There is a problem with the idea of totally encapsulating a building with an external water and vapour and wind proof barrier. That is that the dew point is then within this barrier, in fact just inside it and so condensation would tend to form there and not be able to get away at all easily. The only way out would be to run down to the bottom and hope to find a way out, or to migrate back into the house. Neither of these options is very good and the latter could be extremely slow.

Flat roofs constructed on a warm roof basis have to have a vapour control layer at ceiling level and so there is no good source of condensate available to cause this problem under the roof membrane (the main source of condensate is from the warmer air within the house that inherently contains more moisture) though I think that it still does get damp regularly from condensation there but this is burned away again by the sun on the next sunny day. The void between the ceiling and the insulation is generally far from air tight having major access to outside air which reduces the problem if not largely eliminates it.

MHRV makes no difference.

Tony,

In this instance a MVHR would make a difference!

Water molecules are not self propelled or pre-programmed with a mission to head to the outside skin. There needs to be an existing pressure. Moisture will buffer throughout the sealed warm envelope house and head for the first available permeable object i.e. furniture, plaster board, carpets, clothes.

A small amount of water vapour will be present within all permeable objects in a sealed warm envelope house including the wall/roof structure it can never be totally dry. However, if the interior is maintained at around 45 to 55% RH with a MVHR system the only vapour present within the wall/roof fabric will be the extent that this level of RH will allow. I do not think this would be enough to cause any deterioration to the structure. If a tiny amount moisture condenses on the outside wall then it will run down the outside edge and cause no damage because it will be far away from the structural elements of the wall. This will increase the durability of the structural elements because they will be dry.

This has not been flagged as a problem by the many experts who have been involved in the design of this system. If it were a problem a simple release valve can be retrospectively fitted into the skin to drain water away.

Tony considering the above please explain the force that will drive the moisture towards the warm insulation, then through the warm insulation to the outside skin?

Steve

The force driving the water vapour to the cold inside surface of the outside skin is analogous to gravity and inexorable. It is there because there is a difference in the partial vapour pressure of water in the air between the outer layer of insulation and that of the vapour in the air in the house. Air always contains some water vapour and the warmer the air generally the more it contains and certainly it is capable of holding more. The water vapour automatically transfers itself from places where its partial vapour pressure is higher to places where its partial vapour pressure is lower, even moving through permeable and semi permeable materials on its way. It is just like a ball rolling down a slope -- unstoppable.

Posted By: tonyThe force driving the water vapour to the cold inside surface of the outside skin is analogous to gravity and inexorable.

This does not explain what the force is. If it is a relentless force similar to gravity please explain.

Posted By: tony
It is there because there is a difference in the partial vapour pressure of water in the air between the outer layer of insulation and that of the vapour in the air in the house. Air always contains some water vapour and the warmer the air generally the more it contains and certainly it is capable of holding more. The water vapour automatically transfers itself from places where its partial vapour pressure is higher to places where its partial vapour pressure is lower, even moving through permeable and semi permeable materials on its way. It is just like a ball rolling down a slope -- unstoppable.

There will be no difference in vapour pressure. The pressure within the envelope fabric will be the same pressure as the interior of the building. There will be no impediments, no vapour or air barriers, nothing can stop the pressure from equalising throughout the whole building within the impermeable skin. It is not possible for water to condense on the extreme outside of the system because water molecules will not migrate through the fabric in a neutral pressure environment. They would probably buffer in the first two or three inches.

Cheers

Steve

I've been trying to bite my lip for months on this. The most airtight buildings that I am aware of have an air leakage of 0.38m3/m2@50pa this has been *repeated* many times in PassivHaus projects (< 0.3 ach/hr @50pa in a domestic building). Whilst Steven's system may offer airtightness it has to deal with real world tricky things like interfaces and services penetrations. Back in the day when he called it ASMET these isses were identified:
http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=174&page=1#Item_0
Also in days of old Tom's fact finding mission even found and recorded an instance where the same product (this time as RoofCrete) has caused damp (and mould?) problems in a rather distinguished building.

