Hello Matt

You have given us timber sizes - 65mm roof, 55mm walls, and have stated that you don't want the ceilings much lower, but you have not given any indication yet of how much insulation you think you might apply. Is that implied - to the rafter/stud depth only - or is it open to debate?

While I accept that, with the kiln going, you'll be pumping a lot of heat in, 65/55mm Rockwool or EPS is not going to give you a result which will excite.

Any insulation you put in will make the building perform better than the 'naked' version, and attention to air-tightness will help a lot, but if you are sticking to insulation depths equal to the depth of the timbers I am not sure you are going to achieve even the sort of minimum standard that 'us lot' on here would like. You appear to rule out closed-cell foam (were you thinking of PIR or similar? I am) on the grounds of cost (and flammability, with no built -in fire retardant to the ceiling in the studio room'?), but it would give you probably between 1.5 and 2 x the insulation value of Rockwool (depending on the specific Rockwool product used - I think a Lambda value of 0.032 is the best performance available with Rockwool, as against approx. 0.022 with PIR).

Your description of the building (' a factory-made 18' x 8' brand new garden room') does not give me much of a 'feel' for it. Is it basically a big garden shed, built as sheds are, or is it a bit more robust? The lack of a breathable membrane under the external cladding suggests more shed than office, but at this stage that's not a factor you can change.

Can we assume also that the floor is uninsulated?

Could you feed us a bit more information in terms of internal heights (at the low and high ends, and maybe confirm that no regular users are over 6ft 6'' high - no, I am really not being flippant) and I am sure that we will come up with suggestions which are a reasonable compromise in terms of space and thermal performance.

Nick P

P.S., just before I post, I imagine for the roof at least weight might well be an issue.

Great questions :-) Thank you!

Here are some images/video here: Please excuse my handy-man skills - we are using thin cladding so I added studs to support them
https://www.dropbox.com/sh/ese89vjh96y5110/AACrpI1yJpD82CDCq1SDPP1ha?dl=0

Q> How much insulation might be needed?

A> I don't know - I'm just looking to make it comfortable as an artists studio. For the kiln room, I'm more concerned about removing heat using ventilation, but am insulating it for good measure.

Q> Limited to rafter depth only?
A> Perhaps a wee-bit below the rafters - we are both less than 6', but don't want to feel cramped either

Q> Kiln
A> Kiln generally will be fired at the end of the day - not while we are using it. So that heat wont benefit us much. Firings last many hours. Electric - had an electrician specify and install all the wiring etc, he will return once we've insulated.

Q> Ventilation - Kiln Room
A> Open window + exhaust extractor connected to kiln (for gases - vents to the rear) + bathroom-style fan removing air to the side

Q> Dimensions
A> Kiln room is 1.8m wide and the studio room is 3.7m wide. Depth is 2.5m. Height is 1.9m at the back and 2.06m at the front

Q> Usage
A> Almost daily usage throughout the year

Q> Floor construction
A> 40mm treated joists running length way and Pine pieces running perpendicular on top of that. Beyond that we will add DPM, underlay and laminate floor. The joists were shuttered in with skirting on each side. I've elevated the skirting to allow air to run underneath. Bad idea?

Q> Floor insulation
A> Nothing planned for as of yet.

Q> Warm Roof issue
A> Warm roofs look brilliant. But this has just been installed and under warranty, we didn't fancy removing the felt and amending the decorative wood around it. My wife said we are at 'maximum height' without planning permission given the concrete base came in higher than planned.

Q> Closed Cell Foam
A> I'm referring to closed cell spray foam - very popular in the USA, less so here. Was quoted about £1700 for it being done in the shed. Professionalys spray it between the rafters/studs to 25mm and it forms a perfect vapour barrier with no other insulation required (except to tackle thermal bridging). It's considered (or so I'm told) the only perfect cold-roof solution for unvented rafter cavities...

Q> Celotex / PIR
A> Celotex themselves told me not to use it in unvented rafters. I understand it's 2x better insulation but as it's not air permeable and is water proof, I've been concerned about water getting trapped in there.

