Air tightness in a block build.

gears

With a lot of talk about air tightness and still a lot of homes bing built using block walls I've been wondering what might be a good way of achieving air tightness over the roof and between the walls and the roof.

What I've been thinking of is probably not a new idea but just wanted to throw it out there and see what problems there might be with it.

Say in a build the walls are up, the roof is on and the next step would to felt (membrane) and batten. If this stage an air tight membrane , Intello or similar was put on the roof and fully wraped over all timber covering the fascia and soffit area and any portions that would extend over the eves then sealed to the top of the block wall and to any flues etc that might extend above the roof.

The next step would then be to install a 50mm batten over and in line with the roof timbers, which would give an air space to remove any moisture which might pass through the membrane and then continue to felt and batten as normal.

I'm sure there are many issues I haven't thought of with this and I'm not proposing to build this way but wanted to put it out there for discussion.

So fire away

li@mo

I think the building regulations require ventilation in the roof space. You can make your cavity wall airtight by using cavity closure blocks at eave level.

However, there are gaps usually in the soffitt or in ridge tiles for air to circulate through the roof space so therefore i don't think you can get away with an airtight roof space.

gears

li@mo wrote:

I think the building regulations require ventilation in the roof space. You can make your cavity wall airtight by using cavity closure blocks at eave level.

If you use cavity closures are you not leaving your self open to massive cold bridging ?

On the air circulation, how do flat roofs accomodate this plus if you have a converted attic with no knee wall as for as I can see any roof ventilation would still only be over the air space in what I propose.

As li@mo pointed out the only area which would cause a problem, in my mind is to make the cavity air tight. I'm thinking you could cut and wrap the membrane over the cavity and seal around reach roof truss, or place membrane over the top of the cavity when putting up the roof

Quack13

Have you looked into using ICF (Insulated Concrete Formwor) at all.

I ask as like you I am hoping to build an airtight concrete build. However someone mentioned ICF to me and I have doing a bit of research on it for the last week or so and as far as I can see the advantages far outweigh the disadvantages and it is much easier to achieve airtightness with this type of build.

Concrete blockwork is the most difficult type of build to achieve airtightness with. It can be done of course
Also isn't there another type of membrane which applied to the outside of the house on block builds for airtightness? I've heard it can be pretty costly. Though I could be wrong here.

Like I said it's something I am currently looking into myself and am seriously leaning towards ICF.

gears

gears wrote: »

I'm sure there are many issues I haven't thought of with this and I'm not proposing to build this way but wanted to put it out there for discussion.

So fire away

Thanks for the ICF suggestion, but as I said this is not for a proposed build but more to see if there might be any problems in using this particular detail. I also think this might be a good solution for renovation projects where the roof has been stripped, so does any one see specific issues with the approach.

williamsrk

gears if you thought that up yourself your a clever bloke because that is the proper way to as they call it wind proof your home as i am an air and wind tight installer and have done many jobs like that glad to see some people can grasp this idea

sas

williamsrk wrote: »

gears if you thought that up yourself your a clever bloke because that is the proper way to as they call it wind proof your home as i am an air and wind tight installer and have done many jobs like that glad to see some people can grasp this idea

So you'd put intello outside the rafters?

gears

sas wrote: »

So you'd put intello outside the rafters?

Yeah came up with this all on me tod...

Basically going from inside the roof out it would be:

Rafters : Intello : 50mm batten : roofing membrane : batten : slate

I think it's a resonably good solution but the only area that I'm concerned about is around the fascia and soffit if there might be problems with air circulation. I don't think there will be but wanted to put it out there for some opinions.

sas

gears wrote: »

Yeah came up with this all on me tod...

Basically going from inside the roof out it would be:

Rafters : Intello : 50mm batten : roofing membrane : batten : slate

I think it's a resonably good solution but the only area that I'm concerned about is around the fascia and soffit if there might be problems with air circulation. I don't think there will be but wanted to put it out there for some opinions.

I'd be concerned about you using intello where you are using it. I'd simply use Solitex Plus and tape the joints in it. This too will be wind tight but it will be considerably more breathable so you don't risk moisture damage in your roof.

So going from inside to outside it would be:

Rafters: Solitex : Batten parallel to rafters: Counter Batten: Slate

Double check this with the suppliers but I'm 95% sure that you shouldn't use intello where you suggested.

You are talking about a warm roof?

Disclaimer: I'm not in anyway involved in construction but I've spent some time researching for my own build and I've never seen the double membrane approach you suggested.

gears

I'm not a builder, architect or whatever either but I'm thinking of doing some renovations and looking into doing as good a job as I can including looking at air tightness.

What I'm looking at is being air tight more than wind tight which I see as two very different things.

In doing a little more thinking about this one potential problem I see is simply having a brethable membrane on the cold side of insulation if you had to insulate between the rafters.

Also in doing what I've proposed and creating a warm air tight roof, presumably there would be a need for a HRV in the home, could you not include the attic area in the HRV circuit and benefit from any heat in this area and remove any moisture from this area ????

sas

I've never seen the airtightness layer put on the outside. Hence my assumption that you were trying to make the roof wind tight.

You described a situation where the walls are already up and the roof structure is already in place.

Would you be fixing the membrane to the outer leaf of block work?

gsxr1

I was under the impression that an open cavity near the soffit was an essential part of the house ventilation system . As was a well vented sofit and rafters.

