Choose the Timber Species for Your Timber Windows

Generally, what do we want in a timber window?

• Thermally Insulating: the main natural benefit of timber for everyone is providing a good U-value, reducing the heat loss from your home
• Durability: installing a house full of high performance windows is quite an expense. Ideally, you only want to do it once
• Environmentally Friendly: another natural benefit, but check for FSC or PEFC certification
• Appearance: it’s got to look the part. Both for what you have in mind, and any conservation restrictions
• Economically Priced: everyone has a point where it doesn’t matter how many bells and whistles exist, the price needs to be viable

Hardwood or Softwood for Windows?

One of the first choices to make when looking at timber is do we want a higher density hardwood or lower density softwood.

Hardwoods:

• Most commonly finished in a stain or oil, displaying the natural grain structure. Oils will need topping up regularly.
• Noticeably expensive; slower growing, less frequent supply.
• Worse thermal performance: the higher the timber density, the higher the thermal conductivity.
• Excellent durability, with better resistance to rot and decay.

Softwoods:

• Normally painted in a solid colour; industry standard is around a 70 micron application.
• Affordable; cheaper and widely available due to faster growth.
• Excellent thermal insulation.
• Will degrade relatively quickly due to reduced strength and higher susceptibility to wear, warping and damage. This means the overall lifespan will be shorter.

Unik Funkis heartwood endgrain showing consistent ring spacing; before lacquer

The Middle Ground: Slow-Grown Nordic Pine

Thermal conductivity increases as the density of the wood increases. That is, the higher the density, the quicker it will transfer heat. This is mostly due to the grain structure being compressed in high density timber. We want to reduce that heat loss as best we can, without compromising too far on longevity.

Most timbers below around the 400kg/m^3 mark are extremely soft; there aren’t that many reasonable options in there (a bamboo window might be interesting though)… They’ll damage easily and have generally low strength. Not fantastic for making windows, but often show up as an option as they are usually less expensive.

What we’re really looking for is something in the middle of hardwood and softwood.

Pine is the most common timber used in windows, but pine doesn’t tell you much about the timber being used.

On the other hand, “slow-grown” pine is nicely self explanatory: the tree grows at a slower rate. This slower growth means that the grain or rings in the timber are much closer together, and that normally creates a timber density on the borderline between softwood and hardwood.

As well as the finer grain and higher density giving us a stronger timber, it allows manufacturers to get a better finish with their paint application and reduces the chances of accidentally denting (a common problem in very soft wood).

Heartwood Timber

Located right at the centre of the tree, our factories prefer to use the heartwood sections of slow-grown pine because it comes with further benefits.

Beyond having a slightly higher density again, and a lower moisture content, heartwood timber has far more natural resistance to rot and fungal damage. Unlike species such as cedar or rosewood, pine is not generally resistant to these types of decay, so using the heartwood for pine windows is a very significant improvement.

Finger-jointed and Laminated Timber

A fantastic way to reduce waste: finger-jointing can seem rather odd when you see it exposed. Beyond the appearance though, it’s rather elegant way to increase the amount of usable timber. Without joining sections together, even top-quality slow-grown lengths would struggle to get through a factory quality control process. Most noticeably, you’ll see knots bleeding through paint layers on products that chose to skip this important finger-jointing selection.

At a window factory, lengths of timber arrive pre-finger-jointed, but they don’t stop there! Dye-processes are employed at the very first stage to make sure that overall quality of the lengths is up to scratch. And lengths do get rejected occasionally if doesn’t meet the standards set out by the DVV.

Large Knot in some inexpensive laminated Pine

Unik Funkis finger jointed timber before being finished with lacquer

When thicker sections of wood are needed, these will be more expensive and much more likely to have defects. Sandwiching layers of timber together (“laminating”) is a brilliant way to reduce cost without compromising on the quality of the end product. Combining thinner sections of timber, problem areas can be removed with less wastage.

These two processes have transformed the window industry over the years; there are very few processes in the world where cost and waste have been reduced, yet overall quality has improved alongside.

Laminating also presents an opportunity to combine a hybrid timber frame, utilising different timber species in the best possible areas. For example, our flagship bifolding door has a pine inner layer, encased between beech and ash (all thermally-modified; another topic!). Brilliantly, this allows the lower density pine to improve the thermal performance whilst being protected by the denser outer layers.

Key Features of our Laminated Slow-grown Finger-jointed Nordic Heartwood Pine:

This is our perfect middle ground. Durability benefits of hardwood, cost and thermal benefits of softwood. All of our Danish factories default to this timber variant.

• Dense Grain: Slow growth in cold climates results in tighter growth rings, providing greater strength and durability.
• High Stability: Less prone to warping, shrinking, or swelling, making it ideal for windows exposed to varying weather conditions.
• Aesthetic Appeal: Fine and uniform grain pattern is visually attractive and works well with finishes.
• Longevity: Naturally more resistant to wear and tear compared to faster-grown varieties.
• Environmental Considerations: Sourced from sustainably managed forests in Nordic countries.

Unik Funkis

Whilst we are clearly biased towards our own products, we recognise the timber idea isn’t absolutely perfect. Alongside a basic pine option, we can still end up looking relatively expensive because it’s often not made clear what sort of timber is being used. Slower growth and long selection processes certainly mean superior quality, but will take more time and cost to achieve, and that makes the final result less widely available. Also, a pine timber will still need protective treatments at factory, such as STMs 2ØKO process, and will require customers to maintain their windows across the windows’ 60+ year lifespan.

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Applying to Replace Existing ‘Modern’ Windows in a Listed Building

Privacy Glass Choices and Bathroom Windows

What We Do on a Window Survey