Planning to insulate bedrooms this weekend

This weekend my wife and 5-month old daughter are going up North so I'm going to take the opportunity to insulate our two bedrooms.  The loft is well insulated (as demonstrated this past weekend by the fact that snow didn't melt on our roof even though we had the heating on most of the weekend) but the walls are currently just 9" solid brick (with a U-value of 2.1 W·m-2·K-1; the "U-value" is simply a measure of how well a building element conducts heat; a lower value means a better insulator. The 2010 building regs specify a U-value of 0.3 W·m-2·K-1 for walls). Adding 65mm of PIR insulation (with thermal conductivity of 0.022 W·m-1·K-1) should bring the U-value of the wall down to  0.3 W·m-2·K-1  (in other words, it should reduce the rate at which the walls lose heat by a factor of just over seven.  To give a concrete example of what this implies: say the external temperature is 0°C and the internal temperature is kept at 18°C then the heat loss in our main bedroom through the external walls prior to insulation would be 780 W; after insulation the heat loss will be just 100 W).

By how much will this reduce our energy consumption?  Let's create a very simple model for just the main bedroom to get a feel for the numbers.  Let's say that, for 50 days of the year, we heat our bedroom to 18°C all day whilst the external air temperature is 0°C.  Prior to insulation, the bedroom walls will leak

0.78 kW × 24 hours per day × 50 days = 940 kWhours per year

after insulation that figure is just 120 kWh per year.  At 8p per kWh for gas, that's a financial saving of

(940-120) kWh × £0.08 per kWh = 820 kWh × £0.08 per kWh = £65 per year

Given a cost of about £600 for the insulation, dry-wall adhesive, frame fixing screws, plasterboard socket enclosures etc, that's a pay-back period of just over 9 years (assuming gas prices stay the same). And the insulation should decrease the running costs of the house for the remainder of the house's lifetime (maybe 50 more years?).

It also represents a CO2 saving of

820 kWh × 0.2 kg CO2 per kWh of gas = 164 kgCO2 per year

Which is a bit more CO2 than a return flight from London to Paris for one person every year.  If we assume the house lasts for another 50 years (and is heated by gas for 50 years) then that's a saving of 8.2 tonnes of CO2.

I'm ordering 11 sheets of Celotex PL4000 from PlanetInsulation (plus bonding compound, screws etc).  Celotex produced a decent enough "how to install" video in Dec 2011.  I'm planning to glue the sheets to the wall and also screw them in place.  I did briefly look into options for installing ply wood over the insulation (to make it easy to put up shelves etc) but it looks like it'll not be worth the effort.  I also started a discussion on the green building forum asking "Best frame fixings for installing insulated plasterboard?".

update 8th Feb 2012

After discussing some more on the Green Building Forum, I've decided not to use insulated plasterboard because my wife and I want a wall surface which is easy to attach things to (shelves, pictures etc).  The problem with insulated plasterboard is that plasterboard can't take much load, so if you want to attach heavy shelves etc then you have to somehow get your screws into the brickwork behind the insulation and plasterboard (the easiest way to do this is with a frame fixing screw (screwfix do frame fixing screws up to 185mm in length); if you need to attach really heavy loads to plasterboard then take a look at these suggestions on GBF).

I considered creating my own laminate consisting of 65mm insulation + 11mm OSB ply + 9.5mm plasterboard.  This costs about £40 per "sandwich".  I would glue the components together to make it easier to cut and install.  Ultimately the sandwich would be secured to the wall using lots of frame fixings (i.e. the glue is just to hold the sandwich together during installation; but when it's installed the glue can completely fail if it wants because all the load will be taken by the frame fixings).  Rather tediously, most OSB is supplied in 2440×1220 dimensions, but I want 2400×1200mm to match the insulation and plasterboard. I phoned our local building control officer and she said that this solution should be fine from a fire regs perspective because the inner layer is plasterboard.

Ultimately I'm planning to go for creating my own laminate of 65mm PIR insulation + 12.5mm Fermcell instead of messing around with OSB.  Insulation + Fermacell works out about the same price as insulation+OSB+PB, is easer to work with, less environmentally damaging and almost as strong.  Fermcall is considerably more expensive than plasterboard but is about the same price as plasterboard + OSB.  Fermacell is apparently pretty heavy stuff so I'm worried I wont be able to install it on my own but we'll see. It's apparently "easy" for a DIYer to achieve a good finish using Fermacell Fine Surface Treatment instead of requiring a wet plaser skim (although a skim is possible if desired).  There's lots of discussion of Fermacell on GBF here.  There's a Fermacell video installation guide here.  I'll glue the insulation to the Fermacell using Evo-Stik 528 Industrial Contact Adhesive and then secure each laminate to the wall with frame fixings (I'm going to try both "Rawlplug Frame Fixings 10×140mm" (£19 for 50) and "Fischer Nylon Frame Fixings 10×160mm" (£29 for 50) and see which I prefer). As with the previous design, the glue is just to aid installation; it is not for mechanical strength (that's what the frame fixings are for).   The mains bits of the shopping list are now:

The total cost is about £600.

Now to do the building control application... (aren't I a good boy)...

Update Nov 2012:

I've finally gotten round to blogging about the installation process itself!

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