Mike

If you're using breathable sarking membrane then it makes more sense to ventilate the batten cavity. This way the sarking membrane protects the insulation from the wind. The belt and braces approach would be to use PUR below the rafters & something vapour open between them. My preferred sloping ceiling is, from the inside:

12.5mm Fermacell;
0.3mm polyethylene VCL, laps & edges taped (530MNs/g);
9mm OSB3 lining board (2.2MNs/g);
400mm Crown Rafter Roll 32 between 400mm Masonite rafters;
22mm Pavatex Isolair woodfibre sarking board (0.55MNs/g);
Tyvek Supro Plus sarking membrane, laps & edges taped (0.17MNs/g);
38mm counter battens, 25mm vent to eaves, 5mm vent to ridge;
25mm tile battens.

David

Posted By: fostertomMikeG, I'm coming to that asap!

Cambridge paper yes, plus any others

So anything [peer reviewed or otherwise] which does not fit with your point of view on how vapour 'behaves' is wrong?

Thanks for your comments Peter. Sorry I haven’t had chance to respond earlier, I was hoping to do a combined one with whatever comments Tom intended to make

Posted By: Peter Clark

The general problem is that IF some water gets in to that construction it may be hard to get out again. That might be all right too, depending on how much gets in and where.

Significant water getting in from above would show itself [somewhere] quickly. Smaller leaks would not affect the construction as the surface area of rafters exposed is such that a high level of ventilation would be adequate to ensure such water would evaporate and be vented away.

Any water vapour which migrates from the internal environment would also be vented in the same way.

Posted By: Peter Clark

If a tile slips and is not noticed for a while, water could run onto the breather and through into the rafter/insulation.

No, if the breather membrane is undamaged and correctly lapped, no water will get in.

Posted By: Peter Clark

Might the damp have a detrimental effect on the rafters? If the insulation in between and below the rafters was hygroscopic, it would protect the rafters. If it had good capillarity, the water could run throughout the structure and not pool. If it was very vapour open in addition, it could dry out faster. This would happen especially if the 5:1 ratio of vapour openness was present.

I don’t believe the moisture necessary to cause such damage can remain hidden within fabric which is ventilated in the way I proposed. Dry rot for example, requires a certain temperature range and moisture content, as well as a distinct lack of ventilation. Ensuring any one of these three things does not happen will make dry rot impossible- One will of course try to ensure all three but the presence of sufficient ventilation is enough to avoid problems resulting from non visible leaks. No need to go to the extent [and cost] of the 5:1 ratio, which no doubt works equally well [not withstanding the debated effect on internal ‘health and well being’

Posted By: Peter Clark

If a hole is created in the plaster/vapour barrier, eg by picture hooks, water vapour could be getting in there, at a very low rate, but maybe continuously, day and night, for months on end, each year, year after year.

This may happen, depending on internal conditions but is unimportant due to my ventilation comments above.

In general, the risk of humidity levels being high enough to cause significant risk of interstitial condensation are exaggerated in my opinion. Bedrooms for example do not continually ‘force’ water vapour into the fabric in a way suggested by some. It’s complete rubbish.

Posted By: Peter Clark

A sloping ceiling/roof is probably the safest bet, as I am sure you were aware when you chose it. What about the same kind of construction for a wall – with electrical sockets and service penetrations through to the outside?

I don’t like timber framed walls personally, due to the risk of someone somewhere along the line getting it wrong. Obviously there is nothing wrong with it when it is designed correctly and installed as such. Unfortunately, the complexity of the 5:1 design style makes problems more likely in my opinion. Masonry on the other hand, is easier for everyone to design and build at least in terms of basic weathering and hence durability. Correct installation of insulation however is another argument, which is why I use partial fill PUR, and rigorously inspect things as they are done. I wonder how many realise that such inspection is now a requirement under Part L of the building regulations?

To answer your question about how the polythene integrates with the walls, it is simply lapped down over the wall plate; trapped behind metal lathing; and rendered over.

Posted By: Peter Clark

It all depends on the plastic vapour retarder being installed perfectly, is this realistic? What about the joints between the plastic sheets? Will they be perfect? How long will the adhesive last?

No adhesive involved. Everything is physically lapped and trapped. Staple holes are insignificant, again because of above rafter ventilation

Posted By: Mike GeorgeSo anything [peer reviewed or otherwise] which does not fit with your point of view on how vapour 'behaves' is wrong?

If I'm right, they're very wrong - as usual my 'listeners' must be the judge of that. Do you feel that you're in my firing line Mike? If so, I wasn't particularly aware of that.

What's all this faith in peers? Peers are by definition the ones who hold the conventional wisdom. Peers' fate is to be invoked when they agree with one, and to be ignored or rubbished when they don't. Majority (or even unanimous) agreement is not the same as the truth. There's no escape from the onerous duty to decide for oneself.

Posted By: fostertomWhat's all this faith in peers? Peers are by definition the ones who hold the conventional wisdom. Peers' fate is to be invoked when they agree with one, and to be ignored or rubbished when they don't. Majority (or even unanimous) agreement is not the same as the truth. There's no escape from the onerous duty to decide for oneself.

