Vapour barrier and insulation for new workshop

Hi All,

I am in the process of buying a small pre-built, sectioned wooden workshop that will have 19mm cladding and 3" x 2" framing. I intend to use Rockwool or similar for insulation and finish with foil backed plasterboard.

After much Googling I am somewhat confused about the location of a vapour barrier, which I understand to be a non-permeable material such as polythene sheeting. I think the confusion is compounded by mention of a breathable membrane in some online postings.

The book Workshop Construction says the vapour barrier should be on the outside of the insulation, i.e. under the cladding, but maybe he's talking about a breathable membrane. Other commentators online say it should be on the warm side of the insulation, i.e. the inside. Clearly I can only put it on the inside as the cladding will be put on by the shed manufacturer.

Could anyone help with some words of wisdom?

Many thanks,

Nick

Hi Nick,

Breathable membrane goes between the framing and the external cladding, you may put vapour barrier behind the internal lining however the foil backing should do this.

Best of luck.

Peter.

If you were building the structure from scratch then it would read cladding, membrane, frame in filled with insulation, vapour barrier, internal finish. As a note the likes of Celotex/Kingspan has far better insulation properties than Rockwool by a factor around 2.

Bob

Hi Nick, I wouldnt use rockwool as you only have 3" to play with, you need to leave a gap against your outer skin and if you compress the rockwool, you squeeze the air out of it, and it is the air that does the insulating, use 2" jablite (polystyrene) or 2" kingspan/cellotex if you are feeling flush with cash! As peter says, breathable membrane on the cold (outer) side of the insulation, or visqueen dpm on the warm side of the insulation. Plasterboard is cheap, but cold, and absorbs moisture, I am using 3.5 mm wbp plywood on my ceiling in the workshop, in the form of damaged caravan/portakabin sheets 7x4 available form the surplus suppliers, they have bumped corners and some banding marks, but they are new, and about £3 to £4 each.. I am putting in 3" jablite between the roof joists, 2" jablite between the wall ones leaving 1" gap against the outer caldding, then a visqueen membrane (builders polythene) and finally the inner skin.

As I understand it Phil is quite right and you ideally need an air gap behind the outer cladding to let any moisture that does penetrate (and some will ,usually a little with 19mm claddingl -Bernouilli and other effects) dry out. Also the vapour barrier should be on the outside of the insulation as moisture will accumulate where the warm inside air is cooled so that it can hold less moisture.

The foil on Kingspan or similar will act as a vapour barrier by default although I admit I do also install a vapour barrier because I've got enough left over from foundations to cover a young airfield. Don't worry overmuch , the fact your thinking about it already puts you a jump ahead of most people you could pay to build.

Regards Alan

When I insulated the Garage/Workshop, I painted the inner walls with a vapour barrier paint, 3 coats, then 2" of EPS then Stirling board, painted white.

Floor painted as well, and covered with those huge interlocking tiles, making it easy on the feet.

What a difference all that has made, couldn't believe it.

Only needs 30min with the fan heater on, on the coldest day, to make it warm enough for shirt sleeves.

Geoff - Still raining, but no wind, yet.

Sounds like you are doing the right things, but don't forget to insulate the roof, to match the walls.

Or to site on a firm stable foundation.

May sound silly, but with insulated walls and roof, single glazed windows will soon be running with moisture.

(My shop has no windows; a) security, b) more storage shelves in a small shop, c) reduces heat loss, or gain when the outside is hot. The sun does radiate a lot of heat through glass)

Also, you need to reduce temperature variations, as much as is practicable.

And try to insulate the floor, if possible. Cold, numb, feet take away a lot of the pleasure of being there.

For ventilation, you need fixed ventilation near floor level, to get rid of any condensation, (even breathing adds moisture to the air), and a high level vent to allow entry for air to replace the moist air exiting low down.

It goes without saying that the vents must be weatherproof, (and spider/bug proof!)

