My front door is surrounded only by pretty thin wooden panels on either side and I can't imagine it's doing a very good job of keeping the cold out. I already sealed up the gaps between sections (in the right weather you could see sunlight through the joins!) with silicone, but I'm now thinking of putting up a second layer internally and having some kind of insulation in between the two. There'd be a good 4cm between the two layers - what sort of insulation could I be looking at putting in? Cheers!