Help re Insulating a wooden shed with Kingspan/Celotex

[cameronjac]

Hi all

I'm new but need a bit of advice...

We've had a new wooden shed and we're going to put a pool table and dartboard in there for our teenage kids to go out there.

We want to insulate it and then just put OSB boards up (or possibly pallet wood) so that hopefully it won't be too chilly in there. The depth of the walls are 64mm...I've had 50mm Celotex so there'll be a 14mm air gap.

What I want to know is , with Celotex, do I also need a (breathable) membrane? I've had differing opinions from friends where some have said I need it and others have said I don't. I didn't think I did....I was just going to put the sheets up against the shed wall and then put the boards up.

Any advice would be greatly appreciated

Thank you in advance

 

[NickN]

Yes, you do want a breathable membrane on the outside (as in, put it up before the insulation) as this will stop the celotex getting wet from any rainwater ingress that could occur, and prevent water sitting there causing problems. I have just taken delivery of a 1.5m x 50m roll of Cromar Vent Classic which can be bought for a good price online (55 quid I paid inc. delivery).

As I have 2" x 3" verticals on which the cladding sits, I could put the breathable wrapped around them and fastened to back of shiplap, but I have read that the membrane works best with an air gap, so am going to go straight across the back of the verticals, then attach 2" x 2" horizontal battens all round, between which my 50mm insulation will sit. That's just a 'belt and braces' approach though.

However if using celotex you *can* apparently skip the non-breathable / damp-proof membrane which would normally go inside the insulation if using rockwool or similar, as the celotex is capable of preventing condensation building up behind it if the gaps are sealed etc. At least, that's what I've been told anyway. As I'm using rockwool slabs I will be using a DPM inside with the OSB on top of that.

 

[RobinBHM]

I assume you mean you want insulate the inside in between the stud framework.

I would pin on some 12mm battens to maintain an air gap between the cladding and the insulation. In theory there should be a breathable membrane between the 2 but its not going to be easy to achieve and anyway the celetex has a non breath eable foil face so the membrane will only act to keep rainwater out at any joints.

The air gap will create a cavity allowing the back of the cladding to breathe, its not quite ideal the the cavity is on the inside and closed off, but will be better than having the insulation tight to the cladding which could encourage condensation .

After fitting the celetex I would tape all the joints with foil tape to act as a vapour barrier then fit your osb board

 

[Adam9453]

As above has mentioned, the air gap is good for letting the cladding dry out and avoids saturating the insulation with water and rotting the shed.
I'd do as suggested, pin some battens in to maintain the air gap on the cladding side of the insulation then just board out the inside.
Should last longer than your kids will want to use it

 

[graduate_owner]

Perhaps you should take the opportunity to treat the timber first while you still have access. Also would it be worth ventilating the 'cavity' by inserting high and low level vents between each upright? You can get round plastic vents with mesh on, about 3" diameter in white black or dark brown, they have a coarse thread on and you just screw them in. I don't know if this is a good idea or not but thought it might be worth a mention. I'm sure other more experienced members will be able to advise.

K

 

[skipdiver]

The celotex and osb will act as a vapour barrier for the inside and the small air gap should suffice with the thin battens as mentioned for the "breather" bit. It's not ideal but it's only a shed and should last a while.

 

[cameronjac]

Thanks all for the replies....

Still not sure whether I should put a breathable membrane in....what if I put in batons to keep the insulation away from the cladding but then maybe tacked or stapled the breathable membrane to the batons? Only trouble is that the membrane would then be against the back side of the celotex? Would this be a bad thing? I could just cut it to size and put it up between the uprights? The lack of membrane behind the framework though...would that mean it defeats the object?

