vapour barrier

I've been wanting to build a workshop in the garden (yard) for a long while. At first, I was only thinking of a glorified shed, with maybe a trombe wall kind of thing for a bit of extra warmth. Then I worked out that the insulation would cost nearly as much as everything else put together. So, now the plan is straw bales under a huge flat roof, which also makes a rammed earth floor more appropriate, which in turn makes a rocket heater very attractive. (No worries about embers/excessive heat setting fire to the floor.) So it could actually be really cosy; cosy enough to want to sleep in there, but for that to be legal, the building would have to comply with building regs., and they seem to require this 'vapour barrier' stuff all over the place.

Looking into exactly what the stuff is supposed to do, has got me confused. Its supposed to be about keeping the structure dry, and you have to leave air gaps so that condensation can't build up, so why the heck do they also make you use stuff that STOPs moisture from passing at all? Is it REALLY a barrier to vapour? That implies that, if properly sealed, it's also airtight . . . I just can't see how they think it's a good idea, I can only see that it's more likely to cause problems than help.

So, what is it supposed to do, what is it made of, and what does it really do?

Vapor barriers and the reason they are not required in strawbale walls is explained here: https://permies.com/t/43637/natural-building/Breathable-Walls

Unfortunately, building and safety(b&S) codes along with architectural control committees (ACC) of American sub-divisions can be a real deterrent to the natural home builder because most do not understand it or it can be difficult in our country to find supporting data to show inspection it is safe. They are also good since they keep people from hurting themselves and others.

The first thing I suggest you do if you have not already is contact you local B&S office and find out what IRC (code) is being enforced and whether an energy code (IECC) is being enforced. If 2015 is being enforced you are in good shape and can follow strawbale code in Appendix R that does not require vapor barriers. Any other code (2006, 2009, 2012) is going to require alot of knowledge on your part, much more than vapor barriers to get adopted and satisfy the building permit, pretty much everything thing in the code depending on what you are building... If an energy code is being enforced you are going to have to prove with existing data, or test you fund, r-value, barriers, retarders,structures, etc. It may be a battle that also depends on the knowledge of the b&s office as well, and how cooperative they are. Some will allow you to use 2015 even they have not offically adopted it. IRC has a path for that in CH 1 "alternative materials....."

There is another path to get around code, you hire an Architect/PE that provides stamped drawings and a private inspector allowed in CH 1 of IRC. If an energy code is being enforced you'll want to make sure the Architect has an in depth knowledge of bales or other method of choice.

It would be UK regs, (sorry, should have made clear) and you might well be right about straw bale not needing it, but the roof almost certainly would. Then again, I'm inclined to say "*bleep* the regs", and sleep in there illegally (if it ever gets built!) I think I have found a way around it but all the blather has made me curious about the stuff. It's got to be danged clever stuff to let air through but not vapour as it's name would suggest, and in short, I don't believe it.
I can see that a fabric like layer could cut down drafts in certain types of construction, but it's name suggests that it lets air through, while it's use dictates that vented air spaces be left between it and the cladding, and a bunch more contradictory stuff that makes my head spin, so I am trying to cut through the mumbo jumbo and find out just what it is to see if it's any genuine use at all, or just a bs thing that big business has managed to get into legislation so joe public has to buy their s***!

. . . what is it supposed to do, what is it made of, and what does it really do?

I suggest you read that thread I posted. You are confused, most including the UK build "air tight and ventilate right" to stop heat/AC transfer in and out of the building and a air seal is not necessarily a vapor seal. Vapor barriers can stop "vapor" completely from getting out of the wall if it gets in and it more than likely will, we do not want nor in a roof so some subscribe to ventilation gaps to promote a drying direction ....Air sealing (not to be confused with vapor) takes ALOT of detailing, blower door test, etc.....We do not want barriers in the breathable walls, roofs, and foundations, for natural building materials usage anyway since they ARE the vapor "regulator", retarder, and air "regulator" needing no addition manufactured products. Code, was designed and influenced by manufacturing $ and therefore requires a barrier. IRC allows an inspector to make the final call, if you know how to convince them they are not required. I have already been down this path. Take the bale code with you ....it is based on prescriptive "international" buildings and design practices. http://thelaststraw.org/a-strawbale-residential-building-code-for-the-united-states/

To answer your question basically barriers attempt to keep moisture out of the wall, but most fail at understanding how they trapped moisture in the wall or envelope. To fully understand it again read the thread, it gets complex! Ventilation gaps can be effective depending, again complex noone has established flow rates and design guides most guess at size and flow.

