Insulating Summerhouse with Celotex
Discussion
After a lot of googling I'm getting rather confused, so hoping I can be set straight...
I have a summerhouse I'm using as a gym and over the winter its cost a small fortune to heat, even just to keep it above 5 degrees. I'm therefore planning on insulating it where possible without going OTT.
I've decided on celotex for walls and roof and secondary glazing for the windows/doors. But I am reading conflicting advice on what's required. Remember, this is only a glorified shed so I really don't want to throw thousands at it.
Most advice seems to suggest filling the gaps between the battens with celotex (5cm as that's the full depth of the battens) then fixing some 6mm or so ply over the top. Also suggested to fix a vent or two or use a dehumidifier. I'm good with this.
But, other advice is that a damp proof membrane is needed on the cold side. Some say it is, some say it isn't. As i don't fancy removing the current wall, I'm not sure how I'd fix this, assuming it is even required.
Others say I should not pack the celotex to the full depth of the battens and that an air gap should be left between the celotex and the outer wood. If so I'd need to use thinner celetex and mount it on thin battens to give the gap. Bit more of a pain but ok. Now some say I need air holes for circulation, especially in the roof void. Not sure how I can do that as the summerhouse construction has wooden battens going around to top so I've nowhere to drill the holes.
Am I overthinking this?!! Would my original plan of stuffing gaps with celotex and overcladding with ply, and being mindful of airflow be good enough?
Some clarification would really be most appreciated!
I have a summerhouse I'm using as a gym and over the winter its cost a small fortune to heat, even just to keep it above 5 degrees. I'm therefore planning on insulating it where possible without going OTT.
I've decided on celotex for walls and roof and secondary glazing for the windows/doors. But I am reading conflicting advice on what's required. Remember, this is only a glorified shed so I really don't want to throw thousands at it.
Most advice seems to suggest filling the gaps between the battens with celotex (5cm as that's the full depth of the battens) then fixing some 6mm or so ply over the top. Also suggested to fix a vent or two or use a dehumidifier. I'm good with this.
But, other advice is that a damp proof membrane is needed on the cold side. Some say it is, some say it isn't. As i don't fancy removing the current wall, I'm not sure how I'd fix this, assuming it is even required.
Others say I should not pack the celotex to the full depth of the battens and that an air gap should be left between the celotex and the outer wood. If so I'd need to use thinner celetex and mount it on thin battens to give the gap. Bit more of a pain but ok. Now some say I need air holes for circulation, especially in the roof void. Not sure how I can do that as the summerhouse construction has wooden battens going around to top so I've nowhere to drill the holes.
Am I overthinking this?!! Would my original plan of stuffing gaps with celotex and overcladding with ply, and being mindful of airflow be good enough?
Some clarification would really be most appreciated!
sfella said:
Personally I would leave an air gap, so rather than 50mm of celotex I would use 35 or 40mm if you can get it. A breathable membrane would also be worth considering to help prevent a build of moisture. Shouldn't cost too much to do
Thanks for the reply. If I was to leave a gap, I assume it should be on the outside (ie: between insulation and outer skin)? Would I need to air the void?Really appreciate all the advice 
If i'm not leaving a gap, I thought I might as well use 50mm as I've room for it and it's not a lot more expensive.
If I do leave a gap, as mentioned above it will be difficult to vent as the summerhouse design doesn't really allow for it.
I did consider the bubble wrap type stuff, but that really does need a gap to work and I didn't think the finished product looks as good.
The room is only used 3 to 4 times a week for a gym. I had the roof redone last autumn (with rubber) but with no insulation at all it's been 0 to 2 deg C even with an oil filled radiator on. It also doubled my household electricity usage in December to February! It doesn't need to be overly warm, but I want to protect the machinery and cold slippery Kettlebells are a danger to use!
If i'm not leaving a gap, I thought I might as well use 50mm as I've room for it and it's not a lot more expensive.
If I do leave a gap, as mentioned above it will be difficult to vent as the summerhouse design doesn't really allow for it.
I did consider the bubble wrap type stuff, but that really does need a gap to work and I didn't think the finished product looks as good.
The room is only used 3 to 4 times a week for a gym. I had the roof redone last autumn (with rubber) but with no insulation at all it's been 0 to 2 deg C even with an oil filled radiator on. It also doubled my household electricity usage in December to February! It doesn't need to be overly warm, but I want to protect the machinery and cold slippery Kettlebells are a danger to use!
S6PNJ said:
Hi OP, where are you planning to buy your Celotex from as I'm currently pricing up some internal insulation as well and am always on the lookout for other suppliers.
