Finally I could start building the actual fishroom!
The outside weather boards were pretty old and there were two windows but I only needed one so I decided to rip the old sheeting down and start again. That way I could be certain that everything was watertight and could board up the unwanted window properly and at the same time.
After the outside was done everything on the inside came down. Old GibBoard, insulation, wiring, the remaining window and the old door - all came out. Starting with a clean canvas the walls got insulated with the best R-rating I could find for a 90mm wall and so got the ceiling. Every cent you spend on insulation will save you money in the long run maintaining the temperatur in the room. Since most heat escapes through the ceiling, I used 15cm polystyrene - means I had to beef up the rafters to take the insulation. Every little hole was filled with expanding foam.
The old door was not insulated so I build a new door with 40mm polystyrene sandwitched in between two 12mm outdoor plywood sheets. Support for the hinges was added to the internal frame and a couple of stainless steel screws later the door was done. It only has 1mm play around the frame to make it realy air tight and it is a struggle to open the door now because the room is so airtight.
The floor got insulated with 40mm polystyrene as well. This was fitted in between a frame that supports and distributes the weight of the two racks holding the tanks.
In the areas for the drain and the exhaust fan I used polystyrene as insulation in the walls as well. The R-value is not as good as the other insulation but it is easier to cut a perfekt hole through polystyrene and use foam to seal it then it is with rock wool.
After that, it was time to line the whole inside with a vapour barrier and seal everything absolutly airtight. This is very important! Otherwise the warm, moist air from the inside will travel through the insulation and cools down depending on temperature and humidity it will reaches a point (dew point) where the now cooler air can't hold the water anymore and it condensates. This means mould plus a wet insulation can't do it's job anymore. By strictly keeping the inside air and the outside air apart, theses problems can be avoided. For the same reason I used cabel conduits and external powerpoints for all the electric installation. That way I didn't have to breach the vapour barrier with holes for the cables.
Cutting a hole for the heat exchanger and installing the extraction fan was easy. Not so easy was installing the lino floor. I made sure it ran up the walls about 80mm so in case there is a leak, the water will flow out the door without damaging anything.