Vapour Barrier + Cavity Walls

YourAverageJoe · 01-10-2008 7:59pm #1

Hi,

Just wondering if someone could explain (in simple terms) the need to use a vapour barrier in a cavity wall. Is this purely for air-tightness purposes.

From reading some other threads I have seen people saying the layer should be placed on the inner side of the inner leaf with insulation then placed over it (if desired). Is this correct?

Also, some people have suggested polyethene (spelling??). Will the polyethene not make the building 'sweat', should the building not be breathable and a breathable membrane used for air tighness purposes instead of the plastic layer.

Heatherview · 01-10-2008 10:28pm

Hi
A vapour Barrier and air tightness are two different consepts in the building. What exactly are you trying to do ? come back give me some details

YourAverageJoe · 01-10-2008 10:48pm

Thanks for the response, and to clear things up I will say what I am trying to figure out.

I am in the process of trying to decide on the make-up of my cavity wall construction. Have considered ICF but would be worried about plastering and also the lack of experienced trades people in this area. So, have downloaded U-value software tool (builddesk) and am working on different combinations to decide on level of insulation. But, i dont really get what the vapour barrier does, i.e. will a sheet of plastic not cause a build up of condensation on the inner side of the plastic, similar to what happens when you wear a cheap pair of water proof trpusers?

As for air tightness, am hoping to use HRV so want to use an airtight membrane on outer walls, i thought the vapur barrier would also serve this purpose.

Thanks

Heatherview · 01-10-2008 11:22pm

Hi what is your construction Block cavity block -- timber frame or other. Do you want to insulate in the cavity between blocks or on the outside of building. ?

YourAverageJoe · 01-10-2008 11:37pm

Well i have just been trying things out in builddesk but am thinking;

Plaster
ext block wall (100 mm)
Cavity (50mm)
Insulation board Kingspan/extratherm etc (100 mm)
int Block wall (100 mm)
Vapour barrier ????? (polyethylene 15 mm picked at random in software)
Insulation board (50mm) dabbed on wall in between timber battons (50x50mm or similar)
Insulated plaster board fixed to battons
Plaster

Am getting a U-Value of 0.17 from the software with this construction.

Heatherview · 02-10-2008 12:32am

Hi Average Joe

See where you are now.
1. Block Construction normally 100mm outside Block - 100mm Cavity - 100mm Inner Block.
2. Insulation between blocks for an A2 Rating = 80mm Kooltherm K8 Cavity Insulation with a 42.5mm Kooltherm insulated board K18 on inner wall will give you a U Value of 0.6kWh/m²/yr.

The detail you sent me is an older way of insulating to do the same thing. No polythene required with K8 plastic plugged to internal wall. No timber required, no extra slab required.

K18 board 2440 x 1200 x 42.5mm ( 12.5mm slab+30mm insulated )

Heatherview

RKQ · 02-10-2008 8:53am

Check out Sigma, Moy and Ecological Building Systems.
Each manufacturer show clear photos on the installation process.
The Pro Clima is a vapour barrier, (not plastic).

The junction of the 1st floor joist and external block work is critical. The membrane should be wrapped around the joist to prevent air infiltration from the cavity.

sydthebeat · 02-10-2008 10:01am

Heatherview wrote: »

Hi Average Joe

See where you are now.
1. Block Construction normally 100mm outside Block - 100mm Cavity - 100mm Inner Block.
2. Insulation between blocks for an A2 Rating = 80mm Kooltherm K8 Cavity Insulation with a 42.5mm Kooltherm insulated board K18 on inner wall will give you a U Value of 0.6kWh/m²/yr.

The detail you sent me is an older way of insulating to do the same thing. No polythene required with K8 plastic plugged to internal wall. No timber required, no extra slab required.

K18 board 2440 x 1200 x 42.5mm ( 12.5mm slab+30mm insulated )

Heatherview

heatherview,
whilst i can appreciate what you are trying to do, its not helpful to give groundless figures without qualification

1. how a BER rating is achieved depends on a lot more than just a wall construction make up, so to say that that construction leads to an A2 rating is incorrect and disingenuous. The Kingspan brochure doesnt exactly scream this issue out!!!
2. your u value calculation is way way out... i can only assume you mean 0.16... as per the kingspan brochure..... a small piece of advice, don't always believe whats in a sales brochure.
3. i feel it would be appropriate to assume that whoever came up with the kingspan figures didnt take into account BR 443 "conventions for u value calculations" which requires that for partial fill cavity walls the following qualifications have to be included in the final u value figures:
mortar joints
air gap corrections
wall ties, frequency and material
cavity resistance for the unfilled part, vented
drylining mechanical fasteners

it may be interesting to note that a buildesk calculation (which includes BR 443) for the following
construction
20mm sand and cement render
100mm medium density block
50mm cavity (slightly vented upward flow)
80mm phenolic foam (ties and air gaps included)
100mm medium density block
30mm phenolic foam (fasteners and air gaps included)
12.5 plasterboard
1mm skim finish

actually gives a u value of 0.25
thats a long way off the quoted 0.16

see the attachment

Komaaas · 03-10-2008 2:03pm

sydthebeat wrote: »

heatherview,
whilst i can appreciate what you are trying to do, its not helpful to give groundless figures without qualification