Terry Ruth wrote:
To answer your question basically barriers attempt to keep moisture out of the wall, but most fail at understanding how they trapped moisture in the wall or envelope. To fully understand it again read the thread, it gets complex! Ventilation gaps can be effective depending, again complex noone has established flow rates and design guides most guess at size and flow.

Yes I am confused, and that thread was too complex for me, but the bit in blue explains why I got confused in the first place. For instance, a typical example of what you're supposed to do: cladding, ventilated gap, vapour barrier, insulation, inner wall coating. AFAICT, it's supposed to stop ingress of moisture from outside, but if the cladding is intact, as it should be, then there is no need for the membrane, and instead it's acting as a barrier to the moisture penetrating from the warm air inside, further the barrier is cold, and more so because of the venting, so the moisture is likely to condense there, with or without another air gap on the inside. Maybe if there was an air gap on the inner side of the mebrane, also vented to the outside, that might keep things dry, but that's not even suggested, nevermind required. I seriously think 'they' haven't really thought about it, and it reminds me of when soft margarine was first invented, and for years told everyone it was healthier than butter . . . even as a kid I wondered how could they know that when it had only been invented at most a few years before.

You missed the sheathing.....mainstream wall from outside in....

Cladding
ventilation gap or "rain plane"
vapor barrier or retarders (zero perm plastic or foam, high perm (20-30) house wrap)
Sheathing
Studs/insulation
Drywall (often with impermeable or low latex paint)

This wall if an exterior plastic and interior barrier is used is designed to trap moisture in the wall. The ventilation gap has several purposes in theory, if the flow is correct usually through soffit-ridge and addict vents, water or dew will not accumulate on a surface (cladding, OSB sheathing especially) that has air and/or water flow (physics gets complex here), a low pressure vacuum is created in the vent cavity to draw out moisture in the wall by capillary action if it can (but the plastic barrier stops this as you pointed out), and it also provides a capillary break as a drain plane. Buoyancy (pressure differences common inside properly vented addict, pascals law) is another but it gets complex. It provides a rain plane from top to bottom. House wrap does the same thing but does not provide much vacuum pressure for capillary action.

If you want to design a build that last centuries hire a building scientist or engineer that understand this. I suggest not listening to all the hype that mainstream building scientist and people that do not understand have developed for manufactured products to support bad designs, and layer after layer of materials like this. The effects of temperature is incorrect above, I just explained that in the last post on my thread so I'm not going to rewrite it here. I suggest if you are seeing post without credible data to support it do not take it as accurate. The data surrounding this topic is complex so do not take me wrong, most do not understand. I'm an Engineer and am struggling with some of it, but I am getting it. The hard part is explaining or making it simple.

If you have questions after trying to understand my thread please ask on it. I'm about ready to show a wall designed to fail like the one above that still out performs the wall above by far when the right material is used on the inside. Foam exteriors are also common and a barrier. I'm on a SIPs project with water & ice "barrier" as you and many suggest all over the roof that is designed to trap water in the vaulted roof unless they keep the interior humidity low with costly AC or a whole building dehumidifier (~70% is when fungi starts)....How low nobody knows since the mechanical device does not understand the walls, roof, building relative humidity/fungi content. Temperature has little effect on hygroscopic materials that can store at depths into the surface based in RH and proper pore size; more on glass, plastic, or impermeable materials. Heat trapped with moisture and a food for fungi = disaster!