Building an insulated fermenting box?

I live in New Zealand and around this time of year the temperature averages around 10-15 degrees.
I was thinking about making a wooden box to store my brew out of some basic materials from the local hardware store.


Let's say I make a plywood box large enough to keep a 25l plastic brewing barrel and my bottled beer during secondary fermentation.
If I use polystyrene walls for the interior of this box, in theory, this should keep the heat at a constant.
I will varnish the plywood to stop any water damage (The box will be outdoors) and, once assembled, fill the gaps of the box with a silicone sealant.

If my brew is at the right temp. when I place it in the box would I be right in assuming that it should be good for the primary fermentation process? Or will I need some sort of heating (Not too keen on this, really I just want something that wont use any electricity) but if needed should I use something along the lines of a light bulb attached to a thermostat controller?


Does this sound like it will work to anyone with experience? If not where have I gone wrong?

 

Use the search function and search for fermentation chamber. Lots and lots of different ways to do this.

You will need to add some type of heat to maintain your proper temps.