Wall insulation 100mm cavity Best form

Jollyman

Guys what is the best u value insulation i can get for a 100mm cavity wall, my blocklayer is trying to avoid the kingspan as the sheets are too small!!!!!:)

sydthebeat

Jollyman wrote: »

Guys what is the best u value insulation i can get for a 100mm cavity wall, my blocklayer is trying to avoid the kingspan as the sheets are too small!!!!!:)

Depends on what block you use.

Typically 60mm rigid PU bord will give you 0.27..... theoretically anyway, in reality this value would be a lot worse for reasons i wont go into now. i wouldnt go near partial fill cavity construction anyway...... read this:
http://www.century.ie/cent/pdfs/PartialFillCavity.pdf

this artical was written for Construct ireland, so dont mind the url.

Some pump in EPS crowds quote a u value of 0.2 for 100mm cavities.

If you were to use an aerated concrete inner block (ytong 150mm or similar) or a calcium silicate autoclaved blocks (quinnlite etc) with pumped in insulation you may achieve values as low as 0.15 -0.17
generally the lower the u value the greater the initial costs.

ardara1

sydthebeat wrote: »

Depends on what block you use.

Typically 60mm rigid PU bord will give you 0.27..... theoretically anyway, in reality this value would be a lot worse for reasons i wont go into now. i wouldnt go near partial fill cavity construction anyway...... read this:
http://www.century.ie/cent/pdfs/PartialFillCavity.pdf

this artical was written for Construct ireland, so dont mind the url.

Some pump in EPS crowds quote a u value of 0.2 for 100mm cavities.

If you were to use an aerated concrete inner block (ytong 150mm or similar) or a calcium silicate autoclaved blocks (quinnlite etc) with pumped in insulation you may achieve values as low as 0.15 -0.17
generally the lower the u value the greater the initial costs.

This article was sponsored by and presented by a timber frame company.
They had a vested interest in the result.
The report upon which the article was based has been subsequently updated to look at onsite practice faults for all wall types.

No pumped cavity installer can meet the 0.27 regulations in a 100mm cavity - NEVER MIND 0.2 ?????

And as for using AC blocks at 150mm (Lambda 0.18) - pumped with the best grey bead available - you'll achieve 0.26

Syd - did you mention you were a BER assessor - where did you get your figures from?

muffler

In fairness i think the OP is referring to concrete block as opposed to tf.

sydthebeat

i got the 0.2 value from the manufacturers own declared values.

reading the website again, i see its a 150mm cavity they are quoting for.
http://www.mchughinsulation.com/new_build_concrete.html
if they claim it then its up to them to achieve it.

I have used the SEI u value calculator to show 150 quinnlite inner block wil achieve 0.23.
It isnt perfect but at least its not biased.

I know you champion the xtratherm calculator, but i find it very inflexible if trying to edit the standard inputs..... i cannot edit TC values that are less than 0.05... whats that about???

As far as im concerned partial fill cavity is a dead duck. There are way too many bad construction practices inherent in the industry to make this type of construction viable in todays climate. I have drawn this opinion from 9 years of experience in the residential construction industry. Ive seen construction practices that range from the 'correct' to downright mad. The concrete and PU lobby groups have enough muscle in the irish building industry to ensure its survival, so its up to architect / engineers to point out its faults (and advantages obviously) to clients and let them make their own mind up. Believe me i have no vested interest in any construction method, i simply allow my opinions to be informed by experience and knowledge.

sydthebeat

http://www.josephlittlearchitects.com/documents/cavity_wall_paper.pdf


I dont see any mention th =e paper of being sponsored by or presented to / by any TF group. Why do you think it is so?
The facts purported in the paper make a lot of sense to me.

ardara1

[HTML]

sydthebeat wrote: »

i got the 0.2 value from the manufacturers own declared values.[/HTML]
Then their claims are wrong - do the calculation

[HTML]reading the website again, i see its a 150mm cavity they are quoting for.
http://www.mchughinsulation.com/new_build_concrete.html
if they claim it then its up to them to achieve it.[/HTML]

150mm grey pumped will achieve 0.20 - if no wall ties are accounted for in the calculation


[HTML]I have used the SEI u value calculator to show 150 quinnlite inner block wil achieve 0.23.
It isnt perfect but at least its not biased.[/HTML]

Yea -if you don't include the thermal bridging through the mortar - which you have to.