Cheapest place I've found so far is (surprisingly) Wickes for both 50mm and 25mm as their prices include delivery, which is often pricey. Squiggs said:
Are you trying to keep it warm 24/7 seven days a week?
I've got a large summer house (5m x 4m) which is insulated along the lines of your thinking and I've got a 2kw fan heater in there.
Even when it's been freezing outside it's only taken a couple of hours to get it up to a comfortable temp.
My summerhouse is 3 x 4 split in to two rooms, the front is my gym the back an extra shed/workshop. I had power, lighting and network installed last year. I also had the crappy felt roof replaced with rubber.I've got a large summer house (5m x 4m) which is insulated along the lines of your thinking and I've got a 2kw fan heater in there.
Even when it's been freezing outside it's only taken a couple of hours to get it up to a comfortable temp.
The idea is to make this a nice place to be to encourage me to use the gym and try to obtain some level of fitness. I'm impressed with myself that I've used it right through the winter period!
It only needs to be kept above freezing (say 5 deg) to protect the machinery and hifi etc. When I'm using the room I'd then warm it a bit more. This winter was the first time I've been using it as a gym and I have a 500 watt heater and dehumidifier in there. The two things I found were:
1. With the oil radiator on low thermostat setting 24/7 I used double the electricity in December, Jan and Feb than I usually do for the whole house.
2. When heated from say 2 C to 12 things like the kettlebells wouldn't be usable as too cold and slippery.
Without spending a fortune, my plan is:
50mm celotex within the battens on walls and roof. Over clad with ply.
Secondary glazing for the three large windows and two doors.
Vents high and low both ends.
Extractor fan.
New rubber floor matting. Good for insulation and gym use.
Hoping to do the whole lot for around £500 or so which will pay for itself in two years.
First (largest) wall done. 50mm celotex packed between joists. Any gaps sealed with tape. 6mm ply over-boarded. The ply is a bit thin, but should be ok given that this is a summerhouse. Most of the time was spent moving the (surface mounted) plug socket, and feeding the cable (which is clipped to the joist) through a hole in the ply.
I found the celotex fine to work with, although I did use a mask just to be safe. Ended up cutting with an old kitchen knife, which I found neater than a saw.
Thinking forward, does anyone have any suggestions as to a decent primer to use internally on the ply, without costing a fortune?
I found the celotex fine to work with, although I did use a mask just to be safe. Ended up cutting with an old kitchen knife, which I found neater than a saw.
Thinking forward, does anyone have any suggestions as to a decent primer to use internally on the ply, without costing a fortune?
It's taken a while, but I'm almost done...
Rubber roof.
Rub-down, fill and paint of outside.
Replaced and extended decking.
50mm celotex over-boarded with 6mm ply.
base sealant, painted.
1.8cm rubber flooring
air-vents
Electricity. LED lights.
Still got to do secondary glazing.
Mostly done by myself. First time DIYer
Before
After
Rubber roof.
Rub-down, fill and paint of outside.
Replaced and extended decking.
50mm celotex over-boarded with 6mm ply.
base sealant, painted.
1.8cm rubber flooring
air-vents
Electricity. LED lights.
Still got to do secondary glazing.
Mostly done by myself. First time DIYer
Before
After
Simon. said:
I will be interested to hear how this works with those big glass windows / doors at the front, have you done anything to those to try and limit heatloss via them?
I’m intending on installing 4mm secondary glazing which will hopefully help. It’s never going to be perfect and I wouldn’t have chosen to start with this sort of summerhouse had it not been there when I purchased the house.Still here 
It was pretty successful, especially considering what we started with. Still looks good and is used regularly (or was before delayed hernia operations due to a closed NHS, but that’s another story).
No leaks or mould issues. In the winter we use a thermostatic radiator with a smart switch to limit heating to nights. We also use a dehumidifier when necessary, again on a timer.
It would have been better with a higher single pitched roof (to give more ceiling height) and less windows, but we had to work with what we had.
As a garden office you’d need to heat it more. For a gym keeping it cool is fine.
It was pretty successful, especially considering what we started with. Still looks good and is used regularly (or was before delayed hernia operations due to a closed NHS, but that’s another story).
No leaks or mould issues. In the winter we use a thermostatic radiator with a smart switch to limit heating to nights. We also use a dehumidifier when necessary, again on a timer.
It would have been better with a higher single pitched roof (to give more ceiling height) and less windows, but we had to work with what we had.
As a garden office you’d need to heat it more. For a gym keeping it cool is fine.
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