Is cavity wall construction obsolete, and if so what's the alternative?

BryanF

just do it wrote: »

Thanks Bryan.
Yeah aware of sas and what he's up to. Very exposed site facing the atlantic. Cavity wall is the standard build around here, but at the moment they're all being pumped so going from partial fill to full fill.

ok, well think about all the usual stuff so: how your dealing with thermal bridging, air-tightness, glazing install, .15w/m2 wall etc... maybe you should consider extra wide cavity say 250 with say 200mm PIR insulation (leaving 50mm cavity for your exposed location). re mvhr consider sheltered flue intake/ extract locations.

just do it

BryanF wrote: »

maybe you should consider extra wide cavity say 250 with say 200mm PIR insulation (leaving 50mm cavity for your exposed location).

Yes I'll be considering this as cavity wall is what the builders around here are familiar with. I'm very interested in what sinnerboy has posted and if we decide on a block built cavity wall, this is what I will be insisting on. Whilst I'm sure timber frame is perfectly fine, I do prefer the idea of a solid wall.

My architect has completed the passive house design course so will be putting different wall build-ups through the PHPP. It will be interesting to see what comes out of that.

Now financing it is another story;)

sinnerboy

There are a number of issues raised by Heinbleod that I would like to address.
He is banned now and so can't respond and further , posters should be aware of that.

On the issue of PUR boards warping


In post # 51 HB has located some research and drawn the wrong conclusion.

The boards in the experiment ( in 1994 - pre dating the switch form CFC to Penthane as blowing agent ) cut boards which were faced with permeable materials and which were cit into 27mm thick specimens.

They were then subjected to Temp / RH conditions quite unlike we expect to find inside an Irish cavity wall. i.e.

Temp C / RH%
22 / 40
60/10
60/40
60/60
60/75

I reckon the appropriate ranges to be
22/90
-10/90

Take if we consider that the cavity partial fill board sold in Ireland is typically
60mm + thick
faced with non permeable foil
acclimatized to high Irish ambient RH from the start
are restrained by clips

.... we are talking

sinnerboy

On the issue of condensation forming on the hidden face of foil tape over the board joints.

I can't use WUFI. It's on my to do list to learn this but for now be aware that I am bringing only my experience and judgement to bear on the matter.

I don't believe this will happen , or if it does , that it will happen to such a small degree that the inner leaf block can easily absorb and desorb the tiny amounts of condensate. ( back diffusion ).

As I say would need to be able to use WUFI software to demonstrate the following

1. the inner leaf being heated by transmission losses through the wall will not drop below 16* degrees
2. The tiny airspaces between the board joints will have a high thermal resistance - the air is physically inhibited from circulating. The air space being so tiny between the boards acts homogeneously with the boards either side. Temperatures here will not fall below 15* degrees. This is my theory , can't verify with another source.
3. Even if my theory is wrong and some condensation forms on the taped joints I believe the amounts of condensate will be tiny , small enough for the warm inner leaf block to absorb safely.

I would prefer to rely on more than my judgement ( educated guessing ) to fully respond to HB on this point.

Anyone "taking notes" here must bear this in mind .

* as measured using Therm where external temp is 0

when temp is -18 then inner leaf temp at cavity is 14 and (cavity) face of insulation is 9

sinnerboy

Please note that this earlier post has been edited to focus on the thread title.

just do it

beyondpassive wrote: »

Every site and client is different. Here's some one off's between 2008 and 2011 and their walls spec relative to heat demand.

The figures in brackets are PHPP calculated Annual Heat Demands

1. Mayo, Wide cavity 180mm with 2 layers of PI board taped (24kWh/m2.a)
   
2. Meath, Timber frame with services cavity and Aquapanenel rendered rainscreen ( 23kWh/m2.a )
3. Clonmel 200 polybead filled wide cavity.
   
4. Wicklow renovation, mixture of timber frame and external insulation (17.8kWh/m2.a)
   
5. Offally 225mm cavity, stainless steel ties, polybead, softboard and cellulose roof, quinnlite thermal bridge (20kWh/m2.a)
6. Cavan. 225 block, 200 EPS external wall insulation (18kWh/m2.a)
   
7. Cork. 2 houses, wide cavity 225mm bead, quinnlite break. ( 19kWh/m2 and 21kWh/m2.a )
   
8. Longford Hemp lime cast wall, EPS raft. (16kWh/m2.a)
   
9. Galway (only involved in groundworks) 2 houses Quinnlite with EWI on EPS Raft, (13kWh/m2.a)
   
10. Kildare wide cavity 275mm quinnlite break, cellulose in attic ( 18kWh/m2.a )
    
11. Wicklow, 2 houses 275 wide cavity, stainless steel ties, quinnlite broken rising walls. ( 16kWh/m2.a )
    
12. Louth, 1 house Timber frame, next generation frame, timber rainscreen, EPS formwork foundation. ( 14kWh/m2.a )
    
13. Kildare house timber frame on EPS raft, block outer leaf . (18kWh/m2.a )
    
14. Dublin, Next generation Timber frame block outer wall, Quinnlite rising wall thermal break. ( 22kWh/m2.a )
    
15. Kildare house Timber frame, 250 polybead filled cavity ss ties, softboard ( 21kWh/m2.a )
    
16. Kildare care home, 225 block, 225 Rockwool EWI, quinnlite thermally broken dense block rising walls. (13kWh/m2.a)
    
17. North Dublin. Next Generation Timber frame with block outer leaf EPS raft ( 14kWh/m2.a )
    
18. Wicklow/Meath 2 houses, EPS EWI on Timber frame ( 15kWh/m2.a )

So 20 houses, all aspiring towards Passive levels of performance and all completely different wall specifications.

It seems to be the case that each project stands on its own unique set of circumstances.


Moderator note. After consulting with the other moderators this post has been edited to focus on the thread title.

Beyondpassive

Thanks for the detailed reply. I appreciate that all the different build-ups above have been used and at the end of the day economics and the client preference will play a large part.

I also appreciate the more modern methods available such as EWI. But builders in my area (as I imagine in the rest of the country) are used to building cavity walls. Therefore I'm very interested in the solution that sinnerboy has shown. I've seen it proposed before, but this is the first time I've seen photos of it. I appreciate its hard to change hard and fast ways of doing things, but in the current environment, selecting a good builder, and ensuring good communication, achieving this is possible.

Unfortunately all the build-ups above will have different foundations, roofs, designs etc so the PHPP figures aren't giving like for like comparision of the different wall build-ups.

I appreciate due to the great variability between projects it's not necessarily that easy. However individuals do tend to have preferences! And that is what I was asking HB, all things being equal, what would his preferred wall build-up be!