I'm looking to use up some plasterboard I've got left over to line one of the PIR-between-the-studs shed internal walls. For the other walls I'll probably use treated ply. <div><div>Anyway, looking in to it all I see frequent advice to install a vapour barrier inside, behind the plasterboard. And a breathable membrane between the external wall and cladding. Insulation is sandwiched between the plasterboard and the external wall. Example here:</div><div></div><div>https://garden-shed.uk/plasterboarding-a-garden-lo g-cabin-or-summer-house-expert-advice/#Plasterboarding_a_Garden_Log_Cabin_or_ Summer_House_rCo_Expert_Advice </div><div></div><div>https://tinyurl.com/98h8dc3s</div><div></div><div>I've asked a few people about this - some very experienced - and I never seem to get an answer explaining why, if you're using impermeable materials like PIR, you bother with barriers and membranes. Well, I can just about see the point of a membrane - it could protect the wall (11mm OSB in this case) from driving
rain if it gets through whatever cladding is used, for example. But what's the
point of a vapour barrier inside if the moisture inside the building is just going to bounce off the PIR? </div><div></div><div>The last bloke I asked told
me it was to allow the new timber to dry out. Which makes no sense to me. What's 'kiln dried' all about in that case? And what happens to the 'trapped moisture' if it's painted?!</div><div></div><div>If I can avoid the expense and hassle of the barriers so much the better . . . !</div><div></div>-- Cheers, Rob, Sheffield UK
</div>
On 26/04/2026 02:57, RJH wrote:
I'm looking to use up some plasterboard I've got left over to line one of the
PIR-between-the-studs shed internal walls. For the other walls I'll probably >> use treated ply. <div><div>Anyway, looking in to it all I see frequent advice
to install a vapour barrier inside, behind the plasterboard. And a breathable
membrane between the external wall and cladding. Insulation is sandwiched
between the plasterboard and the external wall. Example
here:</div><div></div><div>https://garden-shed.uk/plasterboarding-a-garden-lo
g-cabin-or-summer-house-expert-advice/#Plasterboarding_a_Garden_Log_Cabin_or_
Summer_House_rCo_Expert_Advice
</div><div></div><div>https://tinyurl.com/98h8dc3s</div><div></div><div>I've >> asked a few people about this - some very experienced - and I never seem to >> get an answer explaining why, if you're using impermeable materials like PIR,
you bother with barriers and membranes. Well, I can just about see the point >> of a membrane - it could protect the wall (11mm OSB in this case) from driving
rain if it gets through whatever cladding is used, for example. But what's the
point of a vapour barrier inside if the moisture inside the building is just >> going to bounce off the PIR? </div><div></div><div>The last bloke I asked told
me it was to allow the new timber to dry out. Which makes no sense to me.
What's 'kiln dried' all about in that case? And what happens to the 'trapped >> moisture' if it's painted?!</div><div></div><div>If I can avoid the expense >> and hassle of the barriers so much the better . . . !</div><div></div>--
Cheers, Rob, Sheffield UK
</div>
I am guessing that the left over plasterboard has no polythene or foil "vapour check" on one side ?
