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Apart from changing the way you live, by far the most effective measures for both reducing energy costs and thus CO2 emissions from living in a house is to insulate the building. Space heating for a house in the UK typically accounts for 61% of the total energy use in a house and about 51% of the CO2 emissions.
It is useful to look at where heat is lost in a house when thinking about the most effective ways of insulating your building.
Outside insulation
Fairly early on I discounted the idea of providing insulation on the outside of the walls. This was partly cost, partly appearance given that the house is semi-detached and partly the difficulties of details around windows and roof. An example of this kind of insulation done successfully is here: http://www.theyellowhouse.org.uk/
Inside insulation
Insulation on the inside of the walls is a possibility but again pricey. It also means losing floor area inside. If the whole house had had to be gutted or if it were a single brick skin, then it would have been a more feasible and sensible option. An interesting example of this kind of solution is the Parity Projects house in Carshalton. There are various methods of insulating walls from the inside. One product that can be applied to walls is Sempatap. For an idea of the practicalities of putting this on your walls see the section Sempatap wall insulation.
Cavity wall insulation
 The obvious answer for my house was cavity wall insulation and it can provide a very good and cheap way of improving the insulation performance of a house, saving up to 15% on energy bills. Government grants are available as it is recognized as being such an effective and easy solution. There are only currently 3 possible materials used by installers (mineral wool, urea formaldehyde foam or expanded polystyrene beads). Probably the least environmentally harmful is the mineral wool which is what my installers said they used. It took a very long time to get this done because there are few installers and a huge demand. It was worth checking out several installers. My tenant had to apply for the insulation because the grants are only available for occupiers of houses. Some people are averse to using this because they believe there will be consequent damp problems in the walls. It appears that this does happen in some cases but the vast majority experience no particular problems.
Although the converted loft space had some insulation, it had not been installed well or to a sufficient depth to get to a good insulation value (known as U value) by today's standards. It would have been easy to insulate cheaply, effectively and using environmentally sound materials if it were unconverted loft space - all that would be needed is a good thick layer of insulation across the floor (though there are problems subsequently in trying to lay floorboards given the depth of insulation). However, because the attic space is a room, it was more difficult. There seemed to be two options:
The polystyrene
taken out of the roof
 a) fill in the gaps at the eaves and increase the insulation depth where it is possible to get at it (behind the eaves cupboards and doors and the floor in the eaves space) or b) strip out the plasterboard and increase the insulation of the whole roof
I decided it was worth the cost of doing the latter even though it is quite disruptive and means replacing the plasterboard as it cannot be reused. If I hadn't done this, I would not have been able to make a huge difference to the roof insulation value. The problem was deciding on the method of insulation. One of the constraining factors is trying not to lose too much room space by increasing the depth of insulation; another was cost.
Types of insulation The attic stripped prior to installing insulation
I would have like to put in either sheepswool insulation or warmcell (recycled newspaper) being nice friendly materials but both need a huge depth of insulation in order to get a good insulation value. This would mean reducing the room size so much that it might well become unusable. Cheaper insulation, either “eco” as sold in stores such as Homebase or the more normal rockwool have the same problem.
Another option was spray foam insulation. It is applied by specialist installers with spray guns and is supposed to fill every gap so sealing the roof completely. However there seems to be debate on its thermal properties, it can clog up the roof tiles, it can easily spread when applied into areas you do not need it, its ventilation properties are questionable and it is not a very environmentally friendly material.
There is also a product called spacetherm made from a very strong, very lightweight and highly insulating material called aerogel. This may well become one of the most effective insulation materials of the future. It is available but I do not as yet know very much about it.
The two materials that I finally considered were foam boards, such as Celotex or Kingspan, or expensive foil insulation such as Tri Iso.
Tri Iso
The performance of Tri Iso seems to be hotly debated. It is layers of foil, rather like a space blanket and is easy to apply. People like the idea because it takes up little space so the room area is reduced less. Among others, our local building control officer thinks it’s good but some experts who have tested it say it does not perform to anywhere near its claimed values under some conditions. I cannot see myself how it can perform that well when the multifoil layers are crushed every few inches along batten or rafter lines where it is fixed. It is also very expensive.
