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Solid Shed build,vapor barrier question

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yuzi87

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Hi Guys,

i just finished a 2 year project building a 2 story wrap round extension and complete renovation of existing house and now some of the rooms are full of tools which means i need a shed!

As i love building things and wood work i am building my own and i just have a question regarding the vapor barrier.

The shed is 4.5m x 2.2m the floor and roof will be made from 5x2" 400 centres frame sitting on 6 concrete block piers approx 15cm from ground.

I am thinking for the floor i will have 100mm celotex within the floor frame sealed off by expanding foam and aluminium tape flush to bottom, leaving air gap then roofing breathable membrane(upside down) then 18mm osb

The walls starting inside will be 4x2" 400 centre framework, 18mm osb, 100mm insulation, 18mm osb, breathable membrane, battons, feather edge.

My question is do i need vapor barrier on the internal stud side behind the osb? or the floor?

The shed will have an electric distribution board in and thats the only possible heat source.

Many thanks,
some pics attached progress so far



 

MikeG.

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Blocks with expanding foam, on the ground, and then upside- down felt. Why not do the job properly? Air gaps in floors refer to the space below the bottom of the joists....and you should have a 150 space there. I can't begin to think why you are planning to do what you have in mind.

OSB is vapour impermeable (to all practical intents). It therefore should not be used on the outside of insulation. Used on the inside it forms an effective vapour barrier in the correct place in the wall, and you don't need a secondary layer. 18mm is unnecessarily thick for walls. 10mm is more than adequate.

I suggest you have a read of the two threads linked to in my signature, and you'll maybe have a bit of a re-think before you go any further.
 

yuzi87

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Im not putting blocks on the floor........ im putting blocks of 100mm celotex flush within the floor frame which is raised around 150mm from ground....

Then i will seal all gaps between celotex and frame with expandable foam and aluminium tape, the roofing felt repels water while still breathable thus it needs to be reversed when put on the floor under the osb.
10mm may be enough but this is a solid shed build like i said and double layer 18mm osb will certainly achieve that
 

MikeG.

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Apologies, I misread the floor stuff about Celotex....but I note your edit. I ask again, how are you going to keep vermin out from the underside of the shed?

Your choice, but you shouldn't have a vapour barrier on the outside of the insulation, which is what you are proposing with the unnecessary OSB on the outside of the studwork. You risk interstitial condensation, mould, and eventual rotting. Leave it out and put a breather membrane there in its place, and of course, batten out before applying your external wall covering. I just saved you hundreds of pounds.

The felt under the floor achieves nothing (positive) at all. Also, it isn't breathable. The reason it has been superseded as a roofing material is that it didn't allow the passage of vapour out of loft spaces, and without adequate ventilation rooves actually rotted as a result. That's why their replacement, roofing membranes, are known as "breather membrane". Felt where you are proposing it in the floor would be another example of a vapour barrier on the wrong side of the insulation.

Please read the threads I have linked to. You could save yourself some unnecessary problems.
 

yuzi87

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I've watched many shed build videos now and many off them have framed wall, osb on outside then covered in membrane followed by battons and cladding, the inside then has insulation and more ply/osb or plasterboard.

Mine would be the same as this so im not sure why it would not work?

Also i will skip the membrane on the floor as foil backed celotex sealed with metal tape acts as a vapor barrier in itself. Rodents hopefully wont be a problem i'll set traps if they are. Or perhaps i could put a wire mesh on underside of frame

I have already bought sheets of 18mm osb so i wont be saving anything, i've looked at your detailed drawings they are very informative, however the shed is not as solid as i require thus the 2x walls of 18mm osb
 

MikeG.

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You crack on. Youtube obviously trumps architectural training and 20 years experience, and 200- odd workshop builds around the world. I'm never really sure why people come looking for advice and are then impervious to the answers they get.
 

yuzi87

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Hi Mike i'm certainly not discounting your valuble advice, but the internet is a good tool and there are many experienced people as well as yourself and so i am just taking all the info in and questioning everything.

Like these videos here, one who seems very professional - https://youtu.be/UWQqGBsqfI4
https://youtu.be/LP67MAoihZk


Is that a bad shed build because thats pretty much what i was planning apart from i would have battons behind the cladding, but this build has ply both sides.
 

MikeG.

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Yep, it's wrong. As I say, you carry on and do it how you were going to do it anyway. What you aren't understanding is that almost no properly trained people bother working down at the level of domestic outbuildings, so I am the dissenting voice amongst a bunch of builders and DIYers who don't know any better. And there is no comeback, because Youtube hasn't been around long enough for these buildings to start failing yet. I am happy to help people build decent workshops FOC, but I don't have a Youtube channel so clearly can't have a clue what I am talking about.
 

yuzi87

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what about this guy? He seems to have a good company from it? https://youtu.be/UWQqGBsqfI4

I just want some more science behind it, so i can understand better, like there must be a way that you can achieve what i am after surely, can i not add those periscope vents in your drawings? Based on my idea what will happen in regards to vapor etc so we can work a way around it
 

MikeG.

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Good luck with your build.