By sealing everything up, would that not cause big condensation problems? Even if using breathable membrane.

gears

sas wrote: »

I've never seen the airtightness layer put on the outside. Hence my assumption that you were trying to make the roof wind tight.

You described a situation where the walls are already up and the roof structure is already in place.

Would you be fixing the membrane to the outer leaf of block work?

Yes my idea would be to fix the membrane to the blockwork.

I'm just trying to come up with an easy way to make a block build air tight as by using the conventional approach where you would put the membrane on the inside, at some point you will need to put a cable or pipe of some kind through the membrane which needs to be sealed.

gears

I put together some rough (stress rough) drawings of my idea (mad notion) to try and get across what I'm talking about

sinnerboy

The air tight membrane is intended to keep you heated warm air inside the building - so it must be located closest to the interior

look here
http://www.ecologicalbuildingsystems.com/products/intello/IABCertIntello.pdf

and here
http://www.environ.ie/en/Publications/DevelopmentandHousing/BuildingStandards/FileDownLoad,18751,en.pdf

And hire an architect

sydthebeat

gears wrote: »

Rafters : Intello : 50mm batten : roofing membrane : batten : slate

intello in this location could cause it to be permanently vapour closed.... bad idea... this means it is not breathable and will cause your attic to sweat... causing moisture damage....
this is the very same as using a non-breathable felt...

using the intello on the warm side activates the diffusion properties of the intello ie in summer when external humidity and humidity stress (pressure) is equitable to the internal attic, the membrane is diffusion open... in winter when the external humidity of less that of the attic, the membrane is closed.... basically you need to keep the intello warm for it to work properly, as far as ive been led to believe. i will clarify this after lunch...

gears

sydthebeat wrote: »

intello in this location would cause it to be permanently vapour closed.... bad idea... this means it is not breathable and will cause your attic to sweat... causing moisture damage....

surely fixing the air tight membrane to the underside, or under the ceiling joists, is a much simpler option...

The idea is to close off the roof and keep the whole area as air tight as possible. I do see the problems of condensation build up as you would be relying on the breathablility of the membrane to move any moisture from the area. But could the inclusion of HRV extract and ventilation points in the roof eliminate this risk.

Also I'm confused why you say Intello is not breathable in this location, I thought Intello was a breathable membrane. Maybe I'm just getting terms confused as I thought a breathable membrane would allow moisture but not air through, could you correct me if I'm way off here.

sydthebeat

gears wrote: »

The idea is to close off the roof and keep the whole area as air tight as possible. I do see the problems of condensation build up as you would be relying on the breathablility of the membrane to move any moisture from the area. But could the inclusion of HRV extract and ventilation points in the roof eliminate this risk.

Also I'm confused why you say Intello is not breathable in this location, I thought Intello was a breathable membrane. Maybe I'm just getting terms confused as I thought a breathable membrane would allow moisture but not air through, could you correct me if I'm way off here.

intello is both breathable and non-breathable..... depending on the humidity levels.... thats the 'intelligent' aspect of the product.

i realise you are trying to close off the construction, but i think your choice of a continuous air tightness layer is questionable. You should not allow your cavity to be placed within the air tight envelop, as you would be doing in your sketches..... this cavity will be breached all over the place, at opes, at service penetrations and even at path level.

Its better to have your continuous air tight layer towards teh internal where you have more control over it... then you can adequately seal openings and service penetrations etc.

Sparky78

Why not just attach the intello on the inside of the rafter and then attach it to the plaster on the inside of the block work using the orcon sealant.
The plaster on the blocks will create the airtightness on the blocks and the intello on the rafters.

gears

Sparky78 wrote: »

Why not just attach the intello on the inside of the rafter and then attach it to the plaster on the inside of the block work using the orcon sealant.
The plaster on the blocks will create the airtightness on the blocks and the intello on the rafters.

That would be the natural way to go ok, but just got thinking, dangerous thing I know, as to what might be a easier approach instead of trying to deal with joining to internal walls (block or timber) then trying to get all the details around cables, pipes etc.
I'll probably give in eventually that this is not the best idea I've ever come up with But I still think there is some merrit in trying to get a block build air tight by wrapping membrane from the top of the block either interior or exterior leaf over the roof and back again even if it on the interior of the rafters not the exterior as I was proposing.

sydthebeat

gears wrote: »

But I still think there is some merrit in trying to get a block build air tight by wrapping membrane from the top of the block either interior or exterior leaf over the roof and back again even if it on the interior of the rafters not the exterior as I was proposing.

.... is the recommended method anyway......

gears

sydthebeat wrote: »

.... is the recommended method anyway......

But then I'm not much of a rule follower

"There's no such thing as perfection, we only posess the ability to strive for it...."

As the mods have pointed out what I've suggested is not the recommended way to detail a house for air-tightness so I hope no one runs with this suggestion o ntheir own home.

Quazzie

Sorry for dragging up an old thread but I was just having a look through and I was wondering if anyone could post up a rough guide to what price the proclima intello membrane is per square metre.

Nothing has to be a definite figure but I think a rough price might be of benefit

sinnerboy

€12 / m2 installed

soldsold

Sinnerboy do you know current cost for supply only?

Thanks.