What is a Twinskin

 

There is only one economic way of making a log cabin truly usable all year round, which is not to just insulate the roof and floor but also the walls as well by creating a cavity like a modern house, with 2 individual walls separated by an insulation cavity which can then be filled with various types of insulation.
Once insulated, the walls can be the equivalent thermal value of a solid wall which is 5 times thicker. This saves both space and also materials and therefore costs.
But before discussing these options, it is important to understand the construction of a log cabin.

Log cabins rise and fall during the year due to the differing moisture content of the air in the different seasons - there is no vertical framework in a log cabin which holds the building at a set height (the logs are the framing and the cladding all in one). It is fair to assume each of the wall logs will absorb enough air moisture to cause about 1mm of extra height per log in the winter, and there are generally about 16 logs per wall. This means that an average log cabin will be 16mm taller in winter than summer, but this can vary quite a lot from cabin to cabin.


The 3 main ways of insulating a cabin are:

1)  Buy a standard log cabin with a single thickness of log wall, and then line it out with framing and plasterboard (or similar - this of course loses the internal cabin look and feel which is the reason for buying the cabin to start with). The problem with this method is that there needs to be an expansion gap left all around the top of the inner wall to allow for movement of the logs without fouling the roof boards using sliding brackets (a real problem as the roof boards can be lifted off of the supporting side wall if incorrect), but it is a guess as to how big this needs to be:
a) When a cabin is built, it will always settle down for the first few weeks whilst the corners joints and logs fully interlock (no matter how hard they are hit / clamped down at the time of construction, they will always do this).
b) It depends on the time of year of construction. If built in the winter, you have to allow for the first few weeks of settlement as well as the 16mm (average) seasonal movement.
This means that it really is a guess at how to arrive at the correct expansion gap. There is also the issue of how to finish the internal linings around windows and doors because, once again, the windows and doors will rise and fall with the cabin walls but the internal lining doesn't move (and therefore the lining cannot be fixed to the windows and doors for a clean and neat finish).

Many cabin retailers will only mention insulating the roof and floor because they only sell single walled cabins and do not recommend insulating the walls because they are very aware of the problems that this can cause. They will say that the cabin is warm enough without insulating the walls, but this is plainly untrue and just "sales talk" to try and get around the problem.

2) "Twinskin" a log cabin. This method is to build a second, fully interlocking cabin inside. The crucial element here is that the inner logs are fully interlocked with the outer, and will therefore rise and fall in unison (easily spotted as there are 2 logs protruding on each corner where the inner logs are visible as well). This eliminates any of the expansion issues as no expansion gaps are required (at roof height or around windows and doors). It also has the added advantage of forming a very strong box section at each corner for rigidity.

3) Simply build a totally independent log cabin inside another (which is not interlocked with the outer logs), known as a dual walled cabin, and insulate the gap. This method only exacerbates the problems inherent to the framed method because there are 2 independently moving buildings with common elements (windows, doors and roof) which can move in totally opposite directions at any given time, doubling the potential problems.
There is also the added disadvantage that there wouldn't be any additional integral strength in the corners, leaving a much weaker building.