Passivhaus diaries, part two: Why we chose cavity walls
The Green Building Store is building its Passivhaus home with cavity walls - not the usual choice but one that works at Denby Dale
As far as we know, the Denby Dale house will be the first Passivhaus in the UK built using cavity wall construction. European construction is typically solid masonry with external insulation and render or timber frame, and so Passivhaus buildings tend to use one of these two methods. But there are many ways of crossing a river, and we've decided to use cavity wall at Denby Dale, for three main reasons.
First, cavity walls contribute to a high thermal mass within the insulated building envelope – allowing a more even living environment, in terms of both acoustics and temperature. Through use of concrete blocks and concrete ground-floor slab within the thermal envelope, the house will have a greater thermal mass, which will act as a heat store, storing heat gained from passive solar gains.
Second, cavity wall construction is the method that we at Green Building Company are most familiar with (we used it to build the Longwood low-energy house in the 1990s). We wanted to build a Passivhaus using British construction techniques, with, as far as possible, materials you could find in any local builder's yard.
Third, West Yorkshire planning rules require natural stone facing on the exterior of new buildings and so ruled out the option of block-built construction with rendered finishes.
The potential downside of cavity wall construction is that it relies on wet plaster to be the airtightness barrier on walls and junctions with doors, windows, floors and roof, whereas timber frame construction can be lined with vapour barriers and airtightness tapes etc, so our cavity wall Passivhaus will demand greater attention to airtightness detail.
This week I've been finalising choices for some materials on the build. We need to keep space heating needs at or below the magic figure of 15kWh/m2/annum – the prerequisite for Passivhaus certification. Modelling different materials in the Passivhaus planning package (PHPP) can have dramatic effects on that figure, and this week the figure has ranged between 19-14kWh/m2/annum, depending on which type of wall insulation and window configurations we have inputted.
I've been looking at options for mineral wool batts for the cavity wall – with options ranging from 100mm batts with lambda values of 0.037 (at a price of £2.68/m2) to 0.032 (£6.13 m2). We will be using 100mm batts three deep, which means tripling the price and, with 190m2 of walling, potentially we've got a £2,000 price difference between the different mineral wool options.
PHPP helps us to weigh up the cost implications of decisions with performance and allows us to play off one element against another. With PHPP you can decide on which bits of the elements you are going to compromise on and which bits you're going to upgrade in terms of thermal performance. In this instance, we've had to trade the need to accommodate more glazing on the north side of the building against having more expensive wall insulation. The Tunstalls' garden will be on the north side of the house and naturally enough they want to have sizeable windows on the north side to look at it. To give us flexibility in the sizing of the north-facing windows, we've decided to go for the more expensive batts with the lambda value of 0.032. The better insulation I can get in the wall cavity, the more view the Tunstalls will have of their garden.
We have been delayed this week with the excavation work. At the last moment there was a change of mind regarding the position of the house – it is only a matter of one or two metres – but the local planning department have requested that we get statements from all the neighbours saying the new position is okay. Excavation work will hopefully start on Monday. Ralph the driver is breaking off from silage-making and coming over with his JCB and will get stuck in. Let's hope there aren't any more delays!
Oh, and in response to some requests from readers, I've also added a rough overall breakdown of costs on the build.
To visit the Green Building Store, go to http://www.greenbuildingstore.co.uk/denbydalehouse.php.
Postscript:
Bill Butcher is director of the Green Building Store
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Readers' comments (8)
John Carney | 8 Jun 09 8:29 am
I was very interested in Bill's choice of external wall detail. There is such a range of options available that decision making can become difficult. I like the choice of cavity wall for the 3 reasons Bill cited. However, I would like to know more about the cavity wall tie. This must be of a special type to effectively deal with a 300mm wide cavity and not cause thermal bridging problems. I am also interested in the head, sill and cavity closing details at window and door openings together with floor and roof junctions. Perhaps some details of this can be given. I am also interested in the stone facing requirement - will this be added on to an outside block leaf or built directly on to the insulation and tied to the inner leaf? The devil is in the detail and I'm excited with this project and how it will progress.
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Editor's comments
Bill will address these issues as the build progresses.
Brian Quinn | 8 Jun 09 1:13 pm
I am an architect keen to know more about the Passivhaus standard but I have seen too many simplified diagrams with pretty curved blue and red arrows to take anything at face value. Therefore I applaud this initiaitve and congratulate all involved in making it come about. I look forward to getting down to the nitty gritty to see how it really works or doesn't for ultimately that to my mind is the value of it - convincing people like me and others to see PH as anything other than the latest craze. I am certainly encouraged by the detail Bill is going into and look forward to seeing how it all pans out. Good luck.