On 26 Apr 2026 at 07:57:45 BST, Abandoned Trolley wrote:
On 26/04/2026 02:57, RJH wrote:
I'm looking to use up some plasterboard I've got left over to line one of the
PIR-between-the-studs shed internal walls. For the other walls I'll probably
use treated ply. <div><div>Anyway, looking in to it all I see frequent advice
to install a vapour barrier inside, behind the plasterboard. And a breathable
membrane between the external wall and cladding. Insulation is sandwiched >>> between the plasterboard and the external wall. Example
here:</div><div></div><div>https://garden-shed.uk/plasterboarding-a-garden-lo
g-cabin-or-summer-house-expert-advice/#Plasterboarding_a_Garden_Log_Cabin_or_
Summer_House_rCo_Expert_Advice
</div><div></div><div>https://tinyurl.com/98h8dc3s</div><div></div><div>I've
asked a few people about this - some very experienced - and I never seem to >>> get an answer explaining why, if you're using impermeable materials like PIR,
you bother with barriers and membranes. Well, I can just about see the point
of a membrane - it could protect the wall (11mm OSB in this case) from driving
rain if it gets through whatever cladding is used, for example. But what's the
point of a vapour barrier inside if the moisture inside the building is just
going to bounce off the PIR? </div><div></div><div>The last bloke I asked told
me it was to allow the new timber to dry out. Which makes no sense to me. >>> What's 'kiln dried' all about in that case? And what happens to the 'trapped
moisture' if it's painted?!</div><div></div><div>If I can avoid the expense >>> and hassle of the barriers so much the better . . . !</div><div></div>-- >>> Cheers, Rob, Sheffield UK
</div>
No idea what happened with the formatting. Where do this <div> entries come from?
I am guessing that the left over plasterboard has no polythene or foil
"vapour check" on one side ?
Correct, just plain 12.5 mm . . .
On 26 Apr 2026 at 07:57:45 BST, Abandoned Trolley wrote:
On 26/04/2026 02:57, RJH wrote:
I'm looking to use up some plasterboard I've got left over to line
one of the PIR-between-the-studs shed internal walls. For the
other walls I'll probably use treated ply. <div><div>Anyway,
looking in to it all I see frequent advice to install a vapour
barrier inside, behind the plasterboard. And a breathable membrane
between the external wall and cladding. Insulation is sandwiched
between the plasterboard and the external wall. Example
here:</div><div></div><div>https://garden-shed.uk/plasterboarding-a-garden-lo
g-cabin-or-summer-house-expert-advice/#Plasterboarding_a_Garden_Log_Cabin_or_
Summer_House_rCo_Expert_Advice
</div><div></div><div>https://tinyurl.com/98h8dc3s</div><div></div><div>I've
asked a few people about this - some very experienced - and I
never seem to get an answer explaining why, if you're using
impermeable materials like PIR, you bother with barriers and
membranes. Well, I can just about see the point of a membrane - it
could protect the wall (11mm OSB in this case) from driving rain
if it gets through whatever cladding is used, for example. But
what's the point of a vapour barrier inside if the moisture inside
the building is just going to bounce off the PIR?
</div><div></div><div>The last bloke I asked told me it was to
allow the new timber to dry out. Which makes no sense to me.
What's 'kiln dried' all about in that case? And what happens to
the 'trapped moisture' if it's painted?!</div><div></div><div>If I
can avoid the expense and hassle of the barriers so much the
better . . . !</div><div></div>-- Cheers, Rob, Sheffield UK </div>
No idea what happened with the formatting. Where do this <div>
entries come from?
On Sun, 26 Apr 2026 07:25:28 -0000 (UTC)
RJH <patchmoney@gmx.com> wrote:
On 26 Apr 2026 at 07:57:45 BST, Abandoned Trolley wrote:The <div> is a style tag in HTML. Since your post isn't HTML, it isn't obvious where it has come from, but somewhere in its creation is some software that speaks HTML. Maybe when you copied the URLs from a web
On 26/04/2026 02:57, RJH wrote:
I'm looking to use up some plasterboard I've got left over to line
one of the PIR-between-the-studs shed internal walls. For the
other walls I'll probably use treated ply. <div><div>Anyway,
looking in to it all I see frequent advice to install a vapour
barrier inside, behind the plasterboard. And a breathable membrane
between the external wall and cladding. Insulation is sandwiched
between the plasterboard and the external wall. Example
here:</div><div></div><div>https://garden-shed.uk/plasterboarding-a-garden-lo
g-cabin-or-summer-house-expert-advice/#Plasterboarding_a_Garden_Log_Cabin_or_
Summer_House_rCo_Expert_Advice
</div><div></div><div>https://tinyurl.com/98h8dc3s</div><div></div><div>I've
asked a few people about this - some very experienced - and I
never seem to get an answer explaining why, if you're using
impermeable materials like PIR, you bother with barriers and
membranes. Well, I can just about see the point of a membrane - it
could protect the wall (11mm OSB in this case) from driving rain
if it gets through whatever cladding is used, for example. But
what's the point of a vapour barrier inside if the moisture inside
the building is just going to bounce off the PIR?
