Shed: Lining Interior Walls

I am guessing that the left over plasterboard has no polythene or foil "vapour check" on one side ?

No idea what happened with the formatting. Where do this <div> entries come from?

Correct, just plain 12.5 mm . . .

My timber garage came ply lined on the outside with a breathable membrane over that and finished off with T&G weatherboarding. When I created a “mancave” in the rear, I mostly filled the spaces between the studs with

Richard

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 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 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.

Must have been either the direct URL or the Tiny generated URL - cursed things. Thanks, noted for the future.

Probably you've not told Usenapp to send in plaintext, so it's decided to use html?

HTML

-- “Some people like to travel by train because it combines the slowness of a car with the cramped public exposure of 
an airplane.”

Dennis Miller

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.

HTML IS plain text.

You must have cut'n'pasted from some website

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.

1. Air: You want to prevent draughts and expensively heated/cooled air inside from leaking out.

2. 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.

1. 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.

1. 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?

2. 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:

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.

Theo

Thanks very much for the reply - saved as a file.

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 . . .

Possibly - it's something I need to think about when I insulate/line.

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

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.

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 - my home's one of the notorious cavity fill forms-a-bridge - not picked up by the surveyor. One joist is permanently wet . . . no cheap easy fix there.

Yes - I've picked up on that before, thanks.

Indeed. As it's 'just' a shed it's all pretty low risk and (relatively) easily mendable - but thanks for the pointers.

The old way of doing things was that everything was permeable - lime plaster, timber, brick, lime render. You lit your big coal/wood fire in the fireplace and the heat gradient drove through the moisture out to the outside. You didn't get condensation because everything was well ventilated and you kept the fire burning so there was always heat pushing moisture out. You needed the ventilation so the fire burnt properly anyway.

Then suddenly people installed central heating and got fussy about draughts. Now the fire and associated ventilation is gone, meaning the humidity inside rises - especially in winter when it's cold outside and the air is near the dew point. Plus if you start slapping impermeable finishes like vinyl paints or cement render on things then your moisture can't go anywhere, and worse your woodwork can't dry properly. Hence all the damp and mould problems in older houses when people weren't paying attention.

The main thing is that there's an air gap behind so your timber can get drenched in the rain but it won't soak through to the inside.

That sounds ok - if there's a route for moisture to the outside then it's ok to seal the inside.

The old way was very forgiving of moisture control - just as long as you keep that coal fire burning. With central heating what you're doing is substantially turning down the (metaphorical) pumping action that's causing your walls to dry. Reduce ventilation and add internal moisture (humans, showers, cooking, drying clothes inside) and that now-weak pump has a lot more moisture to get rid of. Turn off the heating and that pump is barely functioning at all. It's no surprise that damp is worse in the houses of people who can't afford to heat them.

Extractors and trickle vents are our sticking plaster to manage internal moisture (and air quality), but many people don't understand why they're there. Nowadays we have better solutions (MVHR) but we're often not installing them as standard.

As a non-habitable space you probably don't have a lot of sources of internal moisture (unless it's got plants inside) so you're mostly worried about keeping the rain out. Although insulation may help with daily temperature variations - eg in a metal shed that can be a source of condensation at nightfall - and general comfort.

Theo