Posted By: mattgossA> I don't know - I'm just looking to make it comfortable as an artists studio. For the kiln room, I'm more concerned about removing heat using ventilation, but am insulating it for good measure.

I'd probably not bother with insulation in the kiln room; just add the plasterboard internally. It shouldn't matter much whether it gets a bit cold in there sometimes, or quite hot other times. And you've obviously thought about getting rid of the various gases that might be given off during firing. You could try to insulate it very well and use the heat to warm the studio after the fashion of a storage heater, but that could get complicated and might not work very well.

Warm roofs look brilliant. But this has just been installed and under warranty, we didn't fancy removing the felt and amending the decorative wood around it. My wife said we are at 'maximum height' without planning permission given the concrete base came in higher than planned.

I wouldn't remove the felt but you might want to add some more decorative wood upstand. The max height is 2.5 m and your floor level seems to be not much above ground level, so you might have some free height.

You could also put some spray foam inside as you suggest. Have you seen the soy-based foams? There're also things like warmcel or hemp-based products that can be sprayed in and help with controlling humidity.

Wall insulation you can add as much as you like inside but I'd be tempted to tack a breather membrane inside the studs and leave the cavity open to let the timber cladding breathe.

I can't see much of the ground-level skirt in the photos. Will insects or rodents want to live under there?

Posted By: mattgossIs there any condensation risk of not doing so? One benefit of insulating is keeping heat out during the summer, especially if we decided to use the kiln in the daytime. If the room exceeds 50C the kiln will switch off to protect it's on-board computer, to the detriment of what you're firing.

There's no source of water in the kiln room except what's fired out of the clay, is there? And that should be cleared by the extraction system.

I think the key to keeping the temperature down will be sufficient ventilation.

I share your wife's feelings about foams, FWIW.

I'm not sure I've understood - insulation with a membrane or no insulation + a membrane?

Sorry, I mean to put a breathable membrane on the outside of the insulation, leaving a gap outside it between it and the cladding. You'd also want a VCL on the inside of the insulation.

I suppose they would yes - I didn't think of that! If I put the skirting back down it's completely sealed off - is that okay?

The usual answer is to fill the gap with some mesh - metal mesh to keep rodents out and maybe some insect mesh inside it if you're concerned, but ant traps or powder will control most insect problems.

Posted By: mattgossI'm a bit unclear on how I would install a breathable membrane within a stud cavity but without touching the cladding.

It can be done by stretching it and/or using battens, but I was meaning to leave the space between the studs clear and put the insulation inside.

It does depend on a lot of variables, such as how much sun it gets, how long you're (and how many of you?) likely to be in there, whether you're using lots of water with the clay etc. Doing what I suggested would be bulletproof but may well be overkill. Basically you need the back of the cladding and the studs to be able to dry out if/when it gets wet either by rain making its way through from outside or by condensation from vapour generated inside the room. You'll have to judge how much you need to do to guarantee that drying enviornment.

Hi All,

Thank you again for all your help.

A final update after much research -- hopefully this is useful to others in the future:


We went with mineral wool insulation throughout, using a thickness that was just less than the rafter cavities (e.g. 50mm insulation for a 55mm cavity). We also used some ecotherm glass insulation which was much cheaper, but only worked in the walls and not the ceiling (it didn't stay in place).

Then we added Intello as a vapour control layer, sealed with tescon vana tape and a butyl tape where it meets the floor. We ordered it all from baunativ in germany where we got all the matching products like instaabox for electrical outlets/switches and kaflex for where wires come through.

We did not leave a service void as it would eat into the space. Instead it's wood cladding in one room and plasterboard in the other.

Insulating it properly was a MAJOR job for DIY'er. A great deal of time and care was needed. It went on for months. (especially if you don't have someone working with you)


With such thin walls it was always a compromise against a 'perfect' solution but hopefully this is robust. We've been advised to be extra diligent in keeping the exterior painted and checking our mini guttering for leaves/etc.


If I were doing it again, I'd re-think whether we really needed to insulate it. A thick interior wood cladding and some good heaters may have been enough, saving a ton of money and hard work. But it's hard to know for certain...


Thank you to everyone for your input and good luck with your projects :-)