1. how a BER rating is achieved depends on a lot more than just a wall construction make up, so to say that that construction leads to an A2 rating is incorrect and disingenuous. The Kingspan brochure doesnt exactly scream this issue out!!!
2. your u value calculation is way way out... i can only assume you mean 0.16... as per the kingspan brochure..... a small piece of advice, don't always believe whats in a sales brochure.
3. i feel it would be appropriate to assume that whoever came up with the kingspan figures didnt take into account BR 443 "conventions for u value calculations" which requires that for partial fill cavity walls the following qualifications have to be included in the final u value figures:
mortar joints
air gap corrections
wall ties, frequency and material
cavity resistance for the unfilled part, vented
drylining mechanical fasteners

it may be interesting to note that a buildesk calculation (which includes BR 443) for the following
construction
20mm sand and cement render
100mm medium density block
50mm cavity (slightly vented upward flow)
80mm phenolic foam (ties and air gaps included)
100mm medium density block
30mm phenolic foam (fasteners and air gaps included)
12.5 plasterboard
1mm skim finish

actually gives a u value of 0.25
thats a long way off the quoted 0.16

see the attachment

Please forgive me for getting off the point here as I’m relatively new to this form, In BR443, if the gaps between insulation layers are less than 5mm you take a level 0 correction factor. I can’t imagine you would aim to except more than 5mm gaps.

The conductivity Kingspan declares for there Phenolic foam is 0.021W/mK, also an unventilated airspace with a Low Emissivity Foil face adjacent you can make a correction based under BS 6949, Kingspan declare 0.644m²K/W
On wall ties if the total correction is less than 3% of the calculated value you can ignore the correction factor. If you take standard 2.5 ties wire ties per m2 you can get away without making this adjustment.

I think you will find making these adjustments to your software will take you closer to 0.16W/m2K.

Just to clarify I use Kingspan products for BER ratings and SEI made me do this research to except any of there calculation.

sydthebeat · 03-10-2008 3:04pm

Komaaas wrote: »

Please forgive me for getting off the point here as I’m relatively new to this form,

1. In BR443, if the gaps between insulation layers are less than 5mm you take a level 0 correction factor. I can’t imagine you would aim to except more than 5mm gaps.

The conductivity Kingspan declares for there Phenolic foam is 0.021W/mK, also
2. an unventilated airspace with a Low Emissivity Foil face adjacent you can make a correction based under BS 6949, Kingspan declare 0.644m²K/W

3. On wall ties if the total correction is less than 3% of the calculated value you can ignore the correction factor. If you take standard 2.5 ties wire ties per m2 you can get away without making this adjustment.

I think you will find making these adjustments to your software will take you closer to 0.16W/m2K.

Just to clarify I use Kingspan products for BER ratings and SEI made me do this research to except any of there calculation.

1. in 'practise' more often than not gaps of 5mm or more will and do exist between panels..... mortar snots, warped boards, cut boards, ope jambs etc all lead to this situation happening. It should be best practise for assessors, in absence of site visits, to assume these corrections in calculating u values.
2. any air gap more than 5mm wide will conduct air movement through conduction!!. warmer air at the insulation side will rise and colder air at the external block will fall......its called thermal looping....... thus creating a slightly vented heat upward motion, and corrections should be done ... also these cavites ARE vented as, again, in practise theres never a complete seal at every ope every duct every pipe etc
3. Youll find paragraph 4.9.2 of BR 443 is at odds with this 'less than 3%' allowance in DEAP. Therefore, when the actual wall u value is to be calculated, in accordance with TGD L 2005 and 2007 the calculation HAS TO take BR 443 into account in my opinion. Also if considering the exemption, it can be the difference between stainless steel versus galvanised steel ties. In my experience builders tend to use galvanised. In the absence of the ability of the assessor to specify what type of ties to be used, the correction should be used. The only allowed exemption in BR 443 is if wall ties are plastic. Plus, again in practise, the frequency of wall ties is well in excess of 2.5 per m when opes are taken into account. As cavities are becoming wider, the frequency if wall ties increase. Again, it should be best practise for assessors to make these corrections to give a more accurate rating.

im not sure what you are referring to with BS 6949....??...... is it IS EN ISO 6946?