Your heating, should not emit water vapour, so wood burners, paraffin heaters etc are non starters. This leaves you with electric, convector, radiant, or fan heaters, unless you can connect to the house hot water central heating. My vote is for a thermostatically controlled fan heater, for a quick warm up. When it is frosty or really cold outside, a tubular heater will provide background heat to hold the temperature above the dew point.

(A 60 Watt heater will hold a small 3 metre x 4 metre shop, 2.25 metres high, insulated with 50mm glassfibre between 19mm cladding and 12mm ply wood inner lining, above the dewpoint, to the point where a steel bench is just warm to the touch)

Moisture equals RUST. In my old small shop, the oil on the Myford would be white and emulsifying when entering for the first time on a U K winter day. Hence my near obsession with insulation and ventilation. Over the eleven years of my current shop, rust is virtually unknown I have had more rust problems from displaying stuff, under cover, outside, at a show.

Having mentioned security, here, and on other posts, DO make the shop secure, 5 or 6 lever lock(s) hinge bolts, no external screwheads or hexagons. (My door is a firedoor with a 6 lever lock and three hingebolts)

OTT? Possibly but a lowlife who cannot make off with a 6cwt lathe or mill, or has no use for what you have or are making, will still set fire to it in frustration.

Once you have implemented all the good advice offered, that you can, enjoy yourself!

Howard

I think that was a small slip - Inside please. But can be between the insulation and the inside cladding, ply or plasterboard.
Now the vapour barrier has to be 100%. If you use plastic and put a nail through you should wherever possible cover the nail head with tape. Tape joins. Foil covered board must have tape over all the joins. Even a small hole allows moisture through over time. Don't be sloppy about the fit of solid insulation and fill gaps with foam-in-a-can.
The plus side is that properly installed the vapour barrier also makes it totally draughtproof so no moist air gets in so don't spoil it with ventillation holes. Use a dehumidifier which also provides some heat and distilled water as a byproduct.

You must provide some ventilation else you'll end up in a wooden box of your own!

Bob

I only raise the point again 'cos its quite important to anyone building a shed.,not life and death,but important Reading througn these posts it would appear that practically everyone including myself and apparently "Building a Workshop" (post 1 ) think the vapour barrier should be on the cold side of the insulation. Having looked into this a lot recently (still building ) thats where the manufacturers say it should be for the simple physics reason warm air can hold more moisture than cold air. Put the vapour barrier on the inside and and where does the "precipitated " moisture go , obviously somewhere you don't want it.

There are it is true "commentators" on line who say put it on the inside but I prefer to run with the manufacturers, builders, and the science.

Where it does get a bit complex I admit is when the successive layers listed from inside the shed go (typically) Board Liningl, al.foil (on kingspan or similar), insulation,al.foil,small air gap,vapour barrier, larger air gap, cladding. I may omit the board lining but as according to the manufacturers the al.foil on both sides of the insulation will act as presumably two additional vapour barriers it all gets a little complicated. One thing I do know is that with timber cladding some external water will get through the cladding layer (pressure differences, due to wind gust conditions during storms) and I want that in the air gap between the vapour barrier I will add and the cladding where later drier air movement will dry it out. The added vapour barrier will pretty well keep these cold air movements outside the vapour barrier so that they don't appear as draughts, although I am hoping to achieve a good fit with the Kingsspan accurately cut,aided by a few miles of very cheap neoprene rubber draught seal strip.

The additional vapour barrier outside the insulation I can assure all only has a snowball in the hot place's chance of being installed if I decide when I start insulation operations there is no chance I shall be found a few years hence as a suffocated mummy wrapped in several hundred layers of green vapour barrier plastic . I certainly think it might be tricky as I intend to minimise punctures by nails and staples if at all possible and to tape any punctures (a la Bazyle) I do have to make to keep me from being driven even more insane than usual , by the sq. miles of green plastic.

Do we have a physicist in the house ??

Alan

ref, compressing glass fibre.