A room at 20°C, with an RH of 50% contains, 7.3g/m3 of water. Now imagine this sealed box at 20C and an external temp of 0C, there will a temperature gradient in the insulation. At some point due to the temperature differential the insulation temp will fall to 13.5C; this is the dew point. At and below the dew point water vapour will condense, the conditions are now such that after 3 days at or below 13.5C mould will grow. As I see it there are two choices: -
1) move the airtightness/vapour barrier to the warm side as building science has recommended for the last 30+years
2) encapsulate the insulation.

So far Steven doesn't seem to have mentioned encapsulation, only an external air/vapour barrier......this makes me worry about whether he is accidentally promoting an unhealthy form of construction.

Mark

So essentially, the driving force is the temperature gradient and the result is that natural levels of humidity are 'pulled' to the colder areas where water can condense out of the air? I must admit that managing moisture levels is a part of building design that I find hardest to grasp (though ventilation in general comes a close second!).

Posted By: Mark SiddallI've been trying to bite my lip for months on this.

Why? Your opinion is valued and I've been waiting for technical answers.

Posted By: Mark SiddallWhilst Steven's system may offer airtightness it has to deal with real world tricky things like interfaces and services penetrations.

There are far more interface problems on industrial roofs and we've been sealing these watertight for years. This is our expertise and no problem. All interfaces are accesible and maintainable on the outer surface.

Posted By: Mark SiddallBack in the day when he called it ASMET these isses were identified:

ASMET is made out of the same product but is a complete structural building system. Please stop saying they are the same. We are discussing a total encapsulation using an external render cladding system.

Posted By: Mark Siddall1) move the airtightness/vapour barrier to the warm side as building science has recommended for the last 30+years

I have discussed this with John Manniex and there seems to be some wires crossed with the interpretation of no airtightness/vapour barrier in the wall build up as concluded by the LABC. The LABC were meaning the structural wall should have no restrictions on the flow of vapour because the structural wall needs to breath inwards to be controlled by the MVHR with our self contained system on the outside. If you put an airtight/vapour barrier under the plaster board as recommended for the last 30yrs + . This would cause major problems trapping water in the wall build up.

The reason water would get trapped is because our warm deck roof has always been installed with a vapour barrier and that is the same package we're going to use to encapsulated a building. The weakness of course is that no vapour barrier is 100% and a tiny amount of moisture will get through and maybe end up on the outside. When the airtight render encapsulates the whole building then everything inside the membrane becomes neutral pressure (not influenced by the external climate). Therefore, it would not produce the rivers of water as suggested by some people on this forum. This cannot be classed as unhealthy.

For example, if sealed thermal cladding is fixed to a timber frame this would be the build up from inside.

Wall build up
------------------
plaster board
mineral wool insulation between studs
plywood
Sealed thermal cladding system which includes: vapour barrier, rigid insulation, airtight render

The system sits on the extreme outside of the structure and will provide a maintainable airtight skin which will dramatically improve efficency and durability of all materials inside it.

Posted By: Mark SiddallA room at 20°C, with an RH of 50% contains, 7.3g/m3 of water. Now imagine this sealed box at 20C and an external temp of 0C, there will a temperature gradient in the insulation. At some point due to the temperature differential the insulation temp will fall to 13.5C; this is the dew point. At and below the dew point water vapour will condense, the conditions are now such that after 3 days at or below 13.5C mould will grow.

In the above timber frame example, if the system is installed with 130mm of rigid insulation and considering the point you made about temperature gradient. Where would the dew point be? This seems to be the main question.

Mark Brinkley said an average house could hold as much as 10,000 litres of water in the envelope fabric. (How much does the building fabric suck in on a rainy day especially if the climate gets more moist as predicted). This moisture is fed into the building envelope from the inside and outside. We stop the penetration of moisture from the outside and inside the envelope the moisture can be managed. Surely 7.3g/m3 is a tiny amount of water to manage especially when moisture can buffer throughout the building with no pressure.

Posted By: Mark Siddall2) encapsulate the insulation.

