I want to insulate my loft but i also want to be able to use the space for storage so i want to board it aswell.
Due to the depth needed if i use blanket insulation i wont be able to board it so i thought the best thing to do would be to use foil backed foam boards.
Is this the best thing to do and what depth of board would i need.
Many thx

Too many questions. Is it the roof of the loft or the floor? How thick is the cavity you want to insulate? what part of the country are you in? (This will dictate how much insulation you need)

Iam in the UK actually, the space is four inch high and its the floor of the loft.
The thing is i think there is a minimum ammount that has to be installed if its mineral wool (building regulations) which might end up being to high but i know foam board is more effective for its thickness than what blanket insulation is.
Temperature where i live rarely exceeds -5 in the winter, the norm is -1 to -2 and these are night temperatures.

Plumbob, I'm not familiar with the building codes in the UK, so it's hard to comment. I'm not even sure if the UK uses the same method of measuring resistance to heat loss (R value). The only thing I can tell you is that foam has one of the best resistance to heat loss on an inch by inch comparison. I would talk to an architect or building official in your area, they may be able to help.

Bob,

The next approach is to consider insulating the rafter space above the loft floor. It is recommended that 2" of space be kept between the inside of the roof sheathing and the face of the foam board sheathing to allow for air movement behind the insulation. In th US we have air vents located under the eaves of the roof that connect to this 2" space. At the peak of the roof we employ ridge vents so that natural convection draws the air through the eave vents and out the ridge vent. These vents are important as the air movement through them removes any condensation that might accumulate as the warm humid air in the home rises and meets the cold surface of the roof sheathing.
A benefit to this is that you can achieve the amount of insulation desired and have an insulated loft space as well.

Another approach that we take in the States is to apply a closed cell foam directly to the underside of the roof sheathing, in the rafter space. With this method no air gap is required as the closed cell foam seals directly to the sheathing thereby not allowing condensation to accumulate on the underside of the sheathing. Some people believe that this can cause the roof material to overheat in the summer time but recent studies have shown that not to be the case. Again, you can finish of the loft space and have a well insulated house to boot.

Good luck from across the pond,

Tom Judt