Years ago I bought from a scrap yard an ex-MOD hot food transport box..It was made of ali, double skinned with about 5/8" of incrediblely compressed glass fibre. So I came to the conclusion to a no-expenses spared outfit like the MOD, to keep hot stuff hot , the idea of more glass fibre the better.

Its bit like the foam stuff, the less air movement, the better, but this has to be balanced against the conduction through the actual material, i.e. the plastic of the foam or the glass of glass fibre.

I can not find any articles on the web to give the law between the thermal performance and the weight of glass fibre for a given overall thickness. I have found some articles that actually say that compressing 6" glass fibre into a 4" void gives inferior performance (then using it in a 6"void), but when the sums are done it is still better then 4" glass fibre in a 4" void.

I would say stuff as much glass fibre into the void, subject only to bending the boards in trying to shut it in!!!

The whole subject of glass fibre (v. multi foils for instance) is still not resolved, because the standard BSI test (the "hot box" for the thermal conductivity, is very much a laboratory experiment and does not reflect what happens in real life.

Frank

I think you have a point ,Frank, compressing glass fibre wool or similar must still leave a lot of air voids ,if smallish, with many small voids presumably being better than fewer larger ,possibly convection prone voids.

.I decided against "wool"of any kind as most of the knowledge and I think the science says that with timber cladding you will get under storm or even less conditions some external water getting through the cladding layer. I have used 36 mm cladding well tongued and grooved ,mitred and rubber sealed at key joints and carefully applied which will have 3 coats of Butinox , but was under no illusions about some water ingress. I did n't fancy wet "wool" and with many of the air voids wet I imagine insulation values drop sharply.

I don't know if Kingspan is closed cell but it is certainly I have observed quite resistant to soaking up moisture which would obviously cut insulation values. I also prefer working with any material I can shape to a dimension to guarantee a good fit and where I don't get one 3 or 5mm cheap neoprene draught sealer will go a long way to curing any lack of what I jokingly call craftsmanship on my part. Pretty impossible with wool . Stuffing in glass fibre probably scores as well over looseness in that its less prone to movement and sagging but must be still far more likely to do this over time, than Kingspan boards,especially if wetted then dried ,then wetted etc..

Also should I need to pay a little attention to any weak points after construction and use I would far rather remove a panel of Kinsgpan than rifle through wet or even fairly dry wool trying to find a more severe "leak'". I am using Kingspan here as the generic name, I'm too mean to buy the lead brand , and would like to have indulged my meanness a little more by using "wool`" but.........

Trouble is opinions differ . Everyone else says oi be wrong but oi says oi be right.I guess quality of build varies so much that all you can do is build the best you can with the best advice you can get and suck it and see.

One thing I am sure of is that building it yourself , providing one can find cheap good timber gives you a quality of shed that exceeds all but the most expensive bought ones., and custom made to fit, in my case a fairly complicated floor plan.

Edited By alan frost on 17/11/2014 23:25:13

Edited By alan frost on 17/11/2014 23:32:36

"Put the vapour barrier on the inside and and where does the "precipitated " moisture go , obviously somewhere you don't want it."

Sorry Alan but the vapour barrier goes ON THE WARM SIDE of the insulation eg on the room sideof the studwork.

The reason is as you state the moisture laden warm air will want to pass to the colder outside of the building, as it travels through the wall the temperature of the air will reduce and somewhere reach the "dew point" where the moisture drops out of the air. With no barrier this tends to happen in the insulation resulting in what is known as "interstitual condensation".

Placing a barrier means that the moisture in the air will stay in the warm room side of the barrier as it will not travel far enough through the wall to reach the critical dew point.

"I decided against "wool"of any kind as most of the knowledge and I think the science says that with timber cladding you will get under storm or even less conditions some external water getting through the cladding layer"

This is why it best to fit a breathable membrain such as Tyvec to the outside . it stops large droplets of water entering the insulation filled studwork but will allow any moisure particles out. Fitting a second vapour barrier on the outside is not a good idea and seems to be what you last comment refered to.