To answer the question re treating the timber, the shed is tanalised so its already been treated but I'm going to give it a few coats of Barrantine oil based treatment to waterproof it

 

[NickN]

You do need a continuous 'wall' of membrane for it to work properly - I thought about the noggins between the 2x3 idea and stapling membrane to them with insulation on top, as RobinBHM suggested above, but decided that:
a) cutting all those noggins, let alone fixing them, would be a heck of a tedious job, far easier to just stick some 2x2 horizontally round whole shed, and,
b) each noggin would effectively seal / make smaller the air cavity behind the membrane, preventing full movement of air and thus preventing quicker drying out of any rainwater ingress.

So imho the easiest, quickest and safest (for the health of the shed) method is to stick up the membrane flat onto the 2x3 verticals all round, so you end up with a flat membrane wall. Then screw treated 2x2 horizontally at whatever centres works for the insulation all the way around, only £1-ish a metre so not hugely expensive, then stick the insulation between those battens, OSB on top, job done.

Because the cladding 'panels' between each 2x3 upright will stay as a full top to bottom cavity that way, you should not get any problems hopefully.

 

[RobinBHM]

I was thinking of 12mm battens pinned on vertically full height to every stud, no horizontals. then cut the celetex tight and push the celetex in until it stops against the battens.

You could counter-batten all the way round as Nick suggests but that is losing 50mm all round which is a fair bit on a shed although breatheable membrane, then counterbattens is the most ideal solution, although not perfect as the cavity is closed and finishes to the inside floor meaning any leaks will end up inside not venting outside.

 

[cameronjac]

You do need a continuous 'wall' of membrane for it to work properly - I thought about the noggins between the 2x3 idea and stapling membrane to them with insulation on top, as RobinBHM suggested above, but decided that:
a) cutting all those noggins, let alone fixing them, would be a heck of a tedious job, far easier to just stick some 2x2 horizontally round whole shed, and,
b) each noggin would effectively seal / make smaller the air cavity behind the membrane, preventing full movement of air and thus preventing quicker drying out of any rainwater ingress.

So imho the easiest, quickest and safest (for the health of the shed) method is to stick up the membrane flat onto the 2x3 verticals all round, so you end up with a flat membrane wall. Then screw treated 2x2 horizontally at whatever centres works for the insulation all the way around, only £1-ish a metre so not hugely expensive, then stick the insulation between those battens, OSB on top, job done.

Because the cladding 'panels' between each 2x3 upright will stay as a full top to bottom cavity that way, you should not get any problems hopefully.

only trouble is the shed is 25' x 14' :shock:

 

[NickN]

Hmm.. on the plus side, it means losing 50mm on each wall is not a problem..

You'd need a fair bit of timber whichever way, and yes I now understand what RobinBHM is suggesting and that could work well too.

 

[Fitzroy]

there are two potential membranes:
- outer (breathable) membrane to prevent rain that gets past the cladding getting to the insulation. If is breathable so that if some moisture does get past the cladding (or past the internal vapour barrier) it can escape again.
- inner (barrier) membrane to prevent moist air reaching the parts of the structure beyond the insulation that will be cold, and where the moisture can condense.

You are talking about the inner barrier. One argument is that the insulation (with fully taped joints) acts as this barrier. Or you can by heavy gauge polythene sheet, staple in place, then board over the top. Others say the OSB acts as sufficient a vapour barrier.

My personal view, this is a not oft used space ie a few hours a day, so:
- Batten behind the celotex to give you a gap and to ensure the celotex does not fall away from the OSB.
- Cut the celotex so that it is a snug fit and use expandable foam to fill any gaps where the celotex does not fit very well. If the air can circulate past the ocelot it cannot function as insulation.
- OSB on the inside making sure all joints sit on a stud/noggin so that there is not an easy path for air to get into the space where the celotex is.

If you use pallet wood, as kind of an internal cladding, you would need to put a vapour barrier (internal impermeable membrane, polythene etc) as you will never seal all the gaps between the slats effectively.

F.

PS. wrote this 15mins back so likley duplicates some of the info in the other replies since then.