</div><div></div><div>The last bloke I asked told me it was to
allow the new timber to dry out. Which makes no sense to me.
What's 'kiln dried' all about in that case? And what happens to
the 'trapped moisture' if it's painted?!</div><div></div><div>If I
can avoid the expense and hassle of the barriers so much the
better . . . !</div><div></div>-- Cheers, Rob, Sheffield UK </div>
No idea what happened with the formatting. Where do this <div>
entries come from?
page, you had selected a bit more than you intended, maybe a space at
each end that was preceded or followed by the <div>s.
I copy and paste a bit from websites, but normally from sites where I--- Synchronet 3.21f-Linux NewsLink 1.2
know there will be no extra junk mixed in. If I don't know that, I
usually paste to a text editor to be sure of what I've got. With HTML,
what you see may very definitely not be what you get.
No idea what happened with the formatting. Where do this <div> entries come from?
No idea what happened with the formatting. Where do this <div> entries come from?HTML
RJH wrote:
No idea what happened with the formatting. Where do this <div> entries come >> from?
Probably you've not told Usenapp to send in plaintext, so it's decided
to use html?
On 26 Apr 2026 at 11:12:11 BST, Andy Burns wrote:
RJH wrote:
No idea what happened with the formatting. Where do this <div> entries come >>> from?
Probably you've not told Usenapp to send in plaintext, so it's decided
to use html?
I've not set anything - just kept it at defaults and not noticed that behaviour before. I'm guessing the message I sent picked up some formatting that somehow spread to the whole message.
I'm seeing it here as plain text - but with a bunch of html type tags thrown in. if I could work out how to stop in happening - any suggestions welcome.
I'm looking to use up some plasterboard I've got left over to line one of the PIR-between-the-studs shed internal walls. For the other walls I'll probably use treated ply. <div><div>Anyway, looking in to it all I see frequent advice to install a vapour barrier inside, behind the plasterboard. And a breathable membrane between the external wall and cladding. Insulation is sandwiched between the plasterboard and the external wall. Example here:</div><div></div><div>https://garden-shed.uk/plasterboarding-a-garden-lo g-cabin-or-summer-house-expert-advice/#Plasterboarding_a_Garden_Log_Cabin_or_ Summer_House_rCo_Expert_Advice </div><div></div><div>https://tinyurl.com/98h8dc3s</div><div></div><div>I've asked a few people about this - some very experienced - and I never seem to get an answer explaining why, if you're using impermeable materials like PIR, you bother with barriers and membranes. Well, I can just about see the point of a membrane - it could protect the wall (11mm OSB in this case) from driving
rain if it gets through whatever cladding is used, for example. But what's the
point of a vapour barrier inside if the moisture inside the building is just going to bounce off the PIR? </div><div></div><div>The last bloke I asked told
me it was to allow the new timber to dry out. Which makes no sense to me. What's 'kiln dried' all about in that case? And what happens to the 'trapped moisture' if it's painted?!</div><div></div><div>If I can avoid the expense and hassle of the barriers so much the better . . . !</div><div></div>-- Cheers, Rob, Sheffield UK
RJH <patchmoney@gmx.com> wrote:
I'm looking to use up some plasterboard I've got left over to line one of the
PIR-between-the-studs shed internal walls. For the other walls I'll probably >> use treated ply. <div><div>Anyway, looking in to it all I see frequent advice
to install a vapour barrier inside, behind the plasterboard. And a breathable
membrane between the external wall and cladding. Insulation is sandwiched
between the plasterboard and the external wall. Example
here:</div><div></div><div>https://garden-shed.uk/plasterboarding-a-garden-lo
g-cabin-or-summer-house-expert-advice/#Plasterboarding_a_Garden_Log_Cabin_or_
Summer_House_rCo_Expert_Advice
</div><div></div><div>https://tinyurl.com/98h8dc3s</div><div></div><div>I've >> asked a few people about this - some very experienced - and I never seem to >> get an answer explaining why, if you're using impermeable materials like PIR,
you bother with barriers and membranes. Well, I can just about see the point >> of a membrane - it could protect the wall (11mm OSB in this case) from driving
rain if it gets through whatever cladding is used, for example. But what's the
point of a vapour barrier inside if the moisture inside the building is just >> going to bounce off the PIR? </div><div></div><div>The last bloke I asked told
me it was to allow the new timber to dry out. Which makes no sense to me.