J, not a physicist but do have OND & HNC in building studies and a surveyor by trade.

Edited By JasonB on 18/11/2014 08:17:33

Edited By JasonB on 18/11/2014 08:19:12

OK here's one, Single skin garage built from rendered blockwork, conversion to workshop.

Should the inside of the block work be tanked? then insulated stud walls erected, then vapour barrier then plaster board.

or should the tanking be omitted to allow breathing.

I don't mean to hijack this thread however I think this is relevant.

Shaun

Ideally just treat the rendered block as your cladding so from the outside inwards

Render

Block

Vertical treated battens @ 400 cts (preferably run strip of DPC between blocks and batten)

Breathable membrain

Studwork

Insulation (not full depth of studwork)

2nd layer of insulation over studs (not really needed in a workshop)

Vapour barrier ( separate sheet or taped if using foil covered insulation, 4" tape to cover studs if not using second layer)

Plasterboard though I prefer ply or faced OSB as you can fix things to it easier than plasterboard.

J

Edited By JasonB on 18/11/2014 09:12:30

That's pretty much what I was thinking, but conflicting advice with regards to the tanking, was my problem, my view is the rendering has waterproofer in the mix and SBR slurry prior to application of render, so should stop most of the water entering through the block, and the inside vapour barrier should stop the water condensation in the cavity.

Shaun

I still think the main requirement is ventilation, stopping up every gap can be bad for things, including any timber. Look in some modern houses and see the rot in some timbers. Wood burning stoves, or any stove with a flue , is in my view an asset. The foil on P/B and insulation sheet also reflects heat. It used to be said that foil was the equivelant of 1" fibre glass.

UK building regs stipulate a minimum ventilation for the main building and you would be advised to avoid completely sealing your shed. You need fresh air to breath (solvents etc?) as well as controlling the humidity levels.

I used a "superquilt" type insulator in my loft conversion. There are several brands available but are fairly similar. The 40mm thickness provides broadly the same insulation as 200mm of rockwool and it's vapourproof on both sides, so no need to argue which way to put it(!!). That's quite a saving in terms of how much room you lose, if you can justify the extra cost. You should leave a breathing space (eg battens) on both sides no matter what you use, with any breathable membrane between that and the outside world - it's basically keeping rain, dirt and insects out..

The dew point will be situated somewhere within the insulation (unless you completely screwed up), so moisture vapour getting into the insulator from either side will be able to condense to liquid. Best to have impermeable layers on both sides or an impermeable insulator surely? The only way to completely avoid internal condensation is to keep the RH down and the minimum internal surface temperature above the resulting dew point. It may not always be possible - but if you have sources of vapour (coolant, tea, sweat, steam, kettles, towels etc), you need to control the RH by ensuring adequate ventilation, unless you like living in an oven. Hence the building regs Part F (pages 20-25?)....

Murray

Edited By Muzzer on 18/11/2014 12:02:49

Gosh, are people so stupid nowadays that they have to be told to make provision for fresh air to breathe while they are in the shed? Should they be allowed near moving machinery and sharp tools?

The requirements are failry obvious -
1 zero damp air coming in when not in use,
2 ventillate while occupied through heat exchanger for preference
3 after use seal fully and use dehumidifier to remove your sweat and dampness from the air introduced.

Modern eco houses have to have a pressure test (with chimney and vents temorarily blocked ) with only the equivalent leak to a mousehole allowed. Then ventillation is provided with a heat recovery system. Meanwhile the building regs also require a solid fuelled range to have about a 6 squ in vent that can't be blocked which where I am on an exposed hill would ensure a complete whole house air change in 10 minutes.

For spacing your solid insulaton off the outer wall try making little blocks of Corex - the corrugated plastic used for a lot of estate agents signs these days.
Re: garage and tanking. Consider providing extra air bricks through the outer wall to circulate air into that gap between insulaton and cladding. This is the principle of cavity walls.