Wul, if you have lots of money/wood/time to heat it in winter, or do not intend to use the hut when it is cold outside, you will be fine.....

Otherwise, use 'plastic' insulation, PIR/phenolic/XPS with a VCL on the warm side. Then condensation should not be an issue, and I don't think the mice like Celotex that much?

With the sun on the roof it might be oven-like in there?

Insulate, or you may regret it later I feel.

Good luck

Air in a void isn't trapped; it's free to move around. Convection will transfer heat moderately effectively.

Apart from any legal aspects (planning/building control) I think you'd be mad not to put in some insulation. It doesn't have to be a huge amount, 100 mm of mineral wool wouldn't be that expensive or take up much space, but would make a big difference to the comfort and the amount of heating required. The other thing to do is to make sure you get good airtightness and sensible control of the ventilation (he says, having just spent a winter in a drafty static caravan on the east coast of Scotland). You need good airtightness outside the insulation to stop the wind blowing through the insulation and bypassing it. Also good airtightness inside to prevent moisture getting to the outside surface and condensing.

Note, if you're keeping the inside warm you'd need airtightness on the inside irrespective of the insulation. Even with your 50 mm gap the outer surface will still be at outdoor temperatures and subject to condensation.

http://www.buildingscience.com/documents/digests/bsd-011-thermal-control-in-buildings

Thanks for the comments, that's helpful.
I'm not familiar with these abbreviations, PIR? VLC? (vapour....?)

Now that its been pointed out, I can see that convection would be a problem. In the depths of winter, the outside timber cladding will be frozen solid, any warm air circulationg behind this is going to produce drips like several taps. I'd definately use a vapour barrier.
I would not have been that concerned with airtightness of the outer skin but I clearly need to re-think this.
I was tending towards board and batten (vertically) for the outer rain-skin since it's cheaper and more "rustic" than horizontal shiplap. However I doubt it would be very airtight, its just one piece of off-saw wood nailed in front of another. At least shiplap has an interlocking groove which should help with airtightness a bit?
Having said that, shiplap never fits perfectly, the boards always flex and I've had many sheds where daylighgt passes through the boards in hot, dry weather. So, maybe board and batten after all?

OK, so definately insulation. I'm convinced. (this is for my wife; it has to be right!)

I'm not a fan of the rigid Kingspan/Celotex stuff for this application. I've found it very difficult in real life to cut it accurately and get it to fit snugly between the various studs, dwangs (noggins) etc in a timber frame. Plus it makes a mess when cut.
I'd prefer wool bats, probably sheep, so that I can "stuff" it in a little and know that air gaps are sealed. I'm never confident with the rigid stuff that air won't just bypass gaps. The rigid stuff is so expensive that I find myself loath to re-cut a piece that won't fit properly.

I do sometimes wonder though, about the bogey-man of condensation. Any one got links to actual experiments or pictures of real-life buildings where condensation has been a problem?

I will definately heat with wood. I have access to a woodland, so very cheap fuel. Plus, it is much more pleasant than electric.

Posted By: Ed DaviesYou need good airtightness outside the insulation to stop the wind blowing through the insulation and bypassing it. Also good airtightness inside to prevent moisture getting to the outside surface and condensing.

Yes but there is a distinction. The inner surface needs to be vapour-tight, e.g. polythene sheet sealed with tape. (Somebody: is there a green alternative to this?)

The outer surface should be wind-resistant ('airtight' is overdoing it) and vapour-permeable or 'breathable', otherwise any moisture which does get into the interior of the wall cannot escape.

However you do the rain-skin boarding, there will be some movement gaps, so it might be an idea to fix a breathable membrane and batten before boarding. One of the roofers in this forum can provide detailed instructions, I am sure.

I do understand why you asked the question 'is insulation necessary', since a 3kW woodburner in a well-insulated shed of that size sounds like the spec for a sauna.

Posted By: Wul: “I'm not familiar with these abbreviations, PIR? VLC? (vapour....?)”

PIR ~= PUR = polyisocyanurate http://en.wikipedia.org/wiki/Polyisocyanurate (Celotex, etc)

VCL = yes, vapour control layer. The polythene sheet in rhamdu's comment:

“Yes but there is a distinction. The inner surface needs to be vapour-tight, e.g. polythene sheet sealed with tape.

The outer surface should be wind-resistant ('airtight' is overdoing it) and vapour-permeable or 'breathable', otherwise any moisture which does get into the interior of the wall cannot escape.”

Don't forget ventilation heat losses which are proportionally larger on a small structure when you size things in liters/person·minute rather than air changes per hour.

Vapour is a shorthand for water vapour i.e. water as a gas. It's sometimes helpful to think of it as water 'dissolved' in the air.

Just like sugar in jam going from liquid to solid when you cool it, 'condensation' is the water going from gas to liquid onto a cold surface.

Also, just like the jam analogy - the % of sugar is analogous to the relative humidity. More sugar and it'll start solidifying at a higher temperature. More humidity and it'll start condensing at a higher temperature. The comfortable humidity range for people is about 40% to 60%.

Regarding condensation damage - yes it's definitely real, and I've seen it quite a few times.