 

[cameronjac]

I was thinking of 12mm battens pinned on vertically full height to every stud, no horizontals. then cut the celetex tight and push the celetex in until it stops against the battens.

You could counter-batten all the way round as Nick suggests but that is losing 50mm all round which is a fair bit on a shed although breatheable membrane, then counterbattens is the most ideal solution, although not perfect as the cavity is closed and finishes to the inside floor meaning any leaks will end up inside not venting outside.

The 12mm batons do sound like a quick and easy solution but I'm worried about the lack of membrane. A lot have told me that with Celotex I really don't need a membrane.... Argh lol

 

[cameronjac]

there are two potential membranes:
- outer (breathable) membrane to prevent rain that gets past the cladding getting to the insulation. If is breathable so that if some moisture does get past the cladding (or past the internal vapour barrier) it can escape again.
- inner (barrier) membrane to prevent moist air reaching the parts of the structure beyond the insulation that will be cold, and where the moisture can condense.

You are talking about the inner barrier. One argument is that the insulation (with fully taped joints) acts as this barrier. Or you can by heavy gauge polythene sheet, staple in place, then board over the top. Others say the OSB acts as sufficient a vapour barrier.

My personal view, this is a not oft used space ie a few hours a day, so:
- Batten behind the celotex to give you a gap and to ensure the celotex does not fall away from the OSB.
- Cut the celotex so that it is a snug fit and use expandable foam to fill any gaps where the celotex does not fit very well. If the air can circulate past the ocelot it cannot function as insulation.
- OSB on the inside making sure all joints sit on a stud/noggin so that there is not an easy path for air to get into the space where the celotex is.

If you use pallet wood, as kind of an internal cladding, you would need to put a vapour barrier (internal impermeable membrane, polythene etc) as you will never seal all the gaps between the slats effectively.

F.

PS. wrote this 15mins back so likley duplicates some of the info in the other replies since then.

I might just do this. The only woodwork for me would be the 12mm battens prior to putting in the Celotex and then the boards afterwards.

I did think that if I used pallet wood there would definitely be air getting into the gaps and I'd have to do something about it so this answers my question about that.

Thank you Fitzroy

 

[cameronjac]

Was going to ask in a separate post but may as well ask here ... I want to do the roof as well.

Its just got roofing felt on top of the wooden roof at the moment. I'll use the same 50mm sheets and then possibly OSB's. Anything else you can suggest to use on the roof or anything else I need to think of?

Thanks all

 

[NickN]

Just in case you might have got membranes mixed up... I think everyone here has agreed you ought to have the outer breathable membrane, wrapped around the studs and cladding, with or without battens - it keeps rainwater and minor leaks out of the insulation and boarding. The inner damp-proof or non-breathable membrane is the one where you can get away without it, if the insulation and boarding gaps are sealed well.

But just to clarify further, my suggested 'retro-fit' layout would be this:

OUTSIDE -> Cladding -> Verticals (2x3) unfilled cavity leaving 50mm air gap -> Breathable Membrane -> Counter battens with insulation layer against the membrane -> DPM / Plastic sheet membrane -> OSB / Plywood - > INSIDE

The RobinBHM suggested layout would be:

OUTSIDE -> Cladding -> Verticals (2x3) cavity battened with vertical 12mm against cladding -> Breathable membrane wrapped around and fastened to battens leaving 12mm air gap -> Insulation layer between verticals and against membrane -> DPM / Plastic sheet membrane -> OSB / Plywood -> INSIDE

The optional bit therefore depending on fit would be the DPM / Plastic sheet inner membrane.

Hope that clears things up a little bit.

As far as the roof goes, you might want to look at EPDM or rubber roof when the time comes to replace the felt. Inside, yep, similar applies to the walls, and if you don't need a boarded ceiling you could always staple some of the foil insulation across the whole lot too, for extra heat retention.