Celotex/Kingspan
Celotex and Kingspan are not particularly green materials being manufactured from oil-based chemicals and not being recycleable, but they have a very high insulation values and will last a long time (I hope!). The embodied energy in their manufacture is actually quite low if you spread it over the lifespan of the building. Both companies talk about their environmental awareness, though how much is PR is difficult to tell. They are not very cheap, but less than the Tri Iso.
I did some calculations of the various options. By using Celotex/Kingspan carefully, the difference in space lost for a similar insulation value (if you believe the Tri Iso figures) is about 40mm into the room, which is very little and now it is finished it seems little different from before.
The Kingspan arriving
Overall I decided to go for Kingspan Kooltherm. The method I went for was between and under rafter application (ie insulation between the rafters and then a layer over that fixed across the rafters, then plasterboard over. The big question was what depth of insulation. The constraining factors were the loss of room space by using a thick layer of insulation under the rafters and the doorway into the attic room.
Insulation thickness
Without decreasing the attic usable space noticeably, it is difficult to get to the current Building Regulation specification for attic conversions. However, after doing many calculations I worked out that by using 60mm Kingspan Kooltherm between rafters and 40mm under, I could get the insulation value of the roof approximately to the current standards for new buildings and conversions. This allows a 40mm gap for condensation ventilation between the roof and the insulation board, 10mm less than the recommended 50mm gap, but I was advised by the company selling the Kingspan that this was OK. I also improved the insulation on the gable wall in the attic as well by using 60mm Kingspan on the wall, covered by a  vapour control layer of plastic sheeting we had lying around. Where possible, the insulation on the party wall was increased using the 60mm Kingspan because the neighbours have an unconverted attic which will be cold. This was fiddly to do as it was by the stairwell and took some time to do.
Kingspan being fitted between the rafters. It proved dusty to install once cut and has to be cut accurately to fit snugly. Quite a time-consuming process.
Fitting Kingspan under the rafters
The overall insulation values of the completed space may not be exactly as good as predicted because it is impossible to take the insulation right down to the wall plate at the eaves - this is the difficulty with retrofitting a house. However, despite these imperfections, I have no doubt that there will be a major improvement in heat retention, not far off the best standards.
Plasterboard
I did look at using Fermacell plasterboard which is made from recycled materials but cost and available supplies at short notice meant that it was not practical. However, the British Gypsum standard plasterboard has as good a carbon footprint as any of the standard materials as far as I can make out, and that was what we used to finish off the attic over the insulation. Disposal of plasterboard is now controlled and it is not possible to put it in skips but has to be disposed of separately. The idea is to ensure recycling of the gypsum in plasterboard.
I abandoned plans to insulate under the wooden front floor of the house between the joists, fill the gaps in the floorboards and leave the floor stripped and waxed, which was what I originally intended.  It just seemed too much disruption and cost and getting to the end of the project, I lost some enthusiasm and looked for simpler solutions. So I decided to go for a thick recycled fibreboard underlay, covered with some Cloud 9 (very expensive) underlay got for free on freecycle with a pure wool carpet over.
7mm recycled fibreboard Cloud 9 underlay
With the rear concrete floor, it was really not practical in this case to dig the floor up and re-lay it with an insulating layer. The best solution was to use the same thermal fibreboard underlay and floor over this. I completely failed on my eco ethics here and went for a laminate flooring. The factors in this decision were cost, wear and maintenance in a rented property and ease of installation. Cork, marmoleum and wood would all be preferable in eco terms but they all cost more and need care to maintain their looks and performance which is difficult to expect from a tenant.
Other Insulation
Pipework – I had hoped to lag any water pipes that became exposed at any stage during the refurbishment, but none were!
Curtains – my plan was to line all the curtains with insulating material. We have done this at our house over the outside doors and keep the curtains drawn immediately it starts to get dark. It seems to keep the warmth in better as well as stopping the odd draught. However, I ran out of time at the end of the project and didn't have time to line them beyond normal curtain lining. I have reused curtains where possible and bought others (good quality lined curtains) from charity shops.
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Insulation