Weatherproofing and insulating existing shed

We had a fairly substantial shed (12x8ft) constructed a few years ago by a company. Its was partially built off site, then bolted together on site, roofed, then torched on felt. Its 3x2" frame with 22mm T&G cladding allover, with a few narrow horizontal windows approx 12x4".

I want to insulate the shed and will be going down the celotex route - dont mind paying that bit extra for a good product if its going to last a good few years.

There are a couple of places where water gets in, which mainly seem to be where the T&G has separated slightly due to shrinkage during the summer, and a couple of knots. The far and rear side of the shed are close to boundary walls, so not the easiest to maintain. I had painted these two sides with a bitumen paint, thinking I could just do it and forget it, but as this is in direct sunlight, its gone bery powdery and worn off in places. The two visible sides have been treated with a good protector.

As I want to insulate, board and then finish the inside so I can use it as an occasional office space, Im thinking I need to make sure the inside is as watertight as I can. I plan to use 50mm celotex and leave a 25mm gap between this and the out cladding that fixed straight onto the frame.

Im contemplating overboarding the rear and far side so I can sure as best I can that water isn't going to get in, and am considering bitumen corrugate roofing sheets. Ugly I know, but you dont see these sides anyway, or even the clear pvc sheets.

Ive considered vinyl cladding but this is just too expensive, and even looked at UV resistant, waterproof PVC sheets to cover these sides. Also considered felting the shed walls or epdm.

Or, am I going a bit over the top? I know water is always going to find a way in, but If I can just put an extra layer in the way, it may just reduce the possibility.

Any other suggestions, or am I just going a bit over the top?

I could just use some silicone sealer to fill in some of the gaps between the T&G and knots and any other obvious ingress points.

Its a tricky one because a shed construction is incorrectly configured, so altering it is a bit of a compromise.

A timber frame construction with timber cladding, built to be fully weathertight would have something like the following,

From inside:
Plasterboard
Vapour barrier
Studwork with insulation in between
Sheathing plywood or osb board
Breathable membrane
50mm x 25mm battens laid flat
Timber cladding

If you imagine a structure built as far as and including the breathable membrane, that would be a weathertight structure.

Add the 25mm battens and you have a cavity
Fit the cladding and the building gets protected from rain, sun etc -it doesnt matter if water gets through the cladding, it runs down and out the bottom.


In your situation, if you fit celetex in between the studs with a 25mm cavity, that will create something similar, but any water getting through will run down and land on the bottom plate of the studwork.
Its a fair compromise for converying a shed.

Internally If you fit some small 25mm battens, that will act as a stop for the celetex to form the 25mm cavity. I would be tempted to go for something 20mm thick to make sure the insulation sits back a bit. If you cut the insulation undersize, say by 6 to 8mm, then you can use a foam gun around after. Buy a quality foam gun and practice getting the foam pressure turned right down so the foam goes nicely into the gap without blow back creating a mess.

Then use 75mm foil tape and tape over every stud forming a continuous vapour barrier.

Dont worry about a breathable membrane -there is nowhere to fit one.

Externally on the sides you cant see, you could fit 38mm x 19mm battens and then screw on rendaboard (about £40 for 2400 x 1200 x 12). It would be worth you fitting breatheable membrane before the battens -ie onto the existing cladding.

The first question is, does the shed have gutters? They stop a lot of the water even touching the sides.

Have Epdm offcuts down the rear of any external sheds that have limited access, easy to retrofit and totally water proof, what heat source will you be using?

Notch7 said:

Its a tricky one because a shed construction is incorrectly configured, so altering it is a bit of a compromise.

A timber frame construction with timber cladding, built to be fully weathertight would have something like the following,

From inside:
Plasterboard
Vapour barrier
Studwork with insulation in between
Sheathing plywood or osb board
Breathable membrane
50mm x 25mm battens laid flat
Timber cladding

If you imagine a structure built as far as and including the breathable membrane, that would be a weathertight structure.

Add the 25mm battens and you have a cavity
Fit the cladding and the building gets protected from rain, sun etc -it doesnt matter if water gets through the cladding, it runs down and out the bottom.


In your situation, if you fit celetex in between the studs with a 25mm cavity, that will create something similar, but any water getting through will run down and land on the bottom plate of the studwork.
Its a fair compromise for converying a shed.

Internally If you fit some small 25mm battens, that will act as a stop for the celetex to form the 25mm cavity. I would be tempted to go for something 20mm thick to make sure the insulation sits back a bit. If you cut the insulation undersize, say by 6 to 8mm, then you can use a foam gun around after. Buy a quality foam gun and practice getting the foam pressure turned right down so the foam goes nicely into the gap without blow back creating a mess.

Then use 75mm foil tape and tape over every stud forming a continuous vapour barrier.

Dont worry about a breathable membrane -there is nowhere to fit one.

Externally on the sides you cant see, you could fit 38mm x 19mm battens and then screw on rendaboard (about £40 for 2400 x 1200 x 12). It would be worth you fitting breatheable membrane before the battens -ie onto the existing cladding.

Click to expand...

Cheers fur the advice! I'll apply that for the internal condition.

Your suggestion is a bit over budget for me. I was considering just using either corrugated pvc cladding or even the bitumen.

https://www.wickes.co.uk/Wickes-Mini-Profile-Corrugated-Sheet-660-x-2400mm/p/240170

https://www.wickes.co.uk/Onduline-3mm-Green-Corrugated-Bitumen-Sheet-950-x-2000mm/p/240059

Anything to add a barrier and still allow air flow.

Any reason not to??

In answer to others...

I won't be adding any heart source as it's only for occasional use. Can share wear a jumper!!!

It's a pent shed with a recent overhang, so no guttering is fitted.

Thanks!

MrBreakit said:

Cheers fur the advice! I'll apply that for the internal condition.

Your suggestion is a bit over budget for me. I was considering just using either corrugated pvc cladding or even the bitumen.

https://www.wickes.co.uk/Wickes-Mini-Profile-Corrugated-Sheet-660-x-2400mm/p/240170

https://www.wickes.co.uk/Onduline-3mm-Green-Corrugated-Bitumen-Sheet-950-x-2000mm/p/240059

Anything to add a barrier and still allow air flow.

Any reason not to??

Click to expand...

I dont see why not, as long as you have enough roof overhang.

Internally you dont need continuous spacer battens, short pieces would fine, just enough to stop the insulation flopping back

No point insulating it if you are not heating it.

Surely insulating it would help to maintain a more ambient temp rather than extreme heat or cold from external?

Notch7 said:

I dont see why not, as long as you have enough roof overhang.

Internally you dont need continuous spacer battens, short pieces would fine, just enough to stop the insulation flopping back

Click to expand...

Another option I have considered would be to fit a weatherproof pvc UV resistant heavy duty tarp on two sides with it off the main cladding about an inch with a timber frame around the edge.

I think I'd prefer this than either the bitumen or pvc corrugated sheet. Why of these options should come in <£100 for the two sides. The tarp sample I received looks good enough age is £6sqm.

zadragrun said:

Surely insulating it would help to maintain a more ambient temp rather than extreme heat or cold from external?

Click to expand...

If it’s cold it stays cold, insulation does nothing to warm a building .