granite faced wall to exceed silver standard?

[Dominic Goetz]

Can anybody offer some advice on how to make the following wall structure as energy efficient as possible?  I am proposing to rebuild a ruined granite barn with a block cavity wall inside.  The wall structure would be, from outside:
300mm granite (porous)
100mm ordinary concrete block
200mm cavity
100mm dense concrete block

The above configuration is the result of lots of different factors such as available skills, the desire for thermal mass inside, minimising the change of materials etc etc.  I could try to explain exactly why the above wall is appropriate but it would result in the longest and most boring posting ever.  I think I've read every posting on this website and on the Green Building Forum but am still not sure how to detail the insulation. (sorry!)

If I go for Kingspan or similar then in theory the U values can be very low but there are problems with joint detailing and also high costs.
Using rockwool cavity batts is much cheaper but U values are higher.

The main problem seems to be whether or not I can get away with not having an air gap, acceptable in the east but not here at Land's End.  Full fill cavity with rockwool would give a U value under 0.2, quite acceptable in theory but not if it doesn't actually work due to moisture going into or out of the structure.  Can the granite layer be used somehow to allow full-fill?

If anyone has got any ideas or experience that might help I'd love to hear about them.
Many thanks,
Dom

[David OLIVIER]

Dominic

Before I try to go further, can you explain why granite is porous?! Apart from basalt or dolerite it's one of the densest and least permeable rocks on earth. Perhaps you mean that the mortar is permeable. But does lime mortar make sense between blocks of granite?

Denmark builds fair-faced cavity walls with 100 mm brick on the outside, and 100+ mm fully-filled cavities, on the North Sea coast. It's done so for 30+ years. I suppose one has to judge which is worst, 100 mm brick and mortar joints exposed to the North Sea or 300 mm granite and mortar joints exposed to the Atlantic. 

HTH

David.

[Dominic Goetz]

Hi David,
I'm afraid I can't remember a huge amount from the geology modules from uni and so I'll take your word on the density of the stuff.

However, dense though it, is a lot of the granite used in old Cornish barns is 'field granite' which is fairly degraded and weathered, thereby allowing some moisture penetration.  I think that internal condensation in old granite buildings is also a huge problem and maybe why this granite is perceived as being more porous than it is.  My stonemason says that there's a fair degree of variation locally, granite from St Just , 4 miles away, is particularly porous, whilst St Buryan granite where I am is not so bad, but you still get some bad 'uns.

My feeling is that 300mm of granite against 100mm block is going to give plenty of protection to a fully filled cavity.  Is driving rain really going to get through all that?  Building control down here seem to think so but I wonder if they are open to persuasion if the argument is right.  Should be talking to them in next couple of days so will be interesting to hear what they say.

As to the use of lime mortar, i'm undecided.  If the granite is just a facing then it should'nt matter too much if i use a weak cement mortar and finish it off with lime pointing.  There's always the struggle between cost, effectiveness and the apparent certainty that I'll go to hell for using cement.
Thanks,
Dom

[David OLIVIER]

Dominic

If you're punished for using cement, what on earth happens if you use steel or clay bricks?! (much more CO2 or energy per kg weight than concrete).

I think the condensation could also be partly due to ground moisture evaporating and having some difficulty escaping. Walls made of large, dense boulders - even if the mortar joints leak - won't be very air-permeable.

It seems to me that the risk is more with the mortar joints than the stone. Even weathered granite can't be really porous as long before that it would have collapsed and part of it would have turned into China clay!

As cavity wall batts are now available with lambda = 0.032 W/mK not 0.038 you might even get quite a good U value from 200 mm.

David.

[Dominic Goetz]

Good news.
Just spoken to building control officer who has said that he has no problem with a fully filled 200mm cavity as long as the insulation has the relevant certificate.
This is great news for me as I can now save a fortune by using cavity batts rather than PU boards.  As you said David, can get Dritherm cavity slab with U value of 0.032 so with 200mm full fill I should get a very high performance wall.

I had previously assumed that full filling was not allowed in high exposure areas, however, according to the Building Control Officer, if the cavity is less than 150mm then need an air gap or you have to 'black-jack' the outer face of the blockwork before adding the facing.  With a 200mm cavity you can do without the 'black-jack' and just full fill the cavity.  So there's extra expense involved in the larger cavity (foundations and wall ties) but also a saving through one less process.
Thanks again for the advice,
Dom

[Nick Grant]

Dominic

I had a word with Peter Warm about this yesterday and I think I'm right in saying he favours full fill as well. Said something  about using extra thickness of insulation and assuming that a certain depth of the insulation will be wet and so not insulating when doing your calcs.

[David OLIVIER]

I can't see how much of the insulation will be wet. Damp, maybe. The rainwater that gets through runs down the outer few mm.

Actually had you thought of just building a 150 mm solid wall, 250-250 mm away from the granite, presumably with a solid first floor (airtightness of the wall helps to keep rain out). Solid walls are much easier than cavity walls.

