Strawbale House

This blog is intended to chart our progress through the self-build process, from half-hearted plot-hunting through to completion of the build. The twist is that we're building the house from timber and straw (hence the blog title).

Click on the image at the end of each post to see that day's photos.

Wednesday, 25 April 2007

Weatherproofing and wall design

The wall construction I've conceived (shown below) is designed to provide an enormous amount of thermal insulation with none of the nasty chemicals and only a fraction of the embodied energy of more conventional products like mineral wool or rigid insulation boards. A structural frame of untreated spruce is braced on the outside face with panelvent (a breathable toxin-free alternative to ply or OSB). This provides both anti-racking and draught-proofing, and allows the transmission of water-vapour from the structural timbers through the wall to the outside. Around the outside of this, straw bales are stacked, like giant lego bricks. These are arranged around window and door openings, and split where necessary to fit. To minimise labour-intensive bale-splitting, doors and windows are specified to whole bale-widths (or multiples of bale-widths). A typical bale measures 900 x 450 x 350mm.
Around the outside of the bale walls is constructed a non-structural frame of native larch. This serves mainly as a frame to which the external cladding will be fixed, and has the added effect of stabilizing the bale walls. This larch frame is fixed to the structure of the building wherever possible, either to window / door boxes or through the bales to the structural frame with thin battens.
The next component in the walls is possibly the most crucial to get right. Straw is a very stable and durable material, very similar in molecular make-up to wood. That's as long as it is kept dry. If moisture is allowed to penetrate the bales AND STAY THERE, the bales will rot and the whole idea of the super-insulated wall goes out the window. It is therefore essential to protect the straw from the elements, and to allow any moisture within the walls to escape. To achieve this, a breathable membrane is used over the entire outside of the walls, immediately behind the external cladding. This membrane must be as impervious to water ingress as possible, while at the same time allowing the maximum possible degree of vapour transfer from inside the walls to outside. Think of it as a Goretex coat for the straw.
There are a number of breathable membranes on the market, some designed for installation under roof-tiles or slates and some for timber-frame protection. Because of the particular demands of the strawbale wall, and the criticality of the performnce of the membrane, we decided to go for the most highly specified product we could find. This turned out to be Roofshield from the Proctor Group. With a thickness of 0.6mm and a vapour transfer rate of about 2.4 litres /m² /day it certainly stands a fighting chance of keeping the straw in pristine condition long after I've turned up my toes!
The external cladding could have been cedar imported from Canada, which boasts a typical life-span of 60 years without any need for treatment. This is quite pricey, and using imported timber would have gone rather against the project's stated aim of using local materials where possible. I was very keen to avoid any nasty chemical treatment of the cladding (typical external timber treatments use arsenic and chromium - neither of which you really want to be sprinkling on your cornflakes, or leeching into ground-water in years to come), so something naturally durable was needed. The only locally grown option was native larch. It's not as durable as slower-grown Siberian larch, but the fact that the trees that will be used are currently growing in the Bowmont Forest less than a mile from my house gives it a very cuddly feel, and saves a bundle on transportation costs, both to the environment and to my pocket. With proper detailing and careful installation, there's no reason it shouldn't still be doing its job perfectly satisfactorily in 30 years' time. That's without treatment. After a year or so it will weather to an attractive silvery colour, in much the same way that cedar does. At this point I may decide to apply a boron treatment (a completely non-toxic mineral salt) that protects very well against both woodworm and moisture.

Friday, 20 April 2007

April 20th - Glimmers of progress

We've now exchanged missives, but don't yet own the plot because Planning still haven't produced the goods. I have, however, had a letter saying it's in the offing, and expect it in a very few days. Encouraging chat with Building Control yesterday. I can apply for a phased warrant, which will enable me to get on site more or less straight away and get cracking with the ground-works while I'm waiting for final architectural drawings and frame calculations.
Currently batting drawings back and forward with Chris at Twisted Designs Architects. The structural engineer is toiling with a scheme to span the big open-plan area downstairs, and is talking about using steel beams and posts. I'd much rather stick with timber (engineered timber if necessary), and yesterday I sent over a sketch of a first-floor joist layout I'd been doodling with (see below). No doubt it's pie in the sky, but I just can't help myself. I bet I'm a nightmare client!

Wednesday, 4 April 2007


Today should have been a red-letter day. It's the deadline for exchange of missives, set by the Walkers, who wanted the transaction to take place before the end of the financial year. Unfortunately yesterday I had a call from the Ecology Building Society, wo have noticed, or decided, that the letter from planning I gave them as indication of the soon-to-be-agreed planning consent was inadequate, and that they can't release funds until it has been formally agreed. Don't blame them, really, but it throws us back at the mercy of the Planning department.
Emailed the Head of Planning, Ian Lindley, and received a reply from his immediate subodinate Brian Frater, who seemed a bit uppity that I'd interpreted their letter last month as suggesting that permission was imminent. Raised a load of red herrings in an attempt to imply that there was still a lot of work to be done before a decision could be made. I couldn't believe it. Our application was registered on October 23rd. They have an obligation to give an answer within eight weeks. They have now had something like 22 weeks, and still can't get their act together. A complete bloody shower, the lot of them. They wouldn't last a month in the private sector.
I quickly fired off an email before I had to go out to work, and struggled gamely to keep it non-confrontational. It ran as follows:

"Dear Mr. Frater, Thank you for your prompt reply. The revocation of the original consent is purely an in-house matter to be dealt with however you deem necessary by your department. Charles knows I am perfectly happy for this to be revoked, and I can't begin to understand why it is taking so long. It certainly doesn't require any input from me or from the vendor of the plot. We were told by letter dated 14th March 2007, that we would be contacted 'directly', by Nuala McKinlay [of your legal department]. We have heard nothing. The slate roof I am also perfectly happy with, and my original planning application (which you have had since October 23rd) specifically states that I will have a slate roof. The requirements set out in Alan Scott's letter of 12th January are, as far as I know, all well in hand. I was shown a plan of revised access by Roger Dodd, which he sent on to planning some weeks ago. The passing place has been installed (again, some weeks ago). In addition to the impression I garnered from the letter from Charles, I was also assured verbally by him, on February 13th, that your department had, after lengthy deliberation "decided to support" my application, as indeed we were assured by Frank Bennett before Christmas. Perhaps now you can understand why we 'gained an impression' that planning was about to be granted. Regards, Damon Rodwell cc Councillor Alex Nicol"

Why is it that organizations with more than a few dozen employees are incapable of functioning efficiently? As well as local government, I'm thinking, of course, of BT, who took two months and four site visits to decide that they couldn't give me a broadband connection, despite the fact that there was one in the house until the day before we moved in. A plague on all their houses!

On the bright side, I had a draft copy of the ground-floor drawing from Chris at Twisted Designs, as shown below. He's had to knock my original idea around a bit to comply with disabled access legislation.