Hortons Log Cabins
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What is a "real" Twin Skin Log Cabin?

What is a Twin Skin Log Cabin?

INSULATED CABINS – WHY THE TWIN SKIN LOG CABIN IS THE BEST SOLUTION

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 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 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.


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