Our wodden house from 1978 has only 10cm (4") insulation. This winter I decided to add an additional 10 cm of insulation to the inside of the wall while upgrading our home movie theater in our living room. I wanted to hide all electrical and video/audio wires. I did some research on how to do this correctly, while keeping a tight vapor barrier. Here is how I did it, step by step.
This is a section of the original wall where there is also a window. I started by removing the winow trim and cutting away 10 cm of our wooden floor to give room for the new wall.
With all the dry wall removed you can see the old 0.15 mm thick plastic vapor barrier. It should be placed on the warm side of the wall. In Norway that is the inside of the wall. It is, however, ok to have a maximum of 1/4 of the wall insulation on the inside of the vapor barrier.
Since the original insulation was in the vertical direction, I added the next 5 cm in the horisontal direction. Then I added the new vapor barrier which was taped together with the old one. Any holes found was fixed with some duct tape. I also useed silicon around adjacent walls. It is very satisfaying to have a completely tight barrier.
Here is where you see the advantage of having the inner most 5cm of insualtion inside of the vapor barrier. All electrical wires, can be hidden here without any worries of punctuate the barrier and getting condensing problems in the wall. The inner most 5 cm insulation was in the vertical direction.
Insulation on the inside completed, a full 20 cm in total for the wall. Some of the dry wall in place. To the very left you can see a hole for a speaker cable.
There it is. The completed wall with the first layer of paint.