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I want to better insulate a shed so that I can use it for a garden office. Already have power and WiFi sorted , but it's freezing sitting in there in winter and costing a lot to heat with an oil radiator! Looking for some guidance on insulation from the hive mind.
It's currently got 25mm of celotex type insulation in the walls and roof. The shed frame is made from 45x45mm timbers. I was thinking of battening out the frame internally so I've got more depth, then putting thicker celotex type insulation in - say 80-100mm. Don't want to go much thicker as I'll loose too much internal space. Ill then clad internally to finish it off.
Should I chuck the current 25mm insulation and start afresh with new insulation board in the thickness I want? Or am I ok to get slightly thinner new insulation and sandwich this up to the existing stuff? Is 100mm total insulation thickness in the walls and roof suitable for a 2.4x3.6m internal space with a heater, or am I way off the mark and need to go thicker? I probably need to put some insulation down on the floor, but I'm somewhat limited by ceiling height as the shed has a pent roof. How little insulation can I get away with in the floor?
I chucked 50mm own brand celotex in the walls floor (underneath) and roof
I put an oil heater on for a bit but it gets too warm, you don't notice the heat until suddenly it's too hot. I have a standing desk and no sooner than I've cleared it, it seems full of stuff again.
It all about U or R values isn't it?
Let's do some sums!
Walls:
45mm timber + 25mm celotex
R values: 0.5 + 1.1 = 1.6 K•m2/W
1/R is the U value = 0.625 W/m2•K . This means you will loose 0.625 Watts from each square meter for each degree temperature difference.
Roof:
25mm celotex + 25mm plywood?
R values 1.1 + 0.18 = 1.28 K•m2/W
U value = 1/(1.1+0.18) = 0.78 W/m2•K. This is worse than the walls. But I guess you have more wall area than roof for heat to escape.
Floor:
Let's assume a un insulated 25mm ply?
U value= 1/0.18 = 5.5 W/m2•K.
Let's assume a un insulated 200mm concrete
U value= 1/0.3 = 3.3 W/m2•K.
So based on this I would get some insulation on the floor. You are loosing 4 x more heat through the concrete floor than the roof. 25mm of celotex + 25mm ply would get the floor to 1/(1.1+0.3+0.18) = U=0.63 W/m2•K.
Disclaimer. This is the first time I have performed this type of analysis as thought it would be fun!
For the minimal loss of space, I'd bang a further 50mm all round. Heat rises...seems daft having so little on the ceiling (but what are the windows and door like? A bit of draft excluder may be the cheapest way to save some heat?
Error in wall calc I think. Only the frame is 45mm wood. Most of the walls is 25mm celitwx plus whatever thickness of cladding. 12mm?
I would bond another layer of celotex over the internal walls, floor and ceiling. Don't frame it out unless you want fixing points for heavy stuff in a certain area, if you insulate over the frame it will remove cold bridges. And you should be able to float 18mm OSB over celetox on the floor without any compression issues, its how I did my van (but with 12mm ply). Heat rises and a cold floor equals cold feet.
Draught seals around the door and secondary glazing on the window could also help.
Get a proper IR panel heater rather than an oil radiator. Heat you, not the rest it. Will pay itself back very quickly.
Point to note about PIR and PUR board insulation is that unless it's installed with tight joints and taped, it doesn't perform much better than an equivalent glass fibre batt, so unless you're going to be thorough save yourself the money.
That said, you have a perfect opportunity to just overboard the shed frame and seal the joints so you have an uninterrupted layer of insulation. Which if combined with the roof and floor should be scorchio.
Cheers for all the replies, plenty of food for thought.
Yes, ply floor and 12mm thick external T&G cladding on the walls currently. Roof is ply with corrugated bitumen sheets.
I like the idea of not framing it out and adding another layer of celotex over the top of the existing all round to remove the cold bridges.
The door is half glazed with a very thin double glazed panel, and is draughty, so that'll need fixing up too.
Off-the-shelf insulated sheds tend to come with boards that fit between the floor joists underneath the floor boards to provide some insulation without taking away floor height or creating a step. But I am assuming that'd be too much faff to do now.
So my instinct would be something along the lines of
Reframe the door opening with a 50mm step and fit Double glazed door
30-40mm XPS boards on the floor topped with with 1/2" ply and vinyl floor boards (could also use more insulative PIR boards, but XPS boards have better compression strength)
50-60mm PIR on the walls - bash them up against the existing framing to indent where they are and notch them. Line with plasterboard or ply screwed into the framing.
Similar for the ceiling - ideally you want thicker but you'll probably get head height issues. You could perhaps switch to a warm deck roof when you need to replace the shingle.
If you use low profile batten lighting and surface mounted conduits rather than perforate the insulation in the ceiling then that helps to eliminate cold bridging. Similar for walls but less important.
No matter how much insulation you use you'll have the issue of low thermal inertia that jamiemcf mentions, but the only way to get more inertia is to rebuild the shed onto an insulated concrete deck 🙂
Also I haven't done any actual calculations - I did once make a U/R value calculator for homogenous & non-homogeneous construction but it'd take me a while to find it.
BTW biggles, I assume your thermal transmittance calcs are fine but to actually calculate real world representive quantity of heat loss you'd need to apply a factor to account for the conductance with the internal and external surfaces. This is lower for vertical rather than horizonal surfaces - hence the higher U-values targets for roof rather than wall. It is all a bit complicated though and I forget exactly how it works 🙂
Have a look at Superfoil products. As with celotex it is important to tape all joints.
Have a look at Superfoil products. As with celotex it is important to tape all joints.
Hmm for foils, as far as I am aware they calculate the advertised R values by getting a single layer lab tested as Z and then saying this product has Y layers so it's R value is Z times Y but I doubt things really stack up that way in the real world.
Thanks for this. Every day is a school day!BTW biggles, I assume your thermal transmittance calcs are fine but to actually calculate real world representive quantity of heat loss you'd need to apply a factor to account for the conductance with the internal and external surfaces. This is lower for vertical rather than horizonal surfaces - hence the higher U-values targets for roof rather than wall. It is all a bit complicated though and I forget exactly how it works 🙂
Also for floors, do they loose less heat than a roof since the ground temp is typically higher than the air temperature?