Komaaas · 06-10-2008 9:39am

sydthebeat wrote: »

1. in 'practise' more often than not gaps of 5mm or more will and do exist between panels..... mortar snots, warped boards, cut boards, ope jambs etc all lead to this situation happening. It should be best practise for assessors, in absence of site visits, to assume these corrections in calculating u values.
2. any air gap more than 5mm wide will conduct air movement through conduction!!. warmer air at the insulation side will rise and colder air at the external block will fall......its called thermal looping....... thus creating a slightly vented heat upward motion, and corrections should be done ... also these cavites ARE vented as, again, in practise theres never a complete seal at every ope every duct every pipe etc
3. Youll find paragraph 4.9.2 of BR 443 is at odds with this 'less than 3%' allowance in DEAP. Therefore, when the actual wall u value is to be calculated, in accordance with TGD L 2005 and 2007 the calculation HAS TO take BR 443 into account in my opinion. Also if considering the exemption, it can be the difference between stainless steel versus galvanised steel ties. In my experience builders tend to use galvanised. In the absence of the ability of the assessor to specify what type of ties to be used, the correction should be used. The only allowed exemption in BR 443 is if wall ties are plastic. Plus, again in practise, the frequency of wall ties is well in excess of 2.5 per m when opes are taken into account. As cavities are becoming wider, the frequency if wall ties increase. Again, it should be best practise for assessors to make these corrections to give a more accurate rating.

im not sure what you are referring to with BS 6949....??...... is it IS EN ISO 6946?

Yes Syd, I meant BS 6946.. My typo.. I still believe that with proper site supervision and good quality workmanship you can get boards tightly fitted to less than a 5mm gap..

Okay maybe make these adjustments after a site visit if there is inaccuracies.. I feel it might be a bit unfair to make these presumptions to every builder in the country..

It is in paragraph 4.6 of BR 443 that I'm taken the 3% figure. Certainly with the galvanised tie the correction factor must be accounted for.. All I am saying is if the engineer does what they are expected to do and make the decision to use stainless ties it will have a significant benefit on the u-value.
Most s/s ties will meet the EN845 standard as a result will perform just as well as the strap tie, maybe a slightly increase per m2 but will still have a smaller cross sectional area.

ardara1 · 06-10-2008 7:18pm

Hi Syd/Komaas - -

OK - if your going to assume the gap - you should include the delta factor in BR443

Do you then use a Y-value of 0.08 for the thermal bridging factor in DEAP?

Komaaas · 07-10-2008 9:05am

ardara1 wrote: »

Hi Syd/Komaas - -

OK - if your going to assume the gap - you should include the delta factor in BR443

Do you then use a Y-value of 0.08 for the thermal bridging factor in DEAP?

Hi Adara1 - I find most engineers are not familiar with the UK accredit details, so unless I have being specifically advised I would take 0.11W/m2K. I have only taken 0.08W/m2K in two assessments so far.. I would expect 0.08W/m2K to be more common when the Irish accredit details are released. When ever that is going to happen!!

ardara1 · 07-10-2008 10:42am

Komaaas wrote: »

Hi Adara1 - I find most engineers are not familiar with the UK accredit details, so unless I have being specifically advised I would take 0.11W/m2K. I have only taken 0.08W/m2K in two assessments so far.. I would expect 0.08W/m2K to be more common when the Irish accredit details are released. When ever that is going to happen!!

Hi Komaaas

Apparently they're ready OK and should have been released Sept! - lead to believe they show what works - AND what doesn't, along with an example calc.

From the July Part L - 0.11 is no longer allowed - if accredited/acceptable details are not used - 0.15 of a Y must be input - you'd never get an A rated house.

heinbloed · 07-10-2008 1:24pm

Since gas filled PU boards are gassing out the manufacturer's stated K-value can not be taken for a calculation reflecting reality. To avoid this gassing-out effect the boards must be sealed airtight. No way to do that in a cavity wall with hundreds of wires puncturing the boards!A cavity wider then 16 mm (here the 50mm gap between the outer block wall and the PU boards) is not a layer of insulation. As soon as the 16mm are reached the air will start to move, on it's own -without any air blowing into ventilation gaps. This is a law of physics and nothing can be done about it.Ask your glazer why simple double glazing spacers are not made wider then 16mm.A cavity wall is build to keep the inner parts of the wall dry. So it MUST be ventilated, the entire outer leaf's U-value plus the cavity's U-value and partly the k-value of the PU board insulation(see above) can not be calculated according to a simple mathematic formula.Stay away from such outdated ideas. A good wall uses it's entire potential i.e it's entire material investment for insulating puposes. Building anything else is a waste of money,energy.

