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Twinskin insulated log cabin info

What is a Twin Skin Log Cabin?


In our opinion, there is only one economic way to ensure that a log cabin can be used throughout the year, regardless of the season.
The key lies in the insulation of the log cabin.  A Twinskin construction has insulation in the walls as well as in the roof and floor, which is achieved by building a cavity into the wall that can then be filled with various types of insulation.
The insulated walls can perform at the equivalent thermal value of a solid wall which is 5 times thicker,  thus saving both space, materials and most importantly, cost.

It is worth reviewing how a log cabin is constructed before looking at the various options:

- Log cabins are built without the use of a vertical framework which would hold the walls at a set height. They are composed of horizontal logs which lie on top of each other and are taller during the Autumn/Winter.  

- The walls are usually 17 logs high and each log can absorb enough atmospheric moisture during the Autumn/ Winter that they can each swell by 1mm, so the log cabin could be 17mm higher during the Autumn/Winter.

Log cabins are normally insulated in one of 3 ways:

1. The authentic "Twin Skin"  - an insulated log cabin with a second, fully interlocking cabin inside....

  • The inner logs are fully interlocked with the outer logs, so they will rise and fall in unison. In the winter, when the outer logs are at their highest moisture content (and therefore at their tallest), the inner logs (which remain dry with a low moisture content and therefore do not swell like the outer logs) are evenly spaced accordingly within the tongue & groove joints (usually about 1 - 2mm spacing per joint). These joints then close up again as the weather dries and warms up during Spring and Summer (thus drying the outer logs which means they contract again). It is essential to paint the outside of the cabin with a high quality waterproofing timber treatment (not a simple shed or fence treatment as these are not as waterproofing as required) to stop the outer logs swelling too much during Winter, thus keeping the external and internal log height differential to a minimum. If the inside walls of the cabin have been painted a colour and the outside of the cabin has not been weatherproofed adequately, then bare timber may be visbile between the inner log joints which will need to be touched up with matching paint. This is why we recommend Danish oil for the internal timber - it is a clear finish and therefore no unpainted lines are highlighted when the cabin settles.
  • This eliminates the expansion issues because expansion gaps are not required at roof height or around the windows and doors (because of the even spacing desrcibed in the previous point). The Twinskin cabin is very rigid because of the box section that is formed at each corner - you can see this easily because there are 2 logs that protrude on each corner where the inner logs are also visible.
  • Special metal connectors, straps and timbers are used where the ends of the inner and outer logs are not directly connected (around the window and door reveals) to maintain their alignment.

The photo above show how a typical 45mm Twinskin cabin is constructed.  Log cabins constructed with 35mm or less walls do not have  corner bolts as the timber is too thin to drill large enough bolt holes into without compromising the log strength, therefore conventional timber wind braces are used.

Twinskins of 45mm or thicker logs can have a 90mm insulation cavity for an even better thermal value.

We offer the following Twinskin options:
1. 28mm inner and outer log with 50mm insulation cavity
2. 35mm inner and outer log with 50mm insulation cavity
3. 45mm inner and outer log with 50mm insulation cavity
4. 45mm inner and outer log with 90mm insulation cavity (complies with Part L Building Regulations for habitation)
5. 60mm inner and outer log with 90mm insulation cavity (complies with Part L Building Regulations for habitation)
6. 70mm inner and outer log with 90mm insulation cavity (complies with Part L Building Regulations for habitation)

We can send you an insulation table (backed up by calculations) showing the insulation "U" values of all elements that go into one of our cabins and how they compare his historic Building Regulation requirement over the years - please send an email to request this.

2. Dry lined over studwork - a standard log cabin with a single thickness of log wall is lined out with framing and plasterboard or similar material....

  • This method requires that an expansion gap must be left all around the top of the inner wall to allow for movement of the logs without fouling the roof boards using sliding brackets. This means that the roof boards can be lifted off of the supporting side wall, gaps can appear between the wall logs and doors stop working correctly if it has been incorrectly installed.
  • The expansion gap cannot be defined exactly because the cabin will ‘settle down for the first few weeks after installation whilst the corners joints and logs fully interlock.
  • The season in which the log cabin is installed will affect the building.  If the cabin is built in the Winter, in addition to the first few weeks of settlement there will also be the 16mm average seasonal movement.
  • In addition to the expansion gap issues, this method causes some considerable problems with the finish of the internal linings around the windows because the windows will rise and fall with the cabin walls, but the internal lining doesn't move so it cannot be fixed to the windows for a clean and neat finish. As admitted by advocates of this type of insulating method - "Any window reveals and door reveals should not create a rigid link from the inner wall to the outer wall. They must allow the two walls to move independently of each other." In other words, it's not a very satisfactory way of doing it!

3. The dual walled method...

  • This incorrect method is where an individual cabin is built inside another individual cabin, but the internal cabin is not interlocked (connected) to the external cabin.  The gap between the two cabins is then insulated.
  • We believe that this method substantially increases the likelihood of gaps between the logs and the consequent malfunctions of the doors and windows. 
  • The internal cabin will dry, shrink and settle down whereas the external cabin will move with the seasons.
  • In the Winter, when the external cabin is at its maximum height, the tops of the openings for the windows and doors (reveals) will not be at the same height as each other. 
  • In addition to these problems, there is no additional strength in the corners of the log cabin, which render it a weaker overall building.

