keeping workshop cool in summer

[Matt@]

I work in a 20ft x 10 ft concrete sectional building in the garden. The walls are lined inside with 2" celotex and 18mm chipboard. The roof is single skin cement fibre corrugated sheets. When the suns out, by mid day the inside face of the roof is red hot and the inside of the building is around 30 degrees and we aint even in summer yet! The walls are cool to the touch a) due to sun not shining on them and b) they are insulated and lined anyway.

So my overheating problem stems from the roof. I have experimented with putting an offcut of 30mm celotex across the roof sheet supports and after a couple of hours the heat from the roof has worked its way to the inside face (warm to touch) . Any ideas what I can do to stop the heat coming through the roof? I can only think line with thicker celotex but how thick do I need to go to stop the heat. U values for roofs should be around 0.15 which means I think around 150mm celotex required to stop heat passing out. So does stopping heat coming in work in reverse?! ie I need 150mm lining inside the roof?

any ideas appreciated apart from fans and air con

 

[blackrodd]

At the risk of sounding silly!, What about silver reflective paint? Designed for this very job! repels heat and cold temperatures.
Wickes sell it and others, just go for the cheapest At roughly 25 sq mtrs you'd need about 3 x 5ltrs tins.
Jet wash off the roof and use a sweeping brush.
Regards Rodders

 

[Matt@]

actually just thinking about that, there are perspex skylights and these dont get hot but maybe thats because the sun comes straight through them! certainly I can feel the sun on my back when standing beneath them. The trouble is, the solid roofing only needs to absorb a tiny amount of the suns heat and it warms up underneath thus acting like one giant radiator. I'm not feeling painting will make the difference required?!

 

[blackrodd]

I've just googled solar reflective paint and some australian tests claim between 6 and 10 degrees cooler.
But that seems to be the realy good quality paint and I've not costed it out.
Perhaps the easiest to fix, in you're roof situation would be the rolls of space saver quilt, I did use some from a bloke
in Plymouth for the last loft extension I did about 3 years ago, works very well, and he was cheap and fast delivery.
You must ensure the quality and insulating value is not compromised by price, I recall.
In the searches I've made, heat reflection seems to be the best rather than just green or brown slab or quilt insulation
as in good old Rockwool.
HTH Regards Rodders