Retrofit insulation for timber frame homes (part one)

Introduction

Besides the continual drive to build new dwellings to ever most stringent thermal performance requirements, running in tandem is the push to improve the existing building stock. This latter is, if anything, arguably the greater problem, due to quantities and life expectancy. Successive governments have, in recent years, instigated a variety of schemes to provide incentives to homeowners to carry out retrofit improvement work, including insulating cavity walls and roof voids, replacing inefficient central heating and hot water boilers, and fitting renewable energy generating measures such as photovoltaic panels.

Dwellings that include uninsulated structural timber framing for either the wall or roof construction can present particular challenges, including:

• The avoidance of interstitial condensation and associated timber decay
• Retrospective insulation methods can be more limited in variety, more difficult to install and hence more expensive.

Furthermore, the choice of insulation method needs to be made wisely, in order to avoid further issues such as thermal or moisture bridging, or blocking-up of cavity ventilation. Conversely, summertime overheating due to solar gain can be exacerbated in certain circumstances.

The current schemes available in the UK include the widely-publicised Green Deal, and the Energy Companies Obligation (ECO). However, timber-framed houses are currently falling through something of a loophole, and the dormer bungalow is a particular victim of this problem. There are many of this type of property in the UK, constructed at all periods but a typically popular tranche date to the 1960s and 1970s. The focus of this two-part article is to discuss the various insulation solutions available for this type of property, as an example.

The Green Deal

The Green Deal was launched at the beginning of 2013, as a loan scheme for householders to ‘borrow’ money to invest in thermal improvements to their homes, and pay back the loan through the savings made in their reduced energy bills. Take-up of the scheme never achieved the targets originally anticipated and, despite a kick-start in June 2014 in the form of the Home Improvement Fund, the Green Deal was closed to new applications in July 2015. Similarly, the Home Improvement Fund closed at the end of September 2015.

Energy Companies Obligation (ECO)

An alternative to the Green Deal is the Energy Companies Obligation, which compels the 'big six' energy providers (British Gas, EDF Energy, E.on, Npower, Scottish Power and Scottish & Southern) to install energy saving measures free of charge, and claim the costs back from the government. This is broken down into three separate areas for eligibility.

People living in hard to treat homes - Carbon Emissions Reduction Obligation (CERO): promotes the installation of solid wall and hard-to-treat cavity wall insulation alongside packages of measures. 

People living in low income areas - Carbon Savings Community Obligation (CSCO): promotes the installation of insulating measures and connections to district heating systems in areas of low income and rural areas. Significantly, this is postcode-based.

People in receipt of qualifying benefits - Home heating Cost Reduction Obligation (HHCRO): promotes the installation of measures, including the repair and replacement of boilers, to homes in receipt of certain benefits, to reduce the overall cost of space heating. The benefits include ‘state pension credit’.

The following points should be borne in mind with regard to ECO funding:

• No funding is specifically available for retired persons, unless they are in receipt of qualifying benefits or live in qualifying properties
• Applicants do not need to be customers of one of the ‘big six’ providers, in order to be eligible for their assistance
• Residents who live in flats, need to be able to coordinate works with their neighbours
• Timber- or metal-framed houses, or those constructed from natural stone or concrete (not cavity wall construction) generally aren’t eligible
• Damp problems need to be treated before being considered eligible
• Scaffolding costs may be excluded.

Green Deal and ECO in practice

The Green Deal’s lack of popularity has been the source of much speculation but the fact that (in contrast to previous incentive schemes) it was a loan and not a grant may have been a significant factor; furthermore, householders wishing to sell their property before the payback period had elapsed would have to transfer the repayment to the new purchaser.

Moreover, upon selling a house the vendor must provide an Energy Performance Certificate (EPC) which is accompanied by a schedule of recommended improvements and their estimated costs. According to the Energy Saving Trust, however, the presence of these recommendations on the EPC does not necessarily imply eligibility for funding. Even if funding is theoretically available, the ‘big six’ energy companies are under no obligation to carry out those improvements. For while they are required to improve the UK’s housing stock, they are at liberty to choose which properties they assist, in order to meet the targets set by the government. It is perhaps not surprising therefore that they are more likely to carry out the simplest and fastest measures, and those requiring more complex remedial measures are likely to experience difficulties in finding an installer prepared to carry them out. Needless to say, the retrospective installation of insulation into a timber-framed house falls within the schemes’ definition of ‘hard to treat’ properties.

Summer overheating

Another factor to be mindful of is avoiding increasing the risk of overheating in the summer. This can be caused by a combination of factors, including:

• Warm air rising from the ground floor and being trapped upstairs, particularly by high levels of insulation, combined with low thermal mass
• Dark-faced flat dormer roofs, if uninsulated, give increased exposure to solar heat gain
• Overheating is compounded if the ventilation is inadequate – hot air that builds up during the day should be expelled at night, when the outside air temperature drops lower than that indoors.