[HTML]I know you champion the xtratherm calculator, but i find it very inflexible if trying to edit the standard inputs..... i cannot edit TC values that are less than 0.05... whats that about???[/HTML]

Call Xtratherm - but any calculation software (approved) will/should come out with the same answer if the input is correct.

[HTML]As far as im concerned partial fill cavity is a dead duck. There are way too many bad construction practices inherent in the industry to make this type of construction viable in todays climate. I have drawn this opinion from 9 years of experience in the residential construction industry[/HTML].

I'm completely in agreement with you - the biggest factor for failure on site is the human one - the new Part L specifically targets site practice in this amendment - detailing and commissioning - must have on site inspection and quality control regime in place.

[HTML]Ive seen construction practices that range from the 'correct' to downright mad. The concrete and PU lobby groups have enough muscle in the irish building industry to ensure its survival, so its up to architect / engineers to point out its faults (and advantages obviously) to clients and let them make their own mind up. Believe me i have no vested interest in any construction method, i simply allow my opinions to be informed by experience and knowledge.

[/HTML]

All sectors of the construction industry have had their methods of getting their voices (often mis guided) across to Joe public - what I maintain incessantly is that ALL method of construction and perform as good as any other - any differences in the resultant energy performance of the end building is miniscule - I just wish that when measurement are quoted they are done accurately - whether on manufacturers web sites or magazine or boards.

Jollyman

Hi guys,

Was away from the desk, glad to see ye argued out the case but as mellor stated its for a traditional cavity block wall 100 100 100. I know it might not be the best construction method but thats the one im going for! After that im just wondering should i go full fill blow in, outsulation or partial fill and a insulated dryline slab on inner walls! What is the best someway economic method of achieving a decent u value!!!?

sydthebeat

Jollyman wrote: »

What is the best someway economic method of achieving a decent u value!!!?

build 100mm outer, 50 cavity, 100mm inner insulation.
batten out, incorporate 100mm PU and slab over with 47.5 composite board... totalling 135mm PU insulation.

advantages
good u value
good air tightness
good insulation detailing at windows
high responsiveness from heating system
easier to create continuous insulating envelope

sinnerboy

I would go for this with 1 refinement . don't use CB internally . use PU board , foil faced both side and foil tape the joints . better vapor check . to improve vapour resistance add layer of polythene now . then plasterboard and skim over .

( sinks in eventually A1 ) .;)

bauderline

Folks,

Let me be clear about this... with respect to the previous two posts...

100mm outer leaf
50mm empty cavity
100mm inner leaf
battons
100mm PU insulation between battons
PU board over battons an insulation
polythene
plasterboard and skim ?

... how is the platerboard attached to the wall if you already have PU board attached to the battons ...

sorry for the dumb question !

baud.

sinnerboy

long fixings , through PU . no such thing as dumb q's

AckwelFoley

Next house i build is getting a 100mm Poly iso in an increased size cavity.

Jimbo

Some questions with regard increasing cavity width -

Is it structurally ok to increase you cavity to +100mm in standard 1 1/2 or 2 storey house?
If so, how wide can you go?
Are there special cavity ties available?
Are there new +60mm insulation boards avaiable?
Which one is best?
Are there any other concerns with increasing the cavity?

OP, Sorry for hijacking thread btw

Sparky78

Another dumb question:)

Why would you need battons that are causeing a cold bridge?

Could you not just fix the insulation directly to the inner block leaf?

ardara1

jimbo78 wrote: »

Some questions with regard increasing cavity width -

[HTML]Is it structurally ok to increase you cavity to +100mm in standard 1 1/2 or 2 storey house?[/HTML]
Yes
[HTML]If so, how wide can you go? [/HTML]
Part L allows for 150mm - after that engineers structural calculation need to be produced to prove.

