is more than 30-40 years old and largely unimproved from an efficiency perspective, the U-value of your walls is likely to be 2-3 (10-15 times worse).

That, of course, is a noticeable change – the difference between wearing a T-shirt at night and a jumper. The difference between 0.13 and 0.15, however, is not. The marginal difference is utterly insignificant and while a nice badge of energy efficiency, an outstandingly low U-value in the context of strict requirements anyway is a red herring for those looking to compare different systems.

Potton offers 3 main build systems, Kingspan LOGIC+, Kingspan ULTIMA™ and Kingspan TEK®, each with their own respective benefits, and each with differing U-values, as illustrated in this comparison table.
  Kingspan LOGIC+ Kingspan ULTIMA™ Kingspan TEK®
Build System Timber Frame Timber Frame Timber Frame
U-value 0.22 - 0.16 0.15 - 0.13 0.18 - 0.10
Of much more significance when comparing timber frame wall systems is the total thickness of the wall itself - critical nowadays given the need for lots of insulation in the structure.

The first PassivHaus home built in the UK was actually constructed of blockwork and stone, but given its need for thick insulation, the wall structure ended up being over 500mm thick. One of the key advantages of choosing timber frame is its ability to achieve the same performance on vastly reduced wall thicknesses.

Potton’s Structural Insulated Panel system, Kingspan TEK®, utilises either a 142mm or 172mm thick panel (in addition to the external choice of cladding, giving wall thickness of under 300mm). If you’re building on a tight site, perhaps in an urban or suburban location, (or where planners have insisted on a limited footprint) this can make a significant difference to the useable floor area of the rooms, with thicker external walls accounting for up to 15% of a total footprint.
All modern timber frame systems involve a degree of off-siting - taking work away from the site and into a factory setting. This is generally considered a good thing. A factory is a controllable, programmable, exacting, dry environment.

The amount of off-site manufacture does vary from system to system, however, and particularly between the three major types of timber frame construction:
  • Open panel timber frame (stickbuilt framing)
  • Closed panel timber frame (e.g. Potton's Kingspan LOGIC+ and Kingspan ULTIMA)
  • Structural Insulated Panels (SIPs, e.g. Potton's Kingspan TEK, which strictly speaking aren’t really timber frame at all)

As a general rule, the more enclosed the panel (the latter two systems), the more work can be taken off-site, to the point where extreme ‘modular’ style closed panel timber frame systems can be delivered to site with windows, and electrics and heating/plumbing systems pre-fitted.

Clearly this move to panellisation speeds up the amount of time on site, so not only reduces live ‘build’ time but also ensures it can’t succumb to the typical things that make building projects late: weather, shorter daylight hours in winter, personal circumstances of tradesmen on site, and so on.

While that is highly appealing - and also the main reason why timber systems are popular in Northern climes - it’s worth bearing in mind that the manufacture of these closed panel/SIPs systems will typically take 6-8 weeks. However, that’s time enough for you to get the footings right, so isn’t usually significant in terms of slowing down the build schedule.

It’s also true that more traditional open panel timber framing buildings, whilst not off-siting many build elements, is usually much faster in terms of wall construction than using blockwork, typically saving 2-4 weeks over a whole build programme.
For most people, cost is a very significant factor in choosing between different systems and suppliers. That’s all very well, but there are some things to bear in mind.

Are you comparing the same thing?

And that’s not just to mean that certain suppliers include all the components of the wall system in their price (from membranes to cavity closers and insulation) and others don’t. It also means differences in the actual specification, such as the type of insulation and membrane, and even the timber itself.

Have you accounted for labour costs?

Many companies recommend approved fitters but inevitably that means that different installers will provide different labour costs. And lastly, but most importantly,

Remember the big picture.

Wall systems account for something like 15-25% of the overall cost of construction. Yes, you might be able to save a few £1,000s by choosing one supplier over the other, but will you be that rigorous on costs when it comes to every other decision, from kitchens to windows and tiles?
Structural Insulated Panels being erected on site
There might be relatively little to choose between individual timber frame wall systems, but when it comes to choosing between closed panel timber frames and Structural Insulated Panels (SIPs), the difference is marked.

One of the most notable differences is in the capacity of the wall to accommodate insulation.
Because SIPs panels can be up to 7.5m long, they can provide a vast continuous layer of insulation. Conventional timber framing, with studs at intervals around 10% of that length, mean that your wall won’t have a continuous insulation layer and instead be a mix of timber and insulation.

In a conventional timber frame system the wall has something like 85% insulation: a SIPs wall will enjoy 96%.
Ultimately buying a timber frame is one of the more significant purchases you will make in your life, and for all of the slight variances in the areas mentioned above, one key area is around customer service and the supplier itself.

There is a huge amount at stake in making this decision and you need to rely on suppliers you can trust, with established experience and a skillset able to deal with you, as a customer. Not all timber frame suppliers offer that, so make sure you’re comparing on how they treat you as a self builder, and what they can do to reassure you that your money is safe with them, when you get your prices in.
  • Will they be happy to store a timber frame system for an extra week or so if your groundworker is delayed?
  • Will they keep in touch through the process to reassure you that things are running smoothly?
  • Do they have an established pedigree in delivering systems with success?
  • Can they provide previous clients to provide references?

All of this matter, in a way, much more so than a little constructional detail.
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