Nonsense. Peer review is about people with expertise in a subject looking for errors that laymen would be unlikely to spot, and checking that the article under review passes basic checks for quality of the analysis being undertaken. Peer review is not about following some consensus, it's about critical thinking - which some people seem all to happy to avoid if it doesn't fit their personal world view.

Two extremes there tuna and bot . Maybe the reality is somewhere in between?

Edit: Changed my mind, I think tuna has it about right.

Can I assume you cannot fault my insulated roof scenario then Tom?

Posted By: Mike George This is my standard sloping ceiling scenario. From Inside to outside

Plaster, painted with matt emulsion
12.5mm standard plasterboad fixed with 45mm galvanised clouts at 400mm centers. All board edges fixed into timber noggins
Cheap polythene lapped accross at least 2 rafters and fixed with standard mild steel staples. Polythene lapped down walls to ensure continued air barrier at walls
30mm PUR insulation fixed with 65mm clouts to underside of rafters. Joints filled with expanding PUR foam
150mm x 50mm rafters at 400mm centers. Voids filled with 100mm PUR flush with rafter undersides and deliberately cut to leave 10mm short of rafters. The 10mm filled from above and below with expanding PUR foam.
Clear 50mm cavity between rafters above insulation
Breathable roofing membrane
Slates/Tiles on treated roofing batten
Eaves vented with continuous overfascia vents
Ridge vented with dry ridge ventilation system.

Some would argue that some parts of this construction make-up are unnecessary [such as a breather membrane]. But other than varying the insulation thickness to improve u-value, I believe it to be a belt and braces approach - Is it?

Sounds perfect to me.

The company that makes the breather membrane will say that ventilating the cavity under it is unnecessary but I doubt it makes much difference to the U-Value if the gaps are sealed as proposed.

Wow Peter, that is a rather long list of queries. I'm not going to answer them sorry as I simply don't have the time. In any case I would only be repeating comments I have already made. I think we are going around in circles and it was obviously a mistake [on my part] to digress from the original focus of the thread by posting what I still believe to be my optimum [sloping]cold roof construction.

What I will say is that there are many ways of swinging a cat . For me its about picking the best option for a given scenario. There is much more to consider than breathability and it should never be the number one consideration in my view. In my experience for most people the primary consideration is function [obviously linked to cost]

So we will have to agree to disagree please. I have my view and you have yours.

Just to declare my interest - I have never worked for a PUR manufacturer in any capacity. Neither have I ever sold or comercially promoted any product or particular building system. I have however spent quite a few years putting such constructions together on site. Never had a problem with my detailed roof construction yet. If you were correct in your criticisms I surely would have done by now.

Thanks also for the discussion Peter. Only a mistake in that I started a side debate that I am unable to finish. Can only spend so much time on here.

Going back to the start, I read an ammended/shortened version on the article in Site Lines the other day.

It was saying that the gaps around the insulation (between it and timber studs etc) allows the passage of moisture vapour though the wall.

Funny thing though.... the BBA certificate also states that gaps around boards in installation will allow the passage of moisture vapour, however it also states in that same BBA certificate that all gaps around boards must be filled with an expanding foam!

I wouldn't trust anything that is spouted by the rigid insulation manufacturers regardless of what it is about. Othres are just as bad... did you know that another large insulation manufacturer don't have 3rd party certification for the use of their insulation in pitched roofs?!?! No-one has ever tested it! They stated that they/everyone had been using it for ages like that so they havn't bothered to test it.

Damage to VCLs in timber frame walls will cause issues, simple as that. I may have said in the past that it is not a huge issue, but some recent digging on my part says that anything over about the size of a single socket outlet can result in condensation on the external wall sheathing (if fitted). That is not to say that condensation results in a major faulure but it could if it were really bad.

To be honest, most of what people say and assume about breathability is just opinion and conjecture, and that includes what i say and think as well. Things like condensation risk calcs are all well and good but they are far too simplistic to really give a good and robust understanding, and that is even more undermined when the information from the product manufacturer's is sometimes completely wrong!

What really needs to be done is some indepth hygrothermal tests on different types of wall construction to show how moisture does really interact with the structure.

Timber

Posted By: TimberTo be honest, most of what people say and assume about breathability is just opinion and conjecture, and that includes what i say and think as well. Things like condensation risk calcs are all well and good but they are far too simplistic to really give a good and robust understanding, and that is even more undermined when the information from the product manufacturer's is sometimes completely wrong!

Timber

Well said that man

Ferns and moss clean indoor air in Canadian experiment.

A cure for "sick building syndrome" may be in sight. Canadian researchers and business people are testing an environmentally friendly air-cleaning system nicknamed the "breathing wall."

Although it looks more like a modern art installation than an air-filtering system, the breathing wall is actually an indoor ecosystem composed of rocks, plants, fish, and microorganisms. In effect, it inhales dirty air and exhales clean air.

Modern buildings with closed-air ventilation systems have a nasty habit of circulating stale and polluted air throughout the office. Workers who breathe fumes, dust, fibers, and biological contaminants day after day could use some relief.