And will you stop editing your posts after they've been responded to. You've changed the video in your previous post, and that is potentially misleading for anyone following along. That's really pretty poor behaviour.
 

yuzi87

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I was already editing the post to add another video as well as the original while you replied.

So you have no response or answers to my last post? There is a plethora of shed build videos and many vary, you cant just say "they are all wrong im right listen to me" i would just appreciate some science or facts behind why it wont work and how vapor/condensation would react to that build so i can understand better and find a work around because like everything there is always a way.
 

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Condensation happens when warm, humid air meets a cold surface. To prevent it, you need the vapour barrier, which will keep the humid air on one side of it, to be at the same temperature as that air. Hence, the OSB goes on the inside of the insulation, because the outside of the insulation is colder. If you have two impermeable layers then there's no way for the air in between them to follow seasonal changes in humidity, and you'll still get condensation on the inside of the outer one. This is why double glazing panels are completely sealed and filled with inert gas, because if they weren't then they'd get condensation between the layers whenever it got cold out.

As for why so many professionals build sheds that way, because it's slightly faster and easier, and by the time the problems become apparent they'll be long gone. Mike is telling you, based on formal training and years of experience, the way to build a shed that will last as long as you do. People on YouTube are showing you how to build a shed that will last long enough that the customer won't ask for their money back. Which one you want to follow is up to you.
 

yuzi87

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Thanks for your reply, so can't i just put some vents in the framework to help airflow and any condensation? Or put the vapour barrier on the outside of the outer osb?
 

MikeG.

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There's good reason not to get involved with some people. :roll:

yuzi87":2qs3q7n1 said:
........... can't i just......put the vapour barrier on the outside of the outer osb?
MikeG.":2qs3q7n1 said:
.......you shouldn't have a vapour barrier outside of the insulation.....
 

yuzi87

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Im sorry but i like to understand things not just be told things, instead of explaining what would happen etc your just my way or the highway.
 

MikeG.

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yuzi87":33tlr2ck said:
Im sorry but i like to understand things not just be told things, instead of explaining what would happen etc your just my way or the highway.
MikeG.":33tlr2ck said:
.........You risk interstitial condensation, mould, and eventual rotting.........
spb":33tlr2ck said:
Condensation happens when warm, humid air meets a cold surface. To prevent it, you need the vapour barrier, which will keep the humid air on one side of it, to be at the same temperature as that air. Hence, the OSB goes on the inside of the insulation, because the outside of the insulation is colder. If you have two impermeable layers then there's no way for the air in between them to follow seasonal changes in humidity, and you'll still get condensation on the inside of the outer one......
 

yuzi87

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I know how consendation works but i wanted to know how it would react to my build idea and then i could come up with a work around.

Based on the fact this is literally just a shed that wont be heated and i will hardly be in it do i even need a vapour barrier? Would it not be better to just leave it out and let the air regulate itsself?
 

CHJ

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yuzi87":3rxog6sl said:
Im sorry but i like to understand things not just be told things, instead of explaining what would happen etc your just my way or the highway.
Then you need to learn how to ask politely for clarification of principle, not pose confrontational replies that purport to show that an ‘experts’ advice is wrong.

If after an ‘expert’ has explained the principals behind the reasoning for the advice and the science behind it, you can then go on to prove that the practice as applied in the UK environment is not optimum your research would add to the collective knowledge.

yuzi87":3rxog6sl said:
I know how consendation works but i wanted to know how it would react to my build idea and then i could come up with a work around.
If you KNOW how it works surly you KNOW what the results will be.

You don't need a 'work around' for something if you don't insist on making the mistake in the first instance, you appear to be perpetuating a problem that only exists in your mind.
 

TheTiddles

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Just a quick note/rant about “professionals”....

“Professionals” built my house... missing mortar, missing vents, missing shower tray supports, disconnected extraction hoses, missing screws on hinges, grass over manhole covers I really don’t have enough time to list everything, our neighbours porch fell off the house and would have killed someone if they were under it at the time, based on what I’ve seen of other new builds, we got lucky.

I recently saw a “professional” plumber bodge a neighbours new boiler, the condensation pipe is about 6’ of copper snaking down the front of their house to the drain including some level sections, that’ll work well in the winter...

I had some “professional” landscapers do some work outside, they tried to convince me that steps shouldn’t be level side to side for water runoff (after they failed to put them in level).

So, builders and other “professionals”... you have to repeatedly tell them how to do their own jobs as they don’t know, they have no qualifications and no professional standards and no limitations to them calling themselves these names and no penalties for failure. They are anything but “professional”.

I’ve been watching that guys videos too, I like how he constructs the foundations, that about where the admiration ends, last nights video had “bodge-job” written all over it.

Aidan
 

yuzi87

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You are completely correct, that is why i researched every single aspect of my house build project managed it and built most of it myself.

Please don't take offence to my line of questioning, i dont make chit chat, i say please and thank you and get to the point, until i understand something completely i will continue to ask questions from every angle.

Im a security engineer and i want a double skinned wall for extra security, i can not believe that there is no way of achieving this.

I have yet to be told how a double skinned wall will react to condensation with/without vapor barrier and whether vents etc could bypass any potential problems
 
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