Shed insulation

[jason110]

Right, so I’ve got a shed at home and I want to insulate it. It’s cold and I’m getting condensation in a big way this time of year. 

so if I may, I would like to run an idea past the folk here and get some feedback. 
 

for context; It’s got single skin corrugated steel on it, which I know it’s a termed a “warm roof”. It’s around 30m2 that I need insulating. I’m happy to have a go myself. 
 

I have looked at icyene spray foam insulation but at around £25 -£28 per sq meter, so that’s a non starter due to cost. Most of the cost is since no one in my area does it and the people I’ve spoke to will have to travel and thus drive up the cost. 
 

ive been looking at celotex sheeting and I’ve found a bloke local to me that has seconds - so cheaper.
 

right,  so the plan is to stick the celotex straight to the sheeting with low modulus foam, then hold it with batten and fill any gaps with expanding foam. Then sheet over it with tyvek membrane, with an overlap of around 80-100mm. 
 

 I know I need to prevent warm moist rising and touching the cold roof which leads to condensation and then rot etc. 
 

I’m after some advice if that’s the best way to do it or is there another way to achieve similar results. I’m just after a decent working space that I can heat a bit in winter and will help prevent my tools rusting over. 

[Badger110]

Is i pitched or a flat roof?

 

Can you get some pictures taken of the inside construction and where it meets the walls too

 

A warm roof is where the insulation is sandwiched between the roof joist and the top layer like this

 

Yours will be a standard roof material fixed to the joists, once insulated, it will then become a cold construction roof as the joists act as a bridge between the room and the roof material...the insualtion is fitted between the joists.

 

 

What you want to do is fairly straight forward but not sticking anything to the underside of the roof material. 

 

The key is airflow, as you stated, to stop condensation build up. 

 

We would place an air gap between the celotex and the roof material...celotex can be cut snug to fit inbetween the joists to achieve this. 

 

You will have no need for vapour barrier in a cold roof construction

 

If you can get some pictures it would help 👍

 

 

 

 

[elbekko]

I put closed-cell Recticel against my roof (house, not shed), with the idea that it being closed-cell a bit of moisture wouldn't matter too much. The stuff is hard to find at the moment though...

Have you got a picture of what it looks like? I'm having trouble picturing it.

[jason110]

thanks both. 
 

the roof is pitched. It’s roughly 18” over a 6 metre distance. I’ll attach a few photos in the next post down 

so, @Badger110 I need an air gap between my insulation and steel sheeting? Make sense and sounds simple enough in that case. Is the key still to stop warm air rising, I’m also concerned about condensation and don’t want to create any further issues if I can help it.  
 

@elbekkoim not sure exactly what the fella has just that it’s 50mm or 100mm seconds insulation sheeting. 

[jason110]

Those are what I have on my phone at the moment. If you want more let me know. 
 

its pitched with the joists concreted into the wall. The corrugated sheets are then running perpendicular to the joists. 
 

the small batons in the corrugations are for the roofing teks screws to stop the corrugations being crushed inwards. 
 

Ignore the red circle, beneath it is my halogen spot light and I was showing someone the issue I’ve got 

[Retroanaconda]

The joists look about 150mm deep?

I’d go with 100mm celotex between the joists leaving a 50mm air gap above, then air can come up through the eaves and run along the ridges of the sheets above the insulation. Then a simple plasterboard ceiling to retain it and provide a nice finish.

Is it single skin blockwork? DPM under the slab do you know?

[jason110]

Thanks @Retroanaconda they are indeed 150mm. The block work is 9” hollow block and I’m not sure if the pad has dpm or not. But there is no rising damp and the pad was put down in the ‘80’s 🤷‍♂️  I have the plans somewhere with the house deeds  

sounds like a good plan and simple enough. 
 

 

[Badger110]

You'll need to allow some airflow into the roof space.  On the walls where they meet the roofing material ( highlighted here in red )

You can either core drill ( 41mm x 2 should do ) or break out some block work on both walls either side and in between each set of joists, this will allow air to flow through above the celotex and consequencently reduce condensation.  You'll need to cover these on the outside as it looks like your eaves are non existant...you have no protection of rain coming through the holes and making it worse with moisture!

 

Are you getting full sheets? If not, baten off the joists to a required length and screw the celotex onto that...it will reduce your headheight with the thickness of the celotex.

 

Finish the celotex however you want, i'd steer clear of plasterboard though, heavy and prone to moisture in a single skin concrete shed....6mm ply is cheaper and paints up just fine 👍

 

[jason110]

@Badger110 thanks for the advice. It’s invaluable. Sounds like a plan. I have eaves on the outside, but it’s the facing the weather so will cover it accordingly to prevent any water ingress. 
 

I am getting full sheets. He has 120mm thick sheets for £25. 

[Badger110]

Ah a better picture, yup those will be fine.  Mesh over the holes though...nothing worse than finding a wasp's nest in your airgap.

120mm will give you a 30mm gap, but with a 120mm you're well over the required thickness so don't worry about the smaller gap.

To make life easier for you, grab some roofing baten ( 25mm x 18mm ) and screw it along the joist butted up against the roof.  When you place the celotex in between the joists the baten will stop you hitting the roof and ensuring you have at least a 25mm gap, this picture might show it a tad better

ignore the 41mm core drill hole...i'd look at several 25mm holes along the roof line but just make sure there is a flow of air over the top of the celotex and you won't go wrong 👍

[jason110]

@Badger110  thank you mate. will update when it’s done.
 

Off to work next week. So will most likely be looking at getting it done in feb / March when I’m back again.