This can be done but would double the price of the airtight render. However, this would be a perfect solution. This is what is on the Weald and Downland Gridshell with 5 inches of Rockwool.

Personally I think its urgent to find a mass market solution that can be rapidly implemented. I'm all for bespoke houses but we need an affordable mass market solution that will solve the energy problem. The (warmdeck) solution we're offering is almost an encapsulation. The difference of course is the insulation is not sandwiched between two airtight renders. It has vapour barrier on warm side of the insulation and airtight render on the cold surface. This design has proven to work all over the UK. It will be an affordable solution for social housing.

Finally....

Tom's fact finding mission - please clarify? As I'm aware over a 12 year period there has been 7000 + installations and only 3 roofs replaced due to incorrect installation. All working and will never need replacing. Any problems are always resolved through the guarantee.

Tom, can you shed some light on this mystery roof please?

Posted By: steveleigh
I have discussed this with John Manniex and there seems to be some wires crossed with the interpretation of no airtightness/vapour barrier in the wall build up as concluded by the LABC. The LABC were meaning the structural wall should have no restrictions on the flow of vapour because the structural wall needs to breath inwards to be controlled by the MVHR with our self contained system on the outside.

I think you are missing the point being made, steve. MVHR will not force a wall to 'breathe inwards', as the temperature gradient is in the opposite direction, meaning that unless your MVHR achieves Saharan levels of dryness (physically impossible and rather unpleasant to live in), moisture will be drawn towards the external cold surface. Condensation will always form there.

Tony suggests that in roofs this doesn't matter as natural warming during a sunny day will reverse the process and push the moisture back into the building envelope. In walls that are shaded or sheltered, this will not occur and moisture/mould build up could be a problem.

Mark,

As I value all opinions on this forum. Have a look at the picture. Would you be kind enough to tell me where you think the dew point would be?

That would be a great help.

By the way, out of thousands of installations there are no leaky roofs.

Steve

Mike,

I want to get this sorted. I have one set of experts telling me it will work and another set of experts on this forum saying it will produce condensation problems. I can't seem to get a straight answer on this and its driving me crazy!

I'm trying to find answers which at the moment are totally conflicting. What I say on this forum is what I've been told by insulation technical experts, ventilation technical experts and of meetings with building control. They all say it will work without the condensation problems suggested on this forum.

In the next few months I have meetings arranged with RSLs and several technical directors of large house builders. In November I had a two hour meeting at Atkins-Global in Bristol with 6 leading technical guys who assessed the system and didn't raise any of these issues.

I've just got off the phone with a large ventilation manufacturer and they are telling me contrary to what Mark Siddall is saying! So what is going on here.

I think we have pinned the issue down to where the dew point will be. If its somewhere in the middle of the rigid insulation how can it condense? I accept some vapour will get through the vapour barrier

Answer to your question Mike.

In our warmdeck flat roof specification we always use a polyethene vapour barrier. It has been used for years horizontally and vertically.

Cheers Steve

Steve

I don't think there's a conflict. You will have a dew point under some conditions. However, whether the rate of energy flow and/or the rate of production of water vapour within the building is sufficient to make the volume of condensation a problem is perhaps a better question.

Without knowing exactly what your materials are, material thickness and proposed conditions within the building and the external environment, it's not possible to assess with certainty (though one could make a reasoned stab at it).

It is unlikely that anyone on this forum would be willing to give a professional view that you could rely on (using rely as a legal term). So, if you have a specialist that says it will be OK, then it may be in your best interests to rely on that specialist.

Tony,

I consider myself a good guy and I've took a major beating of you lot and Keith Hall keeps emailing me telling me off.

The vapour barrier does concerns us all because they will inevitably have perforations. The history of the product show polythene is fine in a warmdeck build up over kitchens and bathrooms and they don't have condensation problems.

However, there is no reason why the insulation can't be sandwiched between two layers of airtight render around the whole building envelope (as shown in the diagram). It would be more expensive (about £65 sqm)and I'm trying to get the figures down for mass market Code 5 solution which can be easily upgraded to Code 6. Maybe a double or triple layer of polythene would be a cheaper option (less than a £2 or £3 extra per sqm). It will still have perforations though! What do you all think?

