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Insulation Options
From Building for a Future Spring 2001

In this article I want to compare, amongst other things, the use of cellulose and wool insulation in a breathing wall construction. The examples used for this comparison are two houses built in Cornwall.

The first house, Powes-Va-Yowan, St Agnes, was commissioned by George and Carolyn Watson, who contacted me through the Green Building Bible. The house was built by, Peter Boulton and his fellow workers, Kevin, Bill, Simon and Rob. The second house, formerly The Old Garage, Godolphin, is speculatively built (also by Peter and his crew) for resale on completion.

Powes-Va-Yowan is located at the top of a cliff on the very exposed north coast of Cornwall in an area of outstanding natural beauty and special scientific interest. This led to a lengthy planning procedure involving several redesigns and meetings with the case planning officer.

The structure of untreated timber frame walls with suspended floors is clad in old granite with a slate sill up to ground floor sill height and green oak boarding above that.
We used Fillcrete products, Panelvent for Sheathing, Bitvent to support the Warmcel between floor joists and Sarket on rafters. We used Tyvek on top of the Panelvent and Sarket as an added precaution against the extreme weather in this location. Other ‘eco features’ incorporated in the building include the use of recycled timber, second-hand doors, organic finishes, low E double glazed units with wide cavities, south facing glazed lobby with heat sink slab and walls, provision to connect to a wind turbine on approval of planning application etc.

Problems we incurred using the Fillcrete Warmcel products were; price increase from time of costing /quotation to time of ordering, cost of delivery to Cornwall, delay in delivery time necessity to over order due to inconvenience and cost should we be short and need to order more restriction of having to pre-book and work around Warmcel installer overall cost of products inflexibility of vapour resistant paint / sealer products required to achieve the 1:5 vapour resistivity ratio for a breathing wall. We had used standard plasterboard and wanted to avoid using paint by using white one-coat plaster, we ended up using 2 coats of Keim dilution to achieve the vapour resistance, followed by Keim Biosil wall paint.

The house at Godolphin is situated on the edge of the village, beside the National Trust managed woods of Godolphin House. It has many similar features to Powes-Va-Yowan. But being a commercial project, much is rationalised. In order to keep costs down, and be less-dependant on specialist suppliers and sub-contractors, I decided to look at ways to achieve a wool insulated breathing wall using materials available from local merchants.

Wool has a number of advantages over other insulants, primarily that it is a naturally produced material which has evolved as an insulant over a long time, retaining its insulating properties when wet. It gives a U-value of 0. 17W/m2K in a 2OOmm thick timber frame wall construction. We had looked into using wool at Powes-Va-Yowan but had found it to be very expensive if bought as a packaged building product.

After researching the subject further, I found that raw, low-grade fleeces can be bought for as little as 20p per kg. The fleeces need scouring by a specialist. I was quoted 16p per kg, (apparently 30% is lost in this process) and then treating for protection against moths. This can either be done as part of the scouring process using what the trade call ‘Mittin’ (sulcofuron), or it can be sprayed with diluted Boron as it is installed on site.

A simpler way to get wool suitable for insulation is to buy it ready scoured and treated from a spinning company or wool processor as a by-product from the yarn spinning industry. We went for this option as it seemed less problematic, but cost more (£ 1.00 per kilogram). Bought in this state, the wool arrived in 150 Kg hessian wrapped bales (not very moveable). I estimated we would need 25 Kg per cubic metres (5 Kg per square metre for 200mm thick walls), this was a bit generous but any extra insulation can be added to further insulate flat ceilings in an attic space or to sound insulate internal walls.

Some people suggest that the wool should also be carded (a process which combs the wool fibres into the same direction) but we found that manually teasing the wool apart as it comes from the bale worked very well.

It took one person approximately 80 hours to fluff and install the wool into 220 square metres of 200mm thick wall and roof. We stuffed the wool into the cavity after fixing each sheet of plasterboard, using leftover breather paper to retain the wool in sections where wool could not be stuffed in after (below noggins and veluxes etc). The wool will actually hold itself in place between studs on a vertical wall before plasterboarding.
We used 9mm shuttering ply as sheathing, this in combination with Duplex plasterboard gave the appropriate ratio of vapour resistivity (5MNs/g and 60 MNs/g respectively).
For cladding, we used horizontal green sweet chestnut. This has similar properties to green oak, but is a little softer and has a tendency to split along the grain if not treated carefully. As with green oak, great care has to be taken to avoid contact with ferrous metals, as they react with the tannin causing the timber to go a magnificent blue/black colour. I chose sweet chestnut as it is a personal favourite and because it weathers a bit lighter than oak.

Another development from Powes-Va-Yowan was to sit the timber frame on top of an insulated block plinth instead of behind it, which came up to the underside of the ground floor window sill (see section right). This created an internal step, due to the difference of wall thicknesses, which was then used as a continuous timber shelf/ledge around the external wall, which housed all cabling, switches and power points.
After completing the main structure, it occurred to me that if the 9mm sheathing ply were to be fixed to the inner face of the studs, standard plasterboard could be fixed directly onto the ply, with a tightly stretched Tyvek type breather paper on the outside containing the insulation and preventing moisture penetration. This arrangement would have the advantage of improving the breathability of the wall, provide easy fixing of appliances and pictures etc internally and prevent the sheathing from ever getting wet in construction. I would welcome any comments on this proposal before trying it on our next building.

By using one-coat plaster on the plasterboard and recycled Douglas fir for skirtings, frames and architraves, we totally avoided the use of paint in the house. Pigments can be mixed with what is normally an off white plaster to give different colours/tints.
For commercial projects, which this house is, the BRE offer a grant to pay for a day’s energy consultancy with Exeter University. This we used to have a heat loss assessment carried out on the house. The conclusion was that an 8 kW wood burning stove would provide sufficient space heating to satisfy normal expectations. Water is preheated by a 4 sq. m flat solar panel, boosted by renewable electricity from the mains grid supplied by Green Electron.

Author: Paul Connel