If this is an issue, then there are a few relatively low-cost solutions available, without having to resort to air conditioning:

1. Consider installing a whole-house ventilation system. This comprises a large extract fan which is mounted in the upper-level roof void (subject to access restrictions), with flexible ducting leading from extract points in the ceilings of each of the upper floor rooms and landing. It should be switched on during hot days and evenings, to encourage air movement by drawing cooler air up through the house, and expelling the hot air out through the roof. The outlet grille should ideally be fixed in an east-facing elevation, since prevailing winds and rain in the UK come from the west, and would hamper performance or create obtrusive noise).
2. Fit white reflective-backed ‘blackout’ blinds to the insides of all windows on the south-facing upper-floor. They are relatively inexpensive, and can be left down during the day to reflect heat back out.
3. An even more effective solution is to fit ‘drop-arm’ awnings outside the south-facing upper-floor windows. These are the most effective because they keep the heat off the glass in the first place (once the heat has passed through the glass, then even with internal blinds, some of that heat will never be reflected back outside – and the glass in itself acts as a low-grade radiator). Check if they need planning permission, however.
4. If the existing windows need replacing, then some glass manufacturers offer ‘solar control’ glass which reflects more of the sun’s heat away. It is marketed principally for conservatories in particular, but is equally beneficial to reducing overheating in homes in general.

Insulation

Perhaps the first thing to be said is that there is no miracle cure It simply isn’t possible to retrofit cavity wall insulation in the same way as in a masonry wall; insertion holes would need to be drilled between each timber stud (which are typically spaced between 400mm and 600mm apart), because the studs would block the insulant being sprayed in, from spreading within the cavity. Having said that, the author is aware of at least one company offering a technique that uses this type of treatment. The foam injection system originates from the USA and Canada, and first came to prominence in the late 1970s and early 1980s for masonry cavity wall insulation. However, at that time it attracted negative press owing to concerns including toxic fumes entering the dwelling, and causing discomfort and irritation in some documented cases. The product gradually fell out of common use in favour of inert dry flake or ball insulants. Watch-points to bear in mind when considering specifying this type of treatment include the following:

1. On a practical level, it can make future renovation work more difficult and costly, as the foam expands within the void; bridging the inner and outer faces of the cavity (a particular concern where the timber frame is faced timber weatherboarding), and can increase the difficulty of other remediation or repair work such as the replacement of water or heating pipes, or electrical cables
2. It is still formaldehyde-based, and has no UK certification to date (at the time of writing this article). According to one manufacturer’s website its product is ‘currently being tested’ to BS EN 12667 but this is only in respect of its U-value (i.e. its thermal insulation properties), so has nothing to do with any other performance properties such as breathability or water resistance (bearing in mind the question of bridging the cavity, again)
3. Approved Document C of the Building Regulations paragraph 5.15d states that urea-formaldehyde foam should conform to BS 5617, and should be installed in accordance with BS 5618 – again, compliance with these standards should be sought from the manufacturer, as should the existence of a British Board of Agrément (BBA) certificate, which should be considered as a minimum requirement for construction materials in the UK
4. Paragraph 5.15e of Approved Document C says that BS 8208-1 gives guidance on moisture transition to the interior of a wall construction, caused by retrofitted cavity insulation, but this only covered masonry cavity walls, and the standard has now been withdrawn
5. BS 5617 and BS 5618, mentioned above, only cover masonry cavity walls, i.e. walls with a brick or block or stone outer leaf – not framed walls with lightweight cladding or boarding on them
6. Sprayed expanding foam insulants (even if they do have a BBA certificate), inhibit the breathability of the structure, which can increase the risk of dampness, interstitial (in-cavity) condensation, and mould growth
7. The National Federation of Roofing Contractors currently doesn’t recognise retrofit foam insulation to timber framed structures as a discipline of the industry
8. Building Research Establishment (BRE) Information Paper 25/82 says that it’s ‘not recommended’ for non-masonry inner leaf houses (i.e. timber frame), where the thin linings are more vapour permeable. Ventilation can overcome this while the foam cures, but it is still nevertheless an important consideration particularly where householders may still be resident within the property at the time of installation.

So having considered the principles of fitting insulation retrospectively to timber-framed dwellings, what other options are available? In summary, any remedial work you do will be relatively invasive. The next article will discuss in detail the different options for fitting insulation in timber-framed dwellings retrospectively, with a focus on the relevant performance requirements of the Building Regulations.

Next: Retrofit insulation for timber frame homes (Part two)  

Useful websites

• The Energy Saving Trust
• Green Deal Oversight & Registration Body
• Gov.uk – Green Deal page
• National Federation of Roofing Contractors (NFRC)
• BS EN 12667 Thermal performance of building materials and products – Determination of thermal resistance by means of guarded hot plate and heat flow meter methods – Products of high and medium thermal resistance, 2001
• BS 5617 Specification for urea-formaldehyde (UF) foam systems suitable for thermal insulation of cavity walls with masonry or concrete inner and outer leaves, 1985
• BS 5618 Code for practice for thermal insulation of cavity walls (with masonry or concrete inner and outer leaves) by filling with urea-formaldehyde (UF) foam systems (incorporating amendment Nos. 1 and 2), 1985
• BS 8208-1 Guide to assessment of suitability of external cavity walls for filling with thermal insulants. Existing traditional cavity construction (AMD 4996) (No longer current but cited in Building Regulations), 1985
• British Board of Agrement
• BRE Information Paper 25/82 Formaldehyde vapour from urea-formaldehyde foam insulation, 1982