 

[RobinBHM]

Tip for celetex.....measure as accurately as possible. Measure top middle and bottom, if it tapers cut the insulation with a matching taper.

Mark the celetex on the face you will see once fitted. Make your cut slightly angled that way the insulation will go in nicely and end up snug. Cutting roughly or undersize and then foaming wotks but is not the easy way to go. Foam wont go into gaps under say 7-10mm.

You will soon get a feel for how snug it needs to be, just tight enough so the last 10mm or so needs a bit of a bang to go flush. Too tight and the insulation will break as you bang it in.

 

[cameronjac]

Just in case you might have got membranes mixed up... I think everyone here has agreed you ought to have the outer breathable membrane, wrapped around the studs and cladding, with or without battens - it keeps rainwater and minor leaks out of the insulation and boarding. The inner damp-proof or non-breathable membrane is the one where you can get away without it, if the insulation and boarding gaps are sealed well.

But just to clarify further, my suggested 'retro-fit' layout would be this:

OUTSIDE -> Cladding -> Verticals (2x3) unfilled cavity leaving 50mm air gap -> Breathable Membrane -> Counter battens with insulation layer against the membrane -> DPM / Plastic sheet membrane -> OSB / Plywood - > INSIDE

The RobinBHM suggested layout would be:

OUTSIDE -> Cladding -> Verticals (2x3) cavity battened with vertical 12mm against cladding -> Breathable membrane wrapped around and fastened to battens leaving 12mm air gap -> Insulation layer between verticals and against membrane -> DPM / Plastic sheet membrane -> OSB / Plywood -> INSIDE

The optional bit therefore depending on fit would be the DPM / Plastic sheet inner membrane.

Hope that clears things up a little bit.

As far as the roof goes, you might want to look at EPDM or rubber roof when the time comes to replace the felt. Inside, yep, similar applies to the walls, and if you don't need a boarded ceiling you could always staple some of the foil insulation across the whole lot too, for extra heat retention.

Thank you so much for this . I think I'll do :-

"OUTSIDE -> Cladding -> Verticals (2x3) cavity battened with vertical 12mm against cladding -> Breathable membrane wrapped around and fastened to battens leaving 12mm air gap -> Insulation layer between verticals and against membrane -> DPM / Plastic sheet membrane -> OSB / Plywood -> INSIDE "

Can I ask... breathable membrane wrapped around and fastened to battens... just wrap it all over everything including the verticals?
Also, the air gap will be by the cladding....will it be okay for the celotex to be flush with the DPM and then for the DPM to be up against the OSB? Just checking

Thank you !

 

[Levitric]

Thank you so much for this . I think I'll do :-

"OUTSIDE -> Cladding -> Verticals (2x3) cavity battened with vertical 12mm against cladding -> Breathable membrane wrapped around and fastened to battens leaving 12mm air gap -> Insulation layer between verticals and against membrane -> DPM / Plastic sheet membrane -> OSB / Plywood -> INSIDE "

Can I ask... breathable membrane wrapped around and fastened to battens... just wrap it all over everything including the verticals?
Also, the air gap will be by the cladding....will it be okay for the celotex to be flush with the DPM and then for the DPM to be up against the OSB? Just checking

Thank you !

I know this is an old post but I found this by searching for my dilemma.
I am having a shed built as it works out more cost effective at the moment. If I can get them to, should I ask them to install a breathable membrane on the framework under the cladding? I read so many different opinions I am getting confused. Once I have the shed I intend to insulate and then line inside with ply.

 

[Fitzroy]

Yes. Ideally it’s framework, membrane, 25mm battens to create air gap, then cladding. Top and bottom of the air gap is closed off with insect mesh. However, if the shed design did not include it’s unlikely you can get the battens added as it’ll mess up the roof/walls interface. So just breather membrane under the cladding will still be an improvement. Just make sure it’s breather membrane not a vapour barrier, shed builders may not have done this before and it’s an easy confusion.