What Will it Cost? - Insulation

Thermal insulation is the basic cornerstone of energy efficiency in modern house design, as Mark Brinkley explains.

Wrapping a big thick woolly jacket around a house keeps the heat in during winter, and the thicker the jacket, the less heat is needed to keep the house warm. All building materials insulate to some extent but the dedicated insulation materials available to us are by far the most effective and it therefore pays to become acquainted with what is out there.

The most basic choice is between a synthetic wool – the type of thing we are all familiar with, laid down in our lofts – and a rigid board material, made up of either poly­styrene or one of the polyurethane family of chemicals. The thermal conductivity of each material determines how much depth you need to meet the regulations and, not surpris­ingly, better thermal conductivity is avail­able at a price. The best of the conven­tional materials in this respect are the polyiso­cyanurate boards, produced by the likes of Kingspan, Celotex and Xtratherm, but they are around three times the price of expanded polystyrene or loose mineral wool. These materials can mostly be blown-in on site by specialist subcontractors as an alter­native to fixing rolls or boards. Cellulose fibre is another option that is generally only avail­able blown, as it has to be moistened to a very specific degree in order to be effective. There is also an increasing number of alter­native materials available, most notably a range of reflective foils used increasingly in roofs and walls.

Decisions about insulation are not made in isolation. You don’t start off by choosing the insulation and then deciding what sort of struc­ture to build around it. Nevertheless, as regulation drives insulation levels ever higher, the cost implications become more important: not only do you have to spend a bigger slice of your budget on insulation but, more impor­tantly, the levels of insulation required start to dictate the type of structure you use.

As an item in the overall expenditure, insula­tion itself is insignificant. The benchmark house cost table, around which these articles are constructed, shows that the spend on insulation materials was a little under £2,000. Even by adding in notional amounts for fitting the insulation – a task that often gets ‘lost’ as it tends to get done by trades in the course of their regular work – the overall cost is still just under 2% of the total budget. But the break­down between the different areas is reveal­ing, as is shown in the accompanying table.

What is immediately noticeable is that the mineral wool, placed in the flat ceilings and the external walls, is much cheaper, on a unit area basis, than the polyurethane boards (made by Celotex in this instance) used under the ground floor and in the sloping ceilings in the master bedroom. In ‘depth critical’ areas – and sloping ceilings are the classic example – it is much cheaper and easier overall to specify insulation with a lower (better) U-value, such as polyurethane boards, even though it is more expensive in itself, as it means you can use a thinner layer and save headroom and reduce associ­ated carpentry costs.

As insulation requirements keep rising, these sorts of factors come into play more and more. Standard timber frame walls have always been 90mm deep: under current build­­ing regs, 90mm struggles to cope with the required insulation, but by next year (2006), when the regs tighten again, many timber frame manufacturers will be faced with a switch to 140mm studs or a switch to a more expensive insulation: the expensive insula­tion represents the most economic option.

The situation with sloping roofs is even more extreme: the U-value required is so low that it is impossible to incorporate the insula­tion into the depth of the rafters or trusses. Various alternatives have been used including the warm roof (where the insulation is placed over the rafters), the ‘in-between and under’ option (where insulation is placed under the rafters as well as between them) and, most controversially, the use of radiant heat bar­riers. These are relatively expensive – costing over £12/m2 – but are just 25mm or 30mm thick and so win hands down on the depth stakes. Their use is controversial because they don’t come close to passing the stan­dard tests for thermal conductivity used world­­­wide for determining just how well insula­tion retains heat. Indeed, many countries don’t allow radiant barriers on their own to be used in roofs or walls, but British authorities have been happy to accept them, based on the manufacturers’ own tests which show that they are equivalent to 200mm of mineral wool, the standard depth of mineral wool required in sloping roof designs.

INSULATION OPTIONS

About These Articles

This series is based on a typically constructed, 4/5 bedroom house, with an internal floor area of 200m2 (2,150 sq.ft.) plus an integral garage. Its raw build cost (that is only labour and materials) is £135,000. Professional fees, contractors’ overheads and profit, insurances and warranties would all be in addition to this. The raw build cost presumes a straightforward job finished to a fairly basic standard; the sort of finish you would expect from a professional housebuilder. In this series we will be looking at how the costs break down for all the various component parts of a house, and looking at the cost implications of choosing alternatives. This house is also featured in Mark Brinkley’s latest book, The Housebuilder's Bible (6th edition).

OTHER COST GUIDES:

• Getting out of the ground
• External Joinery
• Constructing Walls
• Installing Services
• Roofing
• Internal Finishes