[HTML]Are there special cavity ties available?[/HTML]
You will obviously need long - but more than likely more wall ties and possibly thicker - more metal mean greater thermal bridging (Metal's cold) eg, changinfg from 5 wire to 5 strap ties worsens U-value by 0.02W/m2K
[HTML]Are there new +60mm insulation boards avaiable?[/HTML]
Must leave a 40mm residual cavity so 110 available for 150mm cavity - but jointing is an important issue.

[HTML]Which one is best?[/HTML]
All have similar performance 0.023 lambda
[HTML]Are there any other concerns with increasing the cavity?[/HTML]
See above



OP, Sorry for hijacking thread btw

Another thing I'd point out is that be getting better and better U-values in a wall & floor actually increases the thermal bridging threat at the wall/floor junctions - reduce the cavity and dryline internally - connect with floor and ceiling insulation.
Avoid breaking the insulation layer with battens - either put directly on wall or ON to battens - not between.

sinnerboy

for mechanical strength of fixing . quite difficult to fix +100mm of ( realtively ) soft material like PU to a wall . that why layering insulation between and accross the timber is important to prevent cold bridging

sydthebeat

ardara1, what do you make of these calculations using both teh SEi calculator and the U valuate one for the proposed construction as outlined above

http://i48.photobucket.com/albums/f242/sydthebeat/135mminner.jpg

any comments??
i know im doing something incorrectly in the SEI one with regard to resistances.... i cant figure out how to change it though.
Do you think such a construction is viable??

ardara1

sydthebeat wrote: »

ardara1, what do you make of these calculations using both teh SEi calculator and the U valuate one for the proposed construction as outlined above

http://i48.photobucket.com/albums/f242/sydthebeat/135mminner.jpg

any comments??
i know im doing something incorrectly in the SEI one with regard to resistances.... i cant figure out how to change it though.
Do you think such a construction is viable??

Hia Syd - the internal; and external resistances are wrong for a wall in the SEI one - are they added twice also? I know that the resistances for walls and roofs are different and you've to change the external to match the internal when a ventilated layer is incorporated eg. a roof slope - apart from that there's not much difference.

Jollyman

Ok i need to order insulation today, i am going to put one form or another into the cavity and dryline the inside, what is the signifacnt difference between a 60mm aeroboard and the 60mm kingspan, which is the better option in terms of u value and is there a hugely significant difference?

Mellor

There is a pretty large difference.
From memory, the conductivities of each are 0.022 and 0.031 respectivity. So the u-value for the insulation is directly proportional to this. Aerobord will have a u-value that is 40%+ higher than kingspan. (lower the value is better, just like golf)

sydthebeat

Jollyman wrote: »

Ok i need to order insulation today, i am going to put one form or another into the cavity and dryline the inside, what is the signifacnt difference between a 60mm aeroboard and the 60mm kingspan, which is the better option in terms of u value and is there a hugely significant difference?

yes, there is a significant difference... which will be reflected in the cost!!!

The Thermal conductivity of Kingspan / Xtratherm PolyUrethane board is 0.023 typically... where as Expanded polystyrene (EPS) is typically 0.035... extruded polystyrene (XEPS) has a TC value of 0.033

Jollyman

One of these two products so is the best available given the method im choosing to insulate the house?

http://www.insulation.kingspan.com/ireland/partial_fill.htm

What about the option below has that any/many advantages on the partial fill cavity? If also dry lining the inside of the build?

http://www.insulation.kingspan.com/ireland/external.htm

sesswhat

Another option might be Knauf Dritherm 100mm Full Fill slabs for the cavity. More info here.

Jollyman

Have just ordered the 60mm Kingspan TW50, at €8.22/M2.

sas

Jollyman wrote: »

Have just ordered the 60mm Kingspan TW50, at €8.22/M2.