heinbloed · 07-10-2008 1:34pm

About the loss of thermal insulation properties of PU boards see this link as well: http://www.building.co.uk/sustain_story.asp?sectioncode=482&storycode=3118591&c=1Headline " Materials ",cellular plastic

Komaaas · 07-10-2008 4:38pm

heinbloed wrote: »

About the loss of thermal insulation properties of PU boards see this link as well: http://www.building.co.uk/sustain_story.asp?sectioncode=482&storycode=3118591&c=1Headline " Materials ",cellular plastic

Heinbloed - The above page says: "This can be limited by gas impermeable facing and should be allowed for in the thermal ratting"
All PU products on the market must declare the AGED K-value!! If there is gas leakage and it is accounted for than the values given are correct?? A foil layer would be regarded as "impermeable" Okay weak areas at joints and services, but not throughout the entire board..

It seems you are mixing up the difference between a ventilated cavity and air moving in an unventilated cavity.!!

heinbloed · 07-10-2008 6:33pm

You are wrong here, Komaas. As the document already says: The board has to be covered totally.What sense does it make to put a roof on the house when forgetting the walls? The law of Gauss might be new to you, check the www. ALL gases will eventually reache equal concentration.NOTHING is impermeable. "Ageing" means how much gas is left in 30-50-100 years? When I cut open a Kingspan board and hold a lighter to the fresh cut a short but visible blue flame can be seen along the fresh cut. After "ageing" the same batch for a year doing the same experiment there is NO blue flame visible.The gas has sept away.These PU boards have not been tested using real conditions, they'rent long enough on the market to allow for a final opinion. Fact is that the gas will be lost totally-law of Gauss.With or without coverage, it's only a question of time. And an exposure to this time lapse was certainly NOT tested. And the PU foam itself is not holding back the gas, there wouldn't be any covering/sealing be necessary at all otherwise. Logic. And there is NO covering/sealing at the edges.Why not? And there is nothing mentioned of sealing the edges and joints by Kingspan on packets, why not? As I said: what sense does it make to build a roof when there are no walls?! ALL moving air is not to be " insulation". No matter why it moves. Another law of physics. Moving air transports energy.Insulation is suposed to stop that energy transport.By fixing the air. Just the oposite.Learn!Practice is done, theory is talked about.

Komaaas · 08-10-2008 12:30pm

I think it's really fascinating that IAB, BBA, DOE and BRE have not figured this out??

sydthebeat · 08-10-2008 12:44pm

HB, you make it sound like theres no insulation property at all if the gas escapes, why??

what could the extreme result be if all the gas was replace by 'air'???? as trapped air is the main insulative property for most 'insulations'....

about 0.02 w/mk...???

heinbloed · 08-10-2008 8:24pm

You have to check that out with the manufacturers of the gas filled PU boards. Some produce boards with pentane and as well as without pentane, as long as they're of the same density and covered on two sides with the same material they would have the same U-value, after gassing out.These gas filled PU boards were "developed" in a rush to save the cavity wall a few years ago when new U-values for the building regulations were adapted to what Kingspan and Aeroboard could deliver-not to what was best available technology, what made more sense and would be cheaper for the consumer to purchase.We're facing now bills in the billions for these favours to the building industry.The U-values were nodded through I supose since the people on board were as well interested to sell twice as many concrete blocks then necessary for a wall. The Chieftain at this board is now head of the ICF,nice job for a farmer. With all this experience....
The maps of the 13th century did not show America, but this didn't mean that it wasn't there.Most people take manufacturer's data for granted, and this is not what a thinking person should do, relying on vested interests.The official data base of the German gouverment mentions this more then once. For gas filled PU foam they declare that very bluntly, a K-value of 0.04 should be taken for gas filled PU foam " ...at the very best " because of gass loss due to the law of Gauss. See <SNIP> Every window glass manufacturer will openly admit that there is gas loss -even with side sealed spacers- of 50% in a certain time.But PU boards are not sealed at the sides.....One should also consider that a cold surface that is permeable will take up moisture with the air as well, such a PU board will not only take up dry air. Water vapour could acumulate(depending on external circumstances) and form droplets-not a very good insulator.

Mod edit: Links to foreign sites is not acceptable.

bauderline · 09-10-2008 9:40am

To ask the question.... and hopefully get a clear answer....

What are the alternatives to using composite board or PU board when drylining and insulating the interior of the home ?

P.

muffler · 09-10-2008 10:23am

If this thread is anything to go by you will get replies but they not be too clear

Vapour Barrier + Cavity Walls

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