What size cavity for new house

cavanmann

I am building new house this year and want to make it as efficient as possible. I want to know should i go for a traditional 100mm, 150mm or 200mm cavity.

its a 2700sqft house with proposed geothermal heating system.

Can anyone advise? Thanks

fclauson

You have to ask a different question - what heat/energy demand do I want to end up with which will lead you to the build spec.

I am using both BER and PHPP to model my house - the wall make up has a much more limited impact (if you are in the 0.1 to 0.2 U-value range) than say the roof insulation, the size & orientation of windows (their U-value and their g-value) etc etc

Francis

BryanF

Cavanmann, can I ask, what is your consultant recommending?

to answer the original question, try to design and construct the lowest U-value construction you can.IF you must go cavity block, in my opinion go for the 200mm, it can be done with some ingenuity, it'll be the standard in the next few years

With geothermal you need to maximise the insulation in the envelope and eliminate thermal bridges so that the system will run efficiently and keep the running costs low.

fclauson wrote: »

You have to ask a different question - what heat/energy demand do I want to end up with which will lead you to the build spec.

I am using both BER and PHPP to model my house - the wall make up has a much more limited impact (if you are in the 0.1 to 0.2 U-value range) than say the roof insulation, the size & orientation of windows (their U-value and their g-value) etc etc

you have to ask your self lots of questionsbut limited impact? : PHPP is a design tool in which with a minimum wall build up of 0.15w/m2k is set. YES all the other elements are important, but that does not mean you put in a poor wall U-value. the idea of the PHPP software is energy consumption is designed below 15kwh/m2/y and at these levels the wall insulation does matter.

fclauson

Bryan

A couple of questions

1. On you comment “…IF you must go cavity block..” – what would you choose given a blank canvas ?

2. And on the 200mm cavity – how would you close these and support the windows across them “in a zero cold bridge way”?

On the other topic
I was not really saying the walls did not matter - but some people focus 80% on them when the other elements also have a significant implication

In my build going from a
0.16 wall to a 0.11 wall reduces the demand from 22.5 to 20.5
so that’s significant – but not if you compare it to the windows going from
1.6 to 0.8 (sticking with the 0.16 walls) which reduces the demand from 41.6 to 22.5


yes is a purest passive house 15Kwh/M2/Annum is the holy grail – but if you get down to say 19Kwh/M2/Annum at a price which fits one budget then that’s a lot better than the current regulations which would put my build at staggering 94KwH/M2/Annum

Francis

BryanF

1. timber frame, ics panels, maybe hemplime (if I could get experienced contractor)

2. I have specified glav straps before, that just go to external leaf to get anchorage/ support and are then are covered and sealed by window, a PIR board is generally used around the window internally.

So NO, I cannot suggest 'a zero cold bridge way', but its a low Psi value in comparison to accredited details and not to bad imputed in PHPP. if you have cavity construction you have a cold bridge where every the wall tie's are located anyway..

A well-known cork (think province) window manufacture has currently got a detail for a 250mm cavity on the market with metal strap proposed,

also see the Demby dale blog/diaries (start with part 8) they used a plywood form. but i dont like the chances of this staying dry..

there is a new "0.7w/m/k" wall tie just launched in the UK also which looks promising: "a composite wall tie comprising pultruded basalt fibres set into an epoxy resin" (havent picked throu the BBA yet)

Francis,
Id be keen to the see the cost comparison/ rough cost % of your 'close to passive kwh budget option' v achieving the PH standard v building reg house.

I take your point re windows reducing kwhs greatly over those of wall. but I'm still adamant its worth while going as low as possible with the walls, what about a 300mm cavity:)

Carlow52

BryanF wrote: »

Cavanmann, can I ask, what is your consultant recommending?

to answer the original question, try to design and construct the lowest U-value construction you can.IF you must go cavity block, in my opinion go for the 200mm, it can be done with some ingenuity, it'll be the standard in the next few years

With geothermal you need to maximise the insulation in the envelope and eliminate thermal bridges so that the system will run efficiently and keep the running costs low.



you have to ask your self lots of questionsbut limited impact? : PHPP is a design tool in which with a minimum wall build up of 0.15w/m2k is set. YES all the other elements are important, but that does not mean you put in a poor wall U-value. the idea of the PHPP software is energy consumption is designed below 15kwh/m2/y and at these levels the wall insulation does matter.

This post refers to a 200 mm cavity block.
Is this correct or should it be cavity wall?

If cavity wall what thickness are the 2 leaves?

Are there structural design issues for cavity walls of these dimensions, especially if using hollowcore?

just do it

Carlow52 wrote: »

This post refers to a 200 mm cavity block.
Is this correct or should it be cavity wall?

It is a 200mm cavity between two block leaves.

Carlow52 wrote: »

If cavity wall what thickness are the 2 leaves?

100mm each

Carlow52 wrote: »

Are there structural design issues for cavity walls of these dimensions, especially if using hollowcore?

Yes, extra wall ties at closer proxity to each other. You'll need a structural engineer to look at these types of issues.

JB81

I have went with a 100mm outer leaf, 200mm cavity ( with larger wall ties obviously, but more off them than usual ) and a 150mm inner leaf. I got a structural engineer to look at it and this is what they came up with.

JB

about2build

JB81 whats the rest of your insulation like and what overall u value to hope to achieve with same?

good logs...