Dear god, why would anyone then ever opt for the K8 variant given the minimal conductivity difference? E.g. priced the 80mm locally and it was 24.75/m2 inc VAT.

Thats quite a difference!

Dano954

I spoke to two builders at the weekend, the first guy goes with Aerobord Platinum in the cavity and drylines the inside, the second reckons the drylining prevents the internal wall from heating up thus preventing longer lasting heat release back into the room and recommends just High Density Kingspan in the cavity and hardwall the inner wall.How do these methods compare?
Great thread BTW.

Mellor

THe second is right, in a way. Drylining does reduce thermal mass, which helps act as a heat store. But in a room, the thermal mass is provided my all four walls and the floor, so at worst drylining eliminates two of these, is about 30% of the thermal mass for a typical room. Even less if it is just one wall. another thing to consider is that the external walls are the worst preforming thermal mass elements, this is due to the fact that most of the heat will escape to the cold side, heat travels to the least resisance, from hot to cold.

coldheat

I am currently planning my new build and trying to decide on two methods of building the walls and need some advice on which would be the best. Both seem to give about the same U value of ~0.17.

Method 1.
Plaster
100mm block
150 cavity filled with 90mm silver beads + 60mm PU.
100mm block
Plaster

Method 2
Plaster
100mm block
110mm cavity with 80mm PU
100mm block
Battens with 37.5PU+12.5 plaster board
Plaster

I have been told that method 2 can have condensation build up and mold growth between the inner block and PU. Also because I am putting in a HP with underfloor heating method 1 would be better suited due to thermal mass of the walls. The first floor internal walls will be stud/rockwool/plasterboard.

What do you think ?

bauderline

Folks,

Just want to do another iteration on the wall insulation. As outlined previously be various parties.....

100mm outer leaf
50mm empty cavity
100mm inner leaf
battons
100mm PU insulation between battons
PU board over battons an insulation
polythene
plasterboard and skim ?


The 100mm PU insulation is placed OVER the batton and not between them as stated above.

The battons should be 50mm x 25mm which are all laid flat. Thus giving a gap of 25mm between the PU and then concrete wall in all places ?

The two layers of PU should be foil taped at the seams, what about floor and roof ?

b.

sydthebeat

bauderline wrote: »

Folks,

Just want to do another iteration on the wall insulation. As outlined previously be various parties.....

100mm outer leaf
50mm empty cavity
100mm inner leaf
battons
100mm PU insulation between battons
PU board over battons an insulation
polythene
plasterboard and skim ?


The 100mm PU insulation is placed OVER the batton and not between them as stated above.

The battons should be 50mm x 25mm which are all laid flat. Thus giving a gap of 25mm between the PU and then concrete wall in all places ?

The two layers of PU should be foil taped at the seams, what about floor and roof ?

b.

No, this would cause condensation to form on the cold face of the inner block. As this 25mm space isnt ventilated, it will lead to a buildup in moisture cause mould etc ... harmful to health.

why leave the cavity empty....??
if you incorporate 60mm Pu board in the cavity, whilst retaining 40mm air cavity, and dryline inside, you are closer to a better contruction.

bauderline

sydthebeat wrote: »

As far as im concerned partial fill cavity is a dead duck. There are way too many bad construction practices inherent in the industry to make this type of construction viable in todays climate. I have drawn this opinion from 9 years of experience in the residential construction industry. Ive seen construction practices that range from the 'correct' to downright mad. The concrete and PU lobby groups have enough muscle in the irish building industry to ensure its survival, so its up to architect / engineers to point out its faults (and advantages obviously) to clients and let them make their own mind up. Believe me i have no vested interest in any construction method, i simply allow my opinions to be informed by experience and knowledge.

build 100mm outer, 50 cavity, 100mm inner insulation.
batten out, incorporate 100mm PU and slab over with 47.5 composite board... totalling 135mm PU insulation.

advantages
good u value
good air tightness
good insulation detailing at windows
high responsiveness from heating system
easier to create continuous insulating envelope

syd,

Your last post appears to be in direct contradiction to your previous posts !