David.

[Dominic Goetz]

David,
The stone I'll be using is 300mm thick, traditionally when building solid walls with this they are built as two faced outer leaves, this results in a wall thickness of 600mm.
If I want the granite cladding on my barn to be separate from the inner leaf it will have to be stable enough to stand up on its own.  I can't do this just with 300mm of granite but I could if it was block backed, ie tied into a 100mm block.   If the inner leaf is a 150mm block and I want 200-250 mm insulation I will end up with a wall thickness of 750-800mm:

300mm granite
100mm block tied to granite
200-250mm insulation
150mm block (structural)

I think that wall thickness is getting too large.  It could be reduced by using foam insulation but then the expense is increased and there's the problem of good seals between boards.
If I could I glue the boards directly to the outside of the 150mm block and then build the outer granite/block afterwards then the build speed would not be limited by the stonemason.  Ie I could get my roof on before finishing the outer leaf.
From the forum I'm very aware of the importance of airtightness and 'buildabillity', any suggestions?

Any ideas most welcome,
Dom

[Nick Donaldson]

Dominic

I entirely understand the reasoning in arriving at the wall build up described having done a couple like this too (one in Ninnesbridge, Lelant Downs, Hayle - which isn't all that far from you).  Regarding exposure, I think, that Rockwool consider that a coat of render covering a standard blockwork full fill cavity wall provides it with sufficient protection to allow use in 'exposed' locations (where fair faced brick would not), so one assumes that 400mm of masonry on the 'wet' side of the insulation would be more than adequate.  'Black jacking' will only encourage interstitial condensation. 

Most important of all will be the detailing aroung the openings.  Normal convention would see the joinery straddled somewhere across the cavity (half closed?), which would mean your windows are 400mm back in the wall!  To get them near the front of the wall without cold bridging is a right fiddle involving bits of wide dpc, timber, insulation, mesh and plaster.......but you can make the reveals inside splayed.

Cherish those rocks! Go on use lime!

Nick

[Peter Wilkinson]

Hi Dominic

We regularly build in a similar way to what you have discribed. 200-300mm limestone (in the Yorkshire Dales) built up against a 100mm thick out cavity skin. 150-200mm cavity fully filled with glass fibre cavity batt. then 100mm insulation block inner skin. Due the limestone being porous (clints & grikes as I recall from school) wind driven rain gets through the stone like little rivers in severe conditions. But with the block outer leaf it finds a vertical barrier and runs down the back of the stone. The glass fibre must be inserted in the correct way as it allows water to pass through it vertically but not horizontally. But if put in the wrong way round it will go horizontal (obviously).

We line barns and old rubble wall buildings like this and even build new construction like this (recent village hall extension). The block outer leaf helps (speed wise) to put up the stone (saves pinning etc).

If you use an insualtion block in place of outer leaf...even better U value.

As for lime mortar. It's a must for all mortar not just pointing. The cement retains the moisture and after freezing breaks up eventually.

Opening reveal details are problematic but I detail so we set back the internal reveal, so that the insualtion can be returned to the back of the stone. With a wide DPC and the internal block returned to meet the stone jamb but allowing the window/door frames to sit 20mm behind the jamb or outer reveal as a rain check (needs a wider section of frame).

All this works really well and gives a good U value and keeps the severe weather out.

Peter Wilkinson

[David OLIVIER]

The main reason I questioned the use of a pure lime mortar is that most granite is harder than concrete; it's different from the limestones which Peter has just mentioned which clearly need a mortar softer than the stone.

Maybe build the inner leaf of 100 mm dense blocks and solid intermediate floors; this is usually self-supporting once loaded from above. If the outer leaf is aerated blocks, thin-jointed, the U-value with a 200 mm fully-filled cavity is quite good.

David.

[Dominic Goetz]

Thank you all for your advice, it is greatly appreciated.
Now I've got to persuade the 'sustainable development' department that renovating a barn to be a warm and sustainable home is a better option than building a freezing granite seive which adheres to some mythical vision of Cornwall in a soft-focus utopian past!  I am hoping that the next few weeks don't turn me into west Cornwall's most cynical self-builder.
Happy New Year one and all.
Dom

[Dominic Goetz]

Following 6 months of badgering I have finally got the planning committee to let me go ahead with my build.  Of course, during that time I've changed my mind a hundred times about the best way to build the walls. 

Currently thinking of replacing outer leaf of cavity with 'Surecav' (a HDPE backing board for the stone which also forms a 2" cavity)to speed up build as there are only 2 of us on site most of the time.  Should also decreas incidence of getting snots in the insulation.

Initial costing of insulation reveals that it costs the same to use 120mm of Kingspan (other PU boards are available) to get a u-value of 0.16 as to use 200mm of dritherm32 which gives a u-value of 0.14
Former option has advantage of keeping wall thickness down to 570mm rather than 650mm so foundations can be smaller and rooms larger.