JB81 wrote: »

I have went with a 100mm outer leaf, 200mm cavity ( with larger wall ties obviously, but more off them than usual ) and a 150mm inner leaf. I got a structural engineer to look at it and this is what they came up with.

JB

you had a block on edge, 200m cavity, inner leaf 150mm???? why have 200mm cavity?? what was your inner block or wood??

gear_ie

Here is what my Arch is recommending....

100mm BLOCK WORK EXTERNAL LEAF,
100mm 'XTRATHERM CavityTherm CT/PIR INSULATION,
215mm BLOCK INTERNAL LEAF WITH 6mm GYPSUM-BASED PARGE COAT (AIR BARRIER LAYER) AND 12.5mm GYPSUM PLASTER FINISH

I've been trying to work out the U-Values for this wall, but keep getting different figures...

just do it

gear_ie wrote: »

Here is what my Arch is recommending....

100mm BLOCK WORK EXTERNAL LEAF,
100mm 'XTRATHERM CavityTherm CT/PIR INSULATION,
215mm BLOCK INTERNAL LEAF WITH 6mm GYPSUM-BASED PARGE COAT (AIR BARRIER LAYER) AND 12.5mm GYPSUM PLASTER FINISH

I've been trying to work out the U-Values for this wall, but keep getting different figures...

Why not go for a wider cavity, have all the insulation in it, and the inner leaf will act as a thermal store. Better the have all the insulation together, and better for hanging things off etc. I don't think it's that onerous to widen the cavity to 150mm, but others on here will know more about that.

fclauson

what type of block is the 215 ?

all numbers not adjusted for wall ties

if it was 100 regular /100 xtratherm/ 215 regular it would be about = 0.187
if it was 100 regular /100 xtratherm/ 215 quinn it would be = 0.156
if it was 100 regular/215 pumped bead/100 regular it would be = 0.145

My preference would be the last - better U value & easier to ensure good detailing.
And if you are going to go 215 - then go for 250
if it was 100 regular/250 pumped bead/100 regular it would be = 0.115

Also I would suggest you speak to beyondpassive about getting the detailing done for the couple of rows of quinn you will need at the base of the wall to stop cold bridging

Troy McClure

We are building with 200mm cavity, 100 mm block inner and outer. There are some structural issues and we had to use steel in places as we have floor slabs. Engineer had to be happy and come up with how to do it. We are using full fill cavity bonded bead insulation. We also used the teplo ties to stop cold bridging large number of traditional ties would have. They are not cheap, but once you get it through the head of a block layer, who thick he knows better than anyone else, how they work and more importantly how to treat me, I think they are worth it if your going this route.

just do it

Troy McClure wrote: »

We also used the teplo ties to stop cold bridging large number of traditional ties would have. They are not cheap,

How much dearer are they? How much per m2 of your build?

Troy McClure

the quantity will be the same. The cost per tie is the issue. You can try longs in Dublin or Belfast. Their belfast branch is cheaper. Make sure you have the correct tie for your particuliar cavity width, as they make different ones. Its one the BBA cert.

BarneyMc

fclauson wrote: »

if it was 100 regular/250 pumped bead/100 regular it would be = 0.115

This seems like a very impressive U value. Any idea what it would be adjusted to using 1. standard and 2. non standard basalt wall ties?

sas

BarneyMc wrote: »

This seems like a very impressive U value. Any idea what it would be adjusted to using 1. standard and 2. non standard basalt wall ties?

Based on my math (ignoring wall ties) the thermal conductivity of the insulating material would have to be 0.029 to get that u-value.
Assuming tc of 2.3 for dense block.

I am not aware of any pumped insulation that goes this low.
The NSAI cert for one of the systems quotes white at 0.04 and grey at 0.033.

0.033 would give a u-value 0.131.

Still very good. We've a tendency to get too hung up on u-values imho.

Where are we differing fclauson?

kboc

sas wrote: »

Based on my math (ignoring wall ties) the thermal conductivity of the insulating material would have to be 0.029 to get that u-value.
Assuming tc of 2.3 for dense block.

I am not aware of any pumped insulation that goes this low.
The NSAI cert for one of the systems quotes white at 0.04 and grey at 0.033.

0.033 would give a u-value 0.131.

Still very good. We've a tendency to get too hung up on u-values imho.

Where are we differing fclauson?

Exactly. Don't over emphasise on u values to the detriment of equally important aspects such as air tightness, and cold bridging to a lesser extent. Quality trades men will be your biggest problem.
Good luck

BarneyMc

Yes good points guys about the other aspects but I just want to get some comparisons of U value first. Certainly air tightness and cold bridging are important also.

fclauson

sas wrote: »

Based on my math (ignoring wall ties) the thermal conductivity of the insulating material would have to be 0.029 to get that u-value.
Assuming tc of 2.3 for dense block.

I am not aware of any pumped insulation that goes this low.
The NSAI cert for one of the systems quotes white at 0.04 and grey at 0.033.

0.033 would give a u-value 0.131.

Still very good. We've a tendency to get too hung up on u-values imho.

Where are we differing fclauson?

My mistake - :mad: - too many spreadsheets - too many numbers - you are correct

But as others have mentioend U-values are only part of the game - so many other factors can affect performance of a build

But as per one of my other posts - stick to the mantra

• Passive
• Pragmatic
• Price sensative
• Practical
• Plausable

and you will not go to far wrong