Maybe you can clarify what you mean by 50mm cavity above ?

You have me utterly confused at this point !!

b.

ircoha

while syd will no doubt answer here, the point I think he is making about the cavity is that fitting the insulation properly to the inner leaf in the cavity can be difficult to get right. It is impossible to inspect after it is done and the cavity closer is in place.

In looking at this thread, I don't see any specific reference to a vapour control barrier.[ if i missed it sorry]

coldheat wrote: »

I am currently planning my new build and trying to decide on two methods of building the walls and need some advice on which would be the best. Both seem to give about the same U value of ~0.17.

Method 1.
Plaster
100mm block
150 cavity filled with 90mm silver beads + 60mm PU.
100mm block
Plaster

Method 2
Plaster
100mm block
110mm cavity with 80mm PU
100mm block
Battens with 37.5PU+12.5 plaster board
Plaster

I have been told that method 2 can have condensation build up and mold growth between the inner block and PU. Also because I am putting in a HP with underfloor heating method 1 would be better suited due to thermal mass of the walls. The first floor internal walls will be stud/rockwool/plasterboard.

What do you think ?

It may be implied but I think it should be explicit as my experience is that the more explicit the detailing the less room for ambiguity.

sydthebeat

the spec i gave didnt have a 25mm cavity behind the PU board, thats the point i was making.

The spec i gave in that post was sort of 'on the fly'.. without being totally detailed out, as ircoha states. To be honest i wouldnt be too happy about that spec at this stage. What i was trying to do was to show that you can keep all the insulation together to get better performance, rather than splitting it up.

bauderline

So the 100mm PU board was attached directly to the wall between the battons ?

To ask the question directly, if I am using cavity wall construction, what IS my best approach.

I understand about 60mm in the cavity with 40mm gap and then drylining. Straightforward enough, but is this the best approach ? How can it be improved ?

Thanks for your time BTW !

baud.

sinnerboy

bauderline wrote: »

To ask the question directly, if I am using cavity wall construction, what IS my best approach.
baud.

IMO -
http://www.boards.ie/vbulletin/showthread.php?t=2055235906

post no 2

bauderline

So SB,

If I take the meaning of your post you say...

60mm in cavity
batten with 50 x 25
50mm over battens
Then another 50mm insulation and then plasterboard...

This would leave us with the 25mm cavity that syd has taken issue with above ?

I am sure the detail behind this is crystal to syd and sinner and a few others on this forum, however reading through the various posts there appears to be a fair level of ambiguity and contradiction. Sorry for eeking this out to the n'th degree but I want to be crystal clear on the respective methods you guys are putting forward.

I appreciate you efforts syd and sinner very much... the penny will likely drop soon enough... it will likely be heard up there in Donegal !

b.

sinnerboy

the importance of the foil taping of the 2 layers of insulation in the spec I posted , together with using foil backed plasterboard is to set up vapour resistance so that the battens cavity will not experience condensation

take care not to compromise the vapour resistance - elec boxes should be mounted through the inner insulation layer but should not penetrate it

where pipes and cables run from the battens cavity through the insulation , make sure to seal them .

sydthebeat

bauderline wrote: »

So SB,

If I take the meaning of your post you say...

60mm in cavity
batten with 50 x 25
50mm over battens
Then another 50mm insulation and then plasterboard...

This would leave us with the 25mm cavity that syd has taken issue with above ?

I am sure the detail behind this is crystal to syd and sinner and a few others on this forum, however reading through the various posts there appears to be a fair level of ambiguity and contradiction. Sorry for eeking this out to the n'th degree but I want to be crystal clear on the respective methods you guys are putting forward.

I appreciate you efforts syd and sinner very much... the penny will likely drop soon enough... it will likely be heard up there in Donegal !

b.

The difference here is SB is including 60mm in the cavity between the block leaves, therefore the inner leaf is 'warm' and the condensation point is at the cold face of the 60mm insulation.