What's 'kiln dried' all about in that case? And what happens to the 'trapped >> moisture' if it's painted?!</div><div></div><div>If I can avoid the expense >> and hassle of the barriers so much the better . . . !</div><div></div>--
Cheers, Rob, Sheffield UK
In building science you have 4 things you want to control, in descending order of importance. The 4 control layers:
1. water
2. air
3. vapour
4. thermal
You want to keep water, air and heat from moving through the wall. You also want the wall to dry out to avoid rot. How you construct your wall assembly governs what happens to these 4 things.
1. Water: this is your rainscreen - on a shed it could be shiplap or cladding. It will get wet, so it needs to be exposed to outside air to dry. eg in shiplap there's typically an air gap behind to allow the back side access to outside air.
2. Air: You want to prevent draughts and expensively heated/cooled air inside from leaking out.
3. Vapour: to prevent moist air from condensing, and to allow wall
assemblies to dry out. There is a temperature gradient from warm inside to cold outside (or vice versa) and at some point the dew point is reached and you get condensation. We either need to prevent moisture getting in (eg vapour barriers) or let it out (vapour permeable materials like timber and brick). Once that moisture meets fresh air (inside or outside) it can evaporate and not be a problem any more.
4. Thermal: to keep a comfortable temperature inside when it's too hot/cold outside. Insulation is doing this job, but also thermal mass (eg timber sheds got hot in the daytime and cold at night while brick sheds less so), shading, etc.
An unheated shed might be a different proposition to a centrally heated house, which is different again to an air conditioned house in the tropics. But in the UK a shed may be heated by the sun during the day even if it has no artificial heating, and then cooled at night. So we do still have to worry about warm moist air inside at nightfall.
Bare PIR as a material is a good insulator, but it's not completely impermeable which means some moisture can pass through it. That's potentially useful if you have a wall assembly where you need some drying potential - if there is a risk of condensation in the middle of the wall,
you need a route for the moisture to dry to the inside of the room or to the outside air, and naked PIR will help a little here. Other insulating materials (rockwool, timber insulation board) are much more permeable which helps more with drying.
The foil on foil-faced PIR is a vapour barrier, so this kind of PIR is impermeable to vapour. If you put it on the inside of a room then warm vapour can't escape into the wall. As long as either you seal everything moisture tight, or the wall can dry to the outside then you're ok.
Gypsum plasterboard is also (mostly) impermeable, so a room boarded out in
PB has a similar vapour barrier.
Paint can either be permeable (most emulsion) or impermeable (oil-based or waterproof vinyl). Impermeable paint can also act as a vapour barrier.
Breatheable membranes are something you put on the *outside*, the layer
below the rainscreen. They stop rain coming in (if your rainscreen failed, say a board slipped) they prevent that soaking into the timbers or rockwool or whatever beneath while at the same time allowing any moisture in your internal wall structure to dry to the outside.
Timber may be dry, as in kiln-dried, or green (still having its original cellular water) but either way it's still permeable to outside moisture. If that moisture gets trapped in it then rot will start to eat away at the timber.