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Giles Hirst | 19 Jun 09 6:06 am
Dear Sirs, Hanse Haus have built and are continuing to build,pre fabricated timber frame Passiv Houses, and look forward to some dialogue on possible working relationships with your business in the future, I can be contacted by email or on 07968 117160 at any time.
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Mark Bennett | 25 Aug 09 1:10 pm
Hmm. Cavity wall is a very strange choice, and the reasons stated don't add up. First, thermal mass. single skin block construction with external wall insulation typically use 200mm+ blocks as the inside layer. This will give at least as much and probably more thermal mass than the inner skin of a cavity wall (typically only ~100mm). Building with cavity wall might seem familiar to the builders, but the degree of care needed for PassivHaus is still likely to be alien to them. Avoiding thermal bridging through mortar snots, ensuring the insulation is all abutted and overlapped, that there are no thermal loops around behind the insulation, extra long non-bridging (but inflexible) wall ties, ensuring airtightness etc. Overall, it's a different game and going with an "established" technique is just as likely to introduce a false sense of complacency leading to errors. Furthermore, it's virtually impossible to inspect the insulation, since it is hidden behind the outer skin. With solid wall the insulation is all put on before the cladding, so is relatively easy to inspect or repair where necessary. Third, there is nothing that prevents an external finish that looks like brick or stone. Plenty of brick slips are available to fit as the external cladding if you need. The airtight barrier for cavity wall is the same as for solid wall construction - the internal plaster coating. This is no harder for cavity wall and has there are plenty of experience wet plasterers out there. I'd argue it is better than relying on a plastic membrane that is very likely to get punctured during build and is virtually impossible to inspect or repair. Cavity wall is one of the biggest mistakes made for UK building and should be relegated to the scrap heap. I don't see the advantage of cavity wall over solid wall with external insulation in this situation.
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Bill Butcher | 12 Sep 09 9:06 pm
Mark, apologies for my not replying to your comments on cavity wall construction before now, but it has only been brought to my attention in the last few days. Firstly we have never intended to say that cavity wall construction is the only way to reach these levels of performance. It may well turn out that solid masonry construction with external insulation becomes the norm in this country. We decided to go down the cavity path because of familiarity. Our intention is to show that there is nothing for a thoughtful builder/designer/client to be frightened of. All the basic materials for this build come from the builders merchant literally over the road. We also believe, at the moment, the solid wall construction you suggest is more expensive. You are correct in pointing out that a solid 225mm concrete has more mass than a 100mm inner leaf, but my point was in reference to comparisons with timber frame. What we have found in building with large cavities is that they are very easy to keep clean of mortar ‘snots’ particularly, as in this case, the coursing of blocks/insulation/stone is designed to be built sequentially. The expected problems because of the rigidity of the basalt wall ties, have not arisen. Due to there length, they have turned out to be relatively flexible, certainly enough to take up any imperfections of coursing. Your observation on the air tightness for solid wall construction being the wet internal plaster is I believe incorrect. The method should rely on a parging external coat before fixing the insulation. This allows for a greater freedom to install services on and in the plastered walls. I believe your observations on quality control are valid with the present culture we have in the UK construction industry. Fortunately we are a small company with good communications and an ethos of quality over quantity, so I can fully trust that the necessary methods are adhered to. High performance buildings, using any method of construction, will be difficult to achieve until we take a long hard look at our working practices. An under trained, under appreciated and patronised workforce will never perform. Good design should include ease of construction, with dare I say, designers accepting that people on the ground might know better sometimes. However, the main enemy of good building is the present preference for sub contracting, an adversarial approach to the Final Account, and an obsession with short term profit.
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Andrew O'Connor | 10 Dec 09 12:42 pm
I'm interested to see how will you achieve (or at this stage have you achieved!) the required airtightness for Passivhaus using conventional cavity block build. I'm being advised even with good workmanship achieving the Passiv standard is at best optimistic! Andrew
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Bill Butcher | 12 Dec 09 3:00 pm
Andrew, A 'wet' plastered block wall is airtight. The more usual contemporary practice of 'dot and dab' plasterboard is certainly not, however sand and cement parging to the blockwork first improves the performance. Junctions between elements such as floor to wall and around window openings cause potential problems. These, I hope, have been dealt with in subsequent blogs. Services through or against the wall also has to be designed for and executed with care.
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Kevin | 30 Dec 09 1:27 pm
Bill, I'm curious about the statement that "West Yorkshire planning rules require natural stone facing on the exterior of new buildings". This is news to me. Can you please elaborate, or point me in the direction of the planning policy which states this?
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