In your example i took issue with, the 25mm air cavity on the inside of the 'cold' inner leaf would trap condensation moisture. You had no insulation between the block leaves.

bakerbhoy

sydthebeat wrote: »

The difference here is SB is including 60mm in the cavity between the block leaves, therefore the inner leaf is 'warm' and the condensation point is at the cold face of the 60mm insulation.

In your example i took issue with, the 25mm air cavity on the inside of the 'cold' inner leaf would trap condensation moisture. You had no insulation between the block leaves.

Syd
I too will be drylining. I have 60mm xtratherm in cavity.I intend/ed to go 50mm between battens flush with masonary and 50mm over. What would be the preferred option be.
1. Second layer of 50mm pu and 12.5mm foil faced plasterboard.
2. The second layer being a composite board.
I have been informed that a cavity between masonary and pu would give better insulation values ,but i don't particularly want this.
I HATE STUD WALLS. :mad:

bauderline

Sinner,

Is your contention that the internal insulation done in that manner is fine with a 50mm empty cavity as long as vapour control is done correctly ?

Syd,

Your contention is that even with sufficient vapour control in place you still believe the risk of condensation is too great and that 60mm insulation should still be used in the cavity ?

Have I got this right guys ?

b.

sinnerboy

bauderline wrote: »

Sinner,

Is your contention that the internal insulation done in that manner is fine with a 50mm empty cavity as long as vapour control is done correctly ?

b.

didn't think i was in contention with anyone

answer - no . keep inner leaf warm , minimise vapour migration

however - to off topic ( rant warning )

The context of this thread is what is best with 100 cavity

but - ( in no particular order )
Porton
ICF
timber frame
SIPs
even the humble hollow block dry lined

are all superior because they don't rely on blockies working with insulation

I have spent many years seeing cavity walls , mostly partial filled , in the UK and Ireland badly built . mortar snots on ties , dislodged boards , gaps in board joints you could park your car in , missing boards, boards snagging what once were stepped dpcs but which become flat dpcs. The result is a massive under performance , thermally .

My Opinion -
It is not fair to expect blockies who have to work in wind, rain, frost, muck to give a s**t about this . They appear not to anyway , that is my continuous observation .

So abandon the concept folks .

sydthebeat

bauderline wrote: »

Syd,

Your contention is that even with sufficient vapour control in place you still believe the risk of condensation is too great and that 60mm insulation should still be used in the cavity ?


b.

Its not a case of a risk of condensation..... normal common knowledge will tell you that when cold air hits a warm face (face of insulation) condensation will be formed. In your example you left a 25mm air cavity between the insulation and the block, this was to be formed out in timber. As this cavity isnt vented the moisture, thats guaranteed to occur, will build up, causing rot to the timber, compromise to the insulation and all the other health problems of mould and fungal growth. The lcation of your polythene 'vapour check' layer was incorrect. The spec i posted wasnt detailed out fully, and didnt include this 'air cavity' so i wouldnt draw too much into it.

Regarding sinnerboys last post.....
i agree completely with his reservations regarding 'partial cavity fill'... see thread i started....

mad m

Builders that are doing my extension have a 100mm outerleaf,100mm cavity,100mm innerleaf. Whats the minimum the insulation has to be away from the outerleaf? Or does it matter....The insulation to go into the cavity is 50mm kingspan, I'm thinking of getting a bigger size put in.

ardara1

mad m wrote: »

Builders that are doing my extension have a 100mm outerleaf,100mm cavity,100mm innerleaf. Whats the minimum the insulation has to be away from the outerleaf? Or does it matter....The insulation to go into the cavity is 50mm kingspan, I'm thinking of getting a bigger size put in.

40MM REQUIRED

mad m

ardara1 wrote: »

40MM REQUIRED

So 60mm insulation then?

archtech

sinnerboy wrote: »

My Opinion -
It is not fair to expect blockies who have to work in wind, rain, frost, muck to give a s**t about this .