I seem to be quoting Building Science quite a few times in this thread (in the US timber frame is king after-all), but these two articles from them are quite a good introduction:

http://www.buildingscience.com/documents/insights/bsi-049-confusion-about-diffusion/#foot03

http://www.buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces/files/BSI-009_Crawlspace_2010r2.pdf


Re Celotex and fitting. I'd recommend cutting it slightly short, and foaming around the edges with something like this:

http://uksealants.co.uk/illbruck-fm330-air-tight-expanding-foam--p281.html

(or just a bog-standard 'gun grade' expanding foam if you're reluctant to go for the posh stuff).

Also - best not to cut rigid insulation boards up very much at all - instead stick it outside of your timber frame to keep the timber frame itself nice and toasty and dry. I'd recommend not bothering with a vapour control layer - just go for something like "figure 7" from the first building science link - this would be something like 40mm of polystyrene, or 25mm or PIR (Celotex) outside the timber frame, and 80mm or so of flexible batt insulation inside of the frame.

If you want to reduce the total amount of insulation, make sure that you do so either proportionally (e.g. 20mm polystrene with 40mm of flexi-batt), or just cut the inner insulation (e.g. 25mm PIR + 50mm batt).

Posted By: WulI'd love someone to create a (preferrably timber faced) product that could form both the outer and inner finsished surface in one fix.

They do. They're called SIPs ...

I was in a corrugated metal producer's factory today and they had a product that was like a very large, metal-skinned, foam filled, shiplap profile wall board. It was ,say 150mm thick with maybe 0.3mm thick aluminium skin and a celotex-like interior rigid foam.

... and they make them with metal skins too. Both types are fairly expensive.

But that doesn't grant your wish because you still need a ventilated gap. You still need to build a rainscreen in front of the SIP.

The advantage of using PIR on the outside is that it is in the zone where you could get condensation, and PIR (or expanded polystrene) is pretty much condensation-proof when used in this way.

If you'd like to make use of the timber frame for alcoves/shelving etc, then you could stick plasterboard, or OSB on the outside of the studs, then the Celotex on the outside of that, then vertical battens (strips of 12.5mm ply will be plenty, if you get the rest of it airtight), to create a ventilated gap and stainless mesh to keep animals out of the gap, then your larch over that.

That should end up being a pretty bullet-proof constructions, especially as with exposed timbers, you'll be able to see any rain water ingress straight away.

Hi Paul,

My budget is around £4k for a finished 6.5ft x 12ft hut, insulated, tin roof, with a woodburner and flue and 3 windows.
I reckon it will take me around 300hrs. (wild guess)
I know from experience, working on my own that it will be slow work because every step is problem solving and learning.

My 8ft x 15ft x 12ft high (to ridge) shed cost about £2500 - £3000 in materials. It's post and beam with 2 x 2x2 vertical and horiz battens between posts to reduce thermal bridging. It has 100mm recyc plas quilt between floor jsts, 100mm sheep's wool (in 2x50) in the walls and 100mm recyc cotton in the roof. The big shortcoming so far, as alluded to earlier in your thread, is unwanted air infiltration. When I get around to finishing off all the unfinished bits the 300W elec rad I have should be more than adequate, particularly as the 'sitty bit' is only 7.3ft x 7ft x 1.8 high with sloping soffits.

With a £4k budget and only 6.5 x 12 to do you will be able to have bells and whistles.

If I had been prepared to use Pu I could have reduced the heat-loss significantly, BTW.

And yes, as it's immediately next to the boundary it needed planning perm.

I'd lose the battens in the floor, just lay EPS over your lower sheeting layer, foam gun any gaps and edges and lay your upper sheeting layer on top. 5mm ply, sounds too thin, would be very bouncy indeed if you didn't end up falling through it along with the stove and other heavy things, the caravan structure will be a fully bonded setup.

I've asked my local timber frame maker (Scotframe) if they would build my walls for me. Cheeky, but we'll see what they say. It would take them about 10 minutes, and me 4 days.

I should have made it clear that this hut will be built on an old caravan chassis. So I will need some "joists" in the floor, running at right angles to the twin spars of the 'van chassis. (see pic)
Willie; agree that 5mm is too thin, I was thinking of two layers of 9mm ply with 50mm kingspan or EPS sandwiched in between and using 50mm x 50mm battens for strength, either screwed or nail gunned together. (maybe some glue in the mix too?) You are right, the caravan stuff is tightly bonded which will add to its strength.
I have my doubts about how strong this will be.
Surprisingly, in the caravan I dismantled, the corner levelling jacks act directly on the plywood floor and not the metal chassis as I would have expected. This leads me to think that the ply sandwich must be a very strong, rigid unit. I will make one and see.
When the whole hut is screwed together it becomes a monocoque structure, I think, which adds to strength.
I bought one of those incredibly cheap and nasty cardboard honeycomb doors from B&Q yesterday ( to use as a desk) and it was amazingly light and strong, it could be used as a desk with just a support at each end of its two metre span. (although I added support under it)
Sorry, no pics, It won't let me upload jpgs or Tif files; ...weird

And here's the door/desk