So you need to consider your wall assembly:
1. Does moisture from the inside have a route through the wall to meet a
cold spot where it might condense? If so, that's a potential rot and mould problem - consider vapour barriers to prevent that.
2. Does the outside protect sufficiently from rain? What happens to moisture that gets past the rainscreen? Does it have a route to dry out again relatively quickly?
3. Assume parts of the wall assembly *will* get wet, via condensation,
leaks, etc. Do they have a route of permeability to either the inside or
the outside?
For example, a traditional cavity wall assembly passes these tests:
- the brick outside is the rainscreen
- the cavity allows outside air to flow behind the brick to dry it out
- the inner block wall is separated from the outside by the cavity, so no rainwater can soak through to the inside
- gypsum plaster and paint/wallpaper on the inside prevents room humidity from soaking through and condensing on the cold blocks
- the room is sealed against outside air
So it controls water, air and vapour but it leaves something to be desired
in terms of thermal performance, hence cavity wall insulation - but if you
do that wrong you can compromise the first three.
I find it helpful to enter my wall layers into an insulation calculator: https://www.ubakus.de/ (use the demo version)
and it gives you a red/yellow/green on condensation and drying time, which
is a handy starting point for understand whether a stackup is sensible. However any assembly is only as good as the detailing - if you try to make a seal but there are gaps, the gaps are where problems may occur.
On 27 Apr 2026 at 11:51:16 BST, Theo wrote:
A thing I've come across in a book I'm reading ('Old House Eco Handbook', Suhr/Hunt) does point out something I hadn't thought of - using absorbent/permeable materials helps deal with 'shock' moisture - say, after a shower or some robust cooking. This is because it absorbs moisture rather than
allowing it the opportunity to condense on a hard surface, and it can then evaporate in a more controlled way.
OK, get that, and they stress the need for (low thermal performance) wood and wool-type insulation materials in pre-cavity buildings. But then there's the risk somebody slaps it behind plasterboard and decorates with who-knows-what wallpaper and paint. Maybe they elaborate later in the book but I just don't get it . . .
2. Does the outside protect sufficiently from rain? What happens to moisture that gets past the rainscreen? Does it have a route to dry out again relatively quickly?
Pretty good in that regard, I'd say. It's 100mm wide 20mm thick rough sawn treated timber, with a style feature 25mm batten between each and a 4mm gap. Fixed to roofing battens, which in turn are fixed to the internal studwork and
11mm OSB. Along the bottom of the walls to just above ground level, behind the
the timber cladding, loosely fixed 40cm high old roof tiles. The roof has a 30cm overhang all round. Pics here:
https://www.icloud.com/sharedalbum/#B1mG60dj6h8Iwd
3. Assume parts of the wall assembly *will* get wet, via condensation, leaks, etc. Do they have a route of permeability to either the inside or the outside?
Outside - yes. Inside, much less so if/when I insulate with foil backed PIR. In a controlled sense there's one opening window, and I'll put some ventilation louvres in. So ventilation will depend on opening/closing as required.
For example, a traditional cavity wall assembly passes these tests:
- the brick outside is the rainscreen
- the cavity allows outside air to flow behind the brick to dry it out
- the inner block wall is separated from the outside by the cavity, so no rainwater can soak through to the inside
- gypsum plaster and paint/wallpaper on the inside prevents room humidity from soaking through and condensing on the cold blocks
- the room is sealed against outside air
Does it pass an internal moisure escape test? I'd have thought mechanical ventilation would be advisable, especially in rooms like kitchens.
I used to work as a social housing manager - even in well designed and built housing internally generated moisture can cause quite profound damp in a building - especially if it isn't heated and the occupants don't know how/when/if to use things like extractors and trickle vents.
Indeed. As it's 'just' a shed it's all pretty low risk and (relatively) easily
mendable - but thanks for the pointers.
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