Understanding Insulation, K-Values, R-Values and U-Values

K-value of insulation materials

The K-value of a material indicates the ability of a material to conduct heat - materials with low thermal conductivities are better insulators as they resist heat (energy) to conduct through them. The lower the K-value, the better insulator it is (and therefore it will keep heat inside your building during the winter and also keep the cool in and the heat out during the summer).

U-Value (W/m2K ) of insulation materials

The U-Value is a measure of heat loss o(r gain) through materials used for walls, floors and roofs. U-Values are always measured in W/m2K (Watts per Metres Squared Kelvin). Basically, the lower the U-Value , the better it is for retaining heat within the building.
Why should you insulate?
Efficient insulation will keep your heating bills lower. Therefore, you should always look at the U-Value of the materials used  to make your garden building.  Heat rises, therefore most heat is lost through the roof, second most important area is the floor – a cold floor will heavily influence the heat of the room it is in. The walls are usually the largest by area in a room or a building, so although not as much heat will be lost per square metre, there are more of them, so this is also very important. 
Comfortable environment
It is also true that the better the building is insulated, the more constant the environment will be within – a non-insulated building means your heater will cut out when it gets to temperature, the building cools down quickly (due to lack of insulation) and the heater cuts back in again, repeating this cycle many times a day – with only 1 uncomfortable room to be in (hot, cold, hot, cold, hot, cold, etc, etc) and higher heating bills to boot!
All round Insulation
We sell both single skin cabins and Twin Skin cabins – even insulating the roof and floor of a single skin cabin dramatically improves their usability, but a Twin Skin is a whole different world! The table below illustrates this (we have used a 5m x 5m cabin for this illustration  - the U values of a whole building is dictated by the relative areas of the roof, walls and floor, so a 4m x 4m cabin using the same materials would have a different U value as a whole even though the U values of the materials used remain the same).
The table below shows the U values that are achieved with our various cabins (the lower the U value, the better it is) -
Hortons Single Skin Log Cabins
Cabin specifications U value of building (W/m2K)
28mm log cabin, no insulation 2.56
45mm log cabin, no insulation 2.28
28mm log cabin, with just roof insulation - no floor insulation 1.78
45mm log cabin, with just roof insulation - no floor insulation 1.48
45mm log cabin with roof and floor insulation 0.84
70mm log cabin with roof and floor insulation 0.81
Hortons Twin Skin Log Cabins
35+35mm Twin Skin log cabin with standard fibreglass insulation kit 0.60
35+35mm Twin Skin log cabin with standard Celotex insulation kit 0.45
45+45mm Twin Skin log cabin with standard Celotex insulation kit 0.45
45+45mm Twin Skin log cabin with UK Building Regulations Celotex insulation kit 0.23
70+70mm Twin Skin log cabin with UK Building Regulations Celotex insulation kit 0.22

For comparison purposes, this is a table showing the U values required by UK building regulations over the years, so you can see how the insulation of our cabins compare with houses built through the decades -

  1965 1970 1980 1990 2000 2010 2016
Whole building U value (W/m2K) 1.95 1.81 1.59 0.99 0.63 0.37 0.25
As you can see, our Twin Skin cabins are far superior to any single skin cabin (contrary to what a lot of cabin companies might say!....probably because they are unable to sell Twin Skin cabins at a rough guess – it's all just typical sales talk that out tables above prove otherwise).
A log cabin with a 70mm wall has a U-Value of 1.41W/m2K . Some companies “boast” of getting theirs down to 0.69 W/m2K.....not very good then!...Even our thinnest 28+28mm Twin Skin is way better than that.
Some companies also say that it is not possible to get a cabin to meet current UK building Regulations requirements for a habitable house – wrong! See the table above, we easily meet this criteria.
People assume that cabins made from really big round logs are far better insulated – wrong again! In fact, to get the level of heat retention from solid timber to comply with UK building regs, it would need to be just over 600mm thick! Not very practical then. Especially when you consider that round logs are quoted at their thickest point but for calculations purposes, you have to take the average thickness of the log (it is usually about 40% thinner at it's thinnest point).
This is the most important element of an insulated cabin (given that heat rises). Nearly all other companies just add a 25mm, 40mm, or sometimes 50mm sheet of Celotex / Kingspan / Exotherm / etc. on top of the existing cabin roof. This obviously greatly improves it's performance. However we chose a totally different route with our insulated roofs – we use the whole depth of the purlins to insulate between (our purlins range from 140mm deep up to 200mm deep, depending on the building and the spans involved) – you can see how superior this is method is. We achieve a U value of between 0.18 to 0.22, again far superior to commonly achieved U values by other cabin companies (usually around 0.65 W/m2K).
Floors are the second most important part of a cabin to insulate – a cold floor leads to a cold room.
As with roofs, other cabin companies use thin insulation (and shout about how good it is!). Our standard Celotex insulated floor achieves a U value of 0.39 and our building regs floor achieves .21 W/m2K
Windows and Doors
We offer a number of options for our log windows and doors, with our ISO house quality having a U value of 1.1 W/m2K (we can even improve on this if required by using triple glazing!), which again is fully compliant with UK Building Regs (in fact a little better than what is required). The U value of the windows and doors also influences the overall building U value (especially if there are a lot of them) - there's no point opting for a high spec Twin Skin cabin but using standard log cabin windows and doors.
As you can see from the above facts and figures – a cabin that is advertised as “insulated” can mean a variety of things and some are more equal than others! Hortons cabins are designed for purpose, not for headlines or “hidden” penny pinching.
All of our log cabins can also be produced as Twin Skin log cabins as well. Therefore you can have a fully insulated log cabin regardless of the size, shape ans designs.