# Walls moisture barrier - needed or not needed?



## Molynoox (26 Jan 2022)

Hi All,
I'm just retrospectively wondering about moisture barriers in walls, I recently built a 9x4m outbuilding, using stud walls, OSB sheathing and 100mm PIR insulation.
I did use a moisture barrier in the roof, as this feels like the key area to control damp, but I didn't bother in the walls, I am now wondering if this was smart or not and wanted your opinions on the matter please

*Wall Detail*
my wall build up is as below - as you can see, the PIR is pushed up tight to the OSB sheathing on the outside, therefore minimising chances of condensate forming as the 25mm air gap is on the inside (warm side). The only thing separating the air void inside the walls from the ambient room temperature is 12.5mm plasterboard so I figure that the temp gradient wont be very significant. You also have 100mm PIR until we get to a cold surface (the OSB sheathing) so I don't see that the air will get much chance to turn to condensate.

Note: the dimensions on the graphic below are not accurate, I used 125mm timber and 100mm PIR (not 100mm timber and 60mm PIR as shown)
Note: the green moisture barrier shown in graphic is the moisture barrier in question - I put it in the original drawing but decided not to use it. I did use the red tyvek house wrap, but I don't think this is relevant to the discussion as that is more to stop water getting in









thoughts?

I also have a heat pump running a heater/air-con unit which circulates the air inside the room, which I assume helps with moisture control, and the doors and windows have trickle vents.

appreciate your thoughts on this, cheers
Martin


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## Rob Cheetham (29 Jan 2022)

Im no expert but by mositure barrier do you mean vapour barrier. I have nearly completed my workshop and I have followed the plan that is provided on how to build a workshop mikes way (the post is pinned to the workshop builds and tours forum)

I followed his diagram to a t except I put in a vapour barrier all around the walls ceiling and even the floor. Also added 22mm chipboard flooring and then 7mm interlocking pvc floor tiles on top of the 50mmm celotex.

My walls from outside go

treated shiplap cladding
treated counter battens to provide air gap circulation
Kingspan breather membrane
100m rockwool insulation
Vapour barrier
11mm osb (didnt bother with plasterboard as its a workshop so going to get knocked about more than likely)

Because you put 100mm PIR insulation board in the walls I have heard that the foil layer on either side acts as a vapour barrier as someone told me not to bother putting it on the floor as I already had 50mm celotex down. I still did it though as I thought it wont hurt.

Anyway basically like I said im no expert but I think you will be ok.

If anyone would like to say weather im correct on the foil layer on PIR board acting as a barrier then go rite ahead. Would love confirmation myself lol

Just one other thing though I have seen a youtube video of a tradesman putting a moisture barrier on walls before boarding and they had PIR insulation in there wall cavitys also so mayby each to there own


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## Molynoox (29 Jan 2022)

thanks rob, I do mean vapour barrier yes
yeah I know about the PIR foil thing, and although i didn't tape mine it is spray foamed all round
I am pretty confident I wont see any problems but I was curious if others had other thoughts or experience to share

by taping the PIR with foil tape you could additionally be protecting the faces of the studs, so you could argue its better than just the PIR foil on its own, but this stud face is right next to the plasterboard which is sitting at room temperature, so problem seems to be pretty minor

Martin


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## Fitzroy (29 Jan 2022)

Martin. 

There are two outcomes depending on many things. You’re screwed or your fine. 

There is definitely a correct (belt and braces) way to do it and your design is different. The cement board will be cold as it is behind the insulation. If moist air reaches it you will get condensation. Leaving out the moisture barrier has increased the likelihood that this will happen.

What is the likelihood you’ll experience a problem is the unknown. If this was a kitchen or bathroom or room in constant use with lots of moisture, cooking, many people breathing etc you’d be much more likely to have a problem. If it’s a seldom occupied building at a lower temperature, like a workshop, then it’s much lower likelihood of a problem. 

If condensation does occur the cement board will not care a jot and won’t rot, it could run down to the base boards and overtime cause them to rot. If all your timber is treated I doubt you’ll ever notice anything, it won’t reach the inner face and damage anything. 

My problem is my shed is the same construction but I have OSB not cement board, with the risk of it all going mouldy if condensation were to occur. So I’ve still not installed the insulation, membrane or inner finish. It’s only been five years since I built it, still plenty of time to make a decision.


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## mikej460 (29 Jan 2022)

Bit late now I guess but the correct method is house wrap around the outside of the timber frame then battens (25mm gap) then cladding. Inside is PIR in between the studwork then OSB3 onto the studs (you don't need vapour barrier as it is moisture resistant).


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## Lard (30 Jan 2022)

Interesting topic and often confusing. Your house wrap is a breathable membrane and so not a vapour barrier…..here’s some info from the web…

_The unique material science behind Tyvek® *allows it to be both water tight and vapor permeable*. ... So while it's highly effective at preventing bulk water intrusion, it's also permeable, or breathable, to allow any water that does find its way into the wall system to find its way back out again as water vapor._

As you haven’t installed a vapour barrier on the inside, you will, theoretically, be allowing any such moisture that exists within the building to reach the insulation. However, the above info does point out that your tyvek will, again theoretically, allow that moisture to “find its way out again” and so suggests that moisture may pass through from inside to outside.

The problem you may have though is the lack of an air space where it touches the particle board and so it may stop the permeability occurring.


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## Molynoox (30 Jan 2022)

Fitzroy said:


> Martin.
> 
> There are two outcomes depending on many things. You’re screwed or your fine.
> 
> ...



lets hope mine (and yours) is the 'turns out fine' option :-D
all the timber is treated, yes

only 5 years eh? I approve of you taking your time, I've taken long enough for mine  

Martin


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## Molynoox (30 Jan 2022)

Lard said:


> Interesting topic and often confusing. Your house wrap is a breathable membrane and so not a vapour barrier…..here’s some info from the web…
> 
> _The unique material science behind Tyvek® *allows it to be both water tight and vapor permeable*. ... So while it's highly effective at preventing bulk water intrusion, it's also permeable, or breathable, to allow any water that does find its way into the wall system to find its way back out again as water vapor._
> 
> ...


thanks Lard, I know the house wrap isn't doing anything to stop moisture entering the wall cavity from inside, I only mentioned it because it was in the drawing - but like you say, it doesn't create an additional problem by trapping moisture because its breathable, which is of course good  

my summary from the feedback is that most peoples thoughts are pretty much aligned with my own:

1. in theory, vapor barrier should always be used, but might not actually be needed for every application
2. although there is plenty of well established theory, we don't actually know what will happen in any given wall, unless there are obvious glaring design errors
3. it seems unlikely, given all the factors with my wall design, that there will be an issue, but time will tell

thanks
Martin


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