shed insulation

[sgough6077]

First post, so please be nice....!

I have read all the related posts on this forum, and many others; so again, apologies for asking the same question again, but while so many of the other post replies are informative, and really useful, there is so much contradictory information out there that I am now even more confused than when I started!

I have bought a softwood, tongue and groove cabin/shed, that is 3m x 3m.

I have a poured concrete base on which I am going to sit 4"x2" joists, and then 2"x2" batons perpendicular to the joists, between which I am putting celotex insulation. The walls then build up on the batons, and a tongue and groove floor is laid inside once the walls are up.

My question is how best to insulate the walls. My basic suggestion is to simply add 2"x2" batons on the inside of the walls, put 50mm celotex into the gaps between the batons and then add ply, screwed to the batons, and over the top of the insulation. This could be done for the walls and the ceiling.

However - my nervousness is about moisture forming either between the external tongue and groove and the celotex, or the celotex and the ply. There will be no air gap in the sandwhich of external timbre, insulation and internal ply, and both internal and external woodwork will be primed, undercoat added, and then painted.

Am I correct to be worried? Should I leave a ventilated gap between the external wood and the insulation? Should I be using breathable membrane somewhere?

All very confused, and will appreciate any guidance,

thanks

Steve

 

[Paul200]

Welcome Steve

I'm about to insulate a cheap and nasty B&Q shed because we've just moved and I need to store my tools over winter. I've decided that the only way to do it is to line out the inside with a breathable membrane tacked onto the pitifully thin (38mm) framework and to then build another frame inside (probably 50mm deep). This will then be insulated in between and clad in OSB3 (Stirling Board).

This gives a vent space equivalent to the depth of the original frame (38mm) - but I then have to provide airways top and bottom to actually ventilate that space. In this respect I'm lucky because we've just replaced all our old house windows and I kept all the plastic trickle vents from them.

I have seen WIP's online where no ventilation space is made, plastic sheeting is put directly against the inside of the cladding with insulation behind and then wallboards - but I'm sure this is asking for trouble.

I've also seen something called 'shed insulation' which seems to be bubblewrap clad in silver foil. I've no clue about it's performance though.

No doubt someone will be along in a minute with other words of wisdom - and I'll be watching!

Cheers

Paul

 

[Wizard9999]

Paul

Welcome.

I believe the answer is yes, you should worry about your current plan as there is no air gap. The basic idea is that you want to make it hard for any moisture to get from the inside of the shed into the wall cavity and easy for any which does to get into the cavity to get out of the outside wall. Typical shed construction to achieve this is from the inside OSB (so much glue in that stuff it is an effective vapour barrier), insulation, breather membrane, air gap, then exterior cladding.

I think you need to find a way to introduce an air gap as Paul is doing with his. I think that ideally the air gap should be at least 30mm, but to be honest when I built my workshop I think it ended up being closer to 25mm. Without the air gap there is no flow of air circulating which you need to carry out the moisture, if it doesn't leave the wall structure it will eventually rot the timber forming the wall.

HTH,
Terry.

 

[the_g_ster]

I too have found this confusing, and am about to do this same, but from what I have gathered, the following as some tips:

Base, though I know you've not asked about:

Consider something to stop any guests making a home under the shed, like some rodent mesh etc round the perimeter so air still circulates, but pests don't.
Make sure the woodwork is well treated with some preservative too

Walls

The key thing you are trying to prevent, is the external cladding getting wet, drying on the outside, staying wet on the inside, and thus rotting/distorting as the timber has different levels of moisture through it. Celotex/and others are pretty not going to be effected by this, as they wont allow moisture through. Any gaps will allow moisture through, and depending on how well constructed, would risk accumulation of water and damage as a result.

Depending on how even and uniform the shed cladding is, you could put strips of something roofing battens on the inside, that would provide a gap for air to circulate behind. Any wind driven water could also be dealt with via a membrane on the inside.

The order would be:

- Outer shed cladding
- Air gap
- Membrane
- Celotex in battened frame
- Ply/OSB

 

[sgough6077]

Thanks all - so seems the consensus is that I need an air gap between the outside cladding and the insulation; another silly question; can that air gap be vented into the shed, or does it have to be vented externally?

The former presumably is quite easy, because I could just leave a gap between the batons that are there to create the gap and the inside line of the roof, all the way round the walls so that that it is vented internally, at the top (although would I need a gap near the bottom to promote air flow?

The latter, vented outside, presumably would need me to cut out strips from the top of the exterior cladding, and replace with vents?

 

[Paul200]

Not sure, but my instinct (and the way I'm doing it) is to vent to the outside. Otherwise you're negating the effect of a impermeable layer inside of that air gap (ie. OSB/Damp Membrane).

One way of doing it would be to cut slots top and bottom and cover them with bug mesh (or something similar) on the inside of the cladding.

Cheers

Paul

 

[RobinBHM]

The air gap directly behind the cladding allows any ingress of rain water as well as damp air to escape. I would have thought you would want this to be vented externally.

 

[Mike Vastano]

I used this stuff

http://www.ecohome-insulation.com/?i=92 ... oCGr3w_wcB

Not cheap but works a treat

 

[the_g_ster]

You must vent outside.

If you vent inside, you get cold air coming in, warm air going out, what's the point of insulation then?

Just leave some gaps at the eaves, and again, put in some mesh to keep free of nasties.


In the UK today houses are ending up too "airtight" so stale air hangs around and there's more moisture in the air, for your man cave though, you should be alright.