Cheers

Steve

Steve,
From the not-to-scale drawing, with no dims, the location of the dew point can not be determined with any accuracy. Anyway I've brainstormed a few issues that spring to mind:-

1) First of all, good to see that you do actually have a vapour check; this hadn't been mentioned earlier.
2) It's a pity that the VCL is in the wrong place, the vapour control layer is located in the centre of the wall, whereas to avoid interstitial condensation the membrane should be no more than 1/3rd into the wall as measured from the warm side. (This said the rigid insulation is likely to have a higher resistance, so the VCL could possibly be located 1/3rd into the walls temperature gradient .....?)
2) I note that it is a VCL not a VB. VCL suggests that it is permeable so I have the same query as Mike, how vapour open is the VCL? (TIP: The more correct answer would be that its a drafting error and that it should be a VB...but this may still be wrong see vapour permeance below.)
3) How are the joints in the VCL/VB sealed? Assuming that the system is built up from the timber member, how have you avoided penetrations through the VCL/VB.... and still secured the rigid insulation and airtight render? Any penetrations that allow vapour to be transmitted into the insulation will be
4) Vapour permeance: Foam insulation 25mm extruded polystyrene has a vapour permeance of 23-92ng/pa m2s and 25mm polyurethane has a vapour permeance of 69ng/pa m2s (I've no data for greater thicknesses). 0.15mm Polythene has a vapour permeance of 3.4ng/pa m2s, thus is not low enough to stop water vapour. Only 0.03mm aluminum foil has a measured vapour permeance of "zero"...as in to more decimal places than my data. Given enough time water vapour can get into this construction... more to the point it is very hard for vapour get out (construction sites are not very dry and materials need time and opportunity to dry out......mould ensues it it can't be vented.)
5) What is the vapour permeance of RoofCrete? If it is 5-15 times less than the polythene membrane then it could work..... IF.

Now after all that I may have found a nod towards a "solution" to the vapour problem, it's the only advanced membrane that I'm only aware of that can be vapour open in the summer and vapour closed in the summer is Solitex DB PLUS from Ecological Building Solutions. Basically the barrier reacts to various differentials in moisture content within the air. In the summer it becomes vapour open and allows the insulated zone to breath whilst during the winter it is predominantly sealed and any vapour that does get in can get back out later in the year. Now that I've got your hopes up its time to crush them again. With all this said what I'm on about here is seasonal permiability. Water vapour could still get trapped for months on end and as a consequence mould and the assoc. health issues can still result. This is why DB PLUS is still used with vapour permeable breather membranes....
http://www.ecologicalbuildingsystems.com/products/solitex/index.shtml#db

So if I were you I would find a qualified building scientist and work with them, say from the BRE or UKAS. No doubt it will cost but at least you can sleep at night knwowing that the product you're trying to supply will not have any unhealthy implications. None of the reps that you have spoken to are qualified to state that your system will work (and I'm not qualified to catagorically say that it won't all I can do is state my concerns. You have to go away an prove beyond doubt that the system works, and this means test not theory).

I hope that this has helped a little. For the time being I've feel as though I've spent enough time on this now. My concerns have been aired and its is for you to convince me with scientific data that the system works. A good start would be to let the forum know the vapour permeance of RoofCrete.

Mark

Steve,
I must say once again that I had/have the same concerns as Mike. If the idea/system for a whole house solution is so good why won't Roofcrete back it? Sounds to me like they may have concerns.

Mark
P.S. Not that it has any bearing on Steve's absolutely,100% airtight box but it's an 'interesting' fact anyway, at 2pa pressure a 1m long, 1mm wide gap will, at 20C 50% RH (7.5g/m3), transport about 360g of water per day. That is a lot of water. Energy matters aside this is why airtightness is important in helping to reserve (timber frame) structures and preventing mould growth. If my home was timberframe, and wasn't 100% airtight, then I'd want to be sure that any water vapour within the insulation zone could be transported out as soon as technically possible.