And charge what they want for laying a block. Some blockies will be the downfall eventually of their trade, I know of a number of developers, who changed to building timber frame houses, solely due to blockies and what they were charging and their poor standard of workmanship.


No matter where you put the insulation, its performance is only as good as how well it is detailed and the standard of workmanship in fixing it correctly. Seemingly there was a study done in the UK which found quilt insulation in timber frame houses had sagged to the extent that 90% of the heat was escaping out through the walls.

bauderline

Okay....

I am not giving up yet....

I know that cavity wall construction severely displeases some you and I understand the issues with it, however lets work on the basis that I can ensure that the insulation is installed to a high standard and meticulous attention is given to detail.

I have 310mm width of wall to play with so ....

1. 100mm block outer leaf.
2. 110mm cavity which contains 70mm insulation and a 40mm cavity.
3. 100mm block inner leaf.
4. Install battens on inner leaf. (50x25)
5. Install 50mm of PU between each batten using dot dab method.
6. Install another 50mm of PU over existing 50mm insulation and battens. Should be foil faced on both sides. Using long fixings.
7. Foil tape all joints in an effort to prevent air leakage.
8. Cover with 12.5mm plasterboard and skim.

This will give me 170mm of insulation and should minimise condensations issues should it not ?

And now to ask the questions...

1. Considering I am hell bent on cavity wall construction is this the way to do it ? If not WHAT specifically have I got wrong.

2. Do I really need a layer of polythene in there ? If so where EXACTLY should it go.

Thanks for your time !

Best regards, Baud.

sydthebeat

bauderline wrote: »

Okay....

I am not giving up yet....

I know that cavity wall construction severely displeases some you and I understand the issues with it, however lets work on the basis that I can ensure that the insulation is installed to a high standard and meticulous attention is given to detail.

I have 310mm width of wall to play with so ....

1. 100mm block outer leaf.
2. 110mm cavity which contains 70mm insulation and a 40mm cavity.
3. 100mm block inner leaf.
4. Install battens on inner leaf. (50x25)
5. Install 50mm of PU between each batten using dot dab method.
6. Install another 50mm of PU over existing 50mm insulation and battens. Should be foil faced on both sides. Using long fixings.
7. Foil tape all joints in an effort to prevent air leakage.
8. Cover with 12.5mm plasterboard and skim.

This will give me 170mm of insulation and should minimise condensations issues should it not ?

And now to ask the questions...

1. Considering I am hell bent on cavity wall construction is this the way to do it ? If not WHAT specifically have I got wrong.

2. Do I really need a layer of polythene in there ? If so where EXACTLY should it go.

Thanks for your time !

Best regards, Baud.

Baud... just did a few quick calculations for you

firstly a u value calc of the construction above.....
http://i48.photobucket.com/albums/f242/sydthebeat/UValue.jpg

now i havent included bridging factors for the timber studs but i wouldnt expect it to be significant

secondly a condensation risk analysis...
http://i48.photobucket.com/albums/f242/sydthebeat/CRA.jpg
this shows it passes the CRA.... no interstitial condensation...

thirdly, a glaser diagram shows the actual 'dew' point of condensation point..
http://i48.photobucket.com/albums/f242/sydthebeat/glaser.jpg
this shows the condensation point is in the cavity, where it should be, and not within the insulation make-up. Therefore no polythene required.

bauderline

syd,

Thanks for that, the wall U values look pretty good. Did you use builddesk for that ?

b.

DLBuild

Okay, I'm delighted you guys are addressing some of the concerns and questions I have regarding drylining also.
I'm stuck with a cavity build so I want to gain best results possible in terms of heat retention within the building.
A few points regarding bauderlines methods above which may be obvious to you gusy but not so obvious to me are
4.1 Why the need for battens at point 4. if the insulation is being dabbed to wall couldn't you do without battens altogether.
5.1 Is this foil backed insulation at point 5 between the battens or the non foil backed stuff.
Also, and vitally, how do you work with openings? i.e. would you do the same detail at window openings i.e. top, bottom, left and right and reduce your window size?
Many thanks.