Tips for roof insulation

In the previous article, I identified key performance criteria which may be used to help pinpoint which insulation should be used in each particular area of a building. Once we have identified which insulation best suits our requirements, the next step is to decide how much (i.e. what thickness) insulation is required.

Technical Guidance Document Part L of the building regulations sets out the minimum standards of insulation required for each element of the building fabric. They stipulate the U value required for each external element of the building envelope. Technical Guidance Document Part L is split into two primary sections:

• Section 1: New Dwellings
• Section 2: Existing Dwellings

There is a separate Part L document specifically published for buildings other than dwellings which will not be covered in this article.

The latest Technical Guidance Documents L are available on the Gov.ie website:

https://www.gov.ie/en/publication/d82ea-technical-guidance-document-l-conservation-of-fuel-and-energy-dwellings/

The principal requirement of Technical Guidance Document Part L (TGD Part L) 2008 is for a 40% reduction in energy demand and carbon dioxide emissions associated with heating, domestic hot water and lighting. Building designers calculate compliance using DEAP, the Dwelling Energy Assessment Procedure software package created as a result of Ireland’s obligations under the Energy Performance of Buildings Directive.

TGD Part L includes a reference house, setting out U-values for each main element of the building – roofs, walls, windows and floors. These U values are summarised in Table 1.

Under the new Part L, compliance cannot be met simply by copying the reference house values. Using DEAP, the primary energy consumption figure of the proposed building is calculated, and divided by that of the reference house, resulting in the energy performance coefficient (EPC).

To demonstrate that an acceptable primary energy consumption rate has been achieved, the calculated EPC of the dwelling being assessed should be no greater than the maximum permitted energy performance coefficient (MPEPC). This MPEPC is 0.6. Further information concerning DEAP can be assessed on the SEAI website.

Table 2 outlines the key U value requirements for existing buildings. As outlined in previous articles, reducing thermal bridging and air infiltration are essential to attaining an efficient energy rating and building design. So while attaining a low U value is critical, these two points should not be ignored.

It should be kept in mind that TGD Part L of the building regulations is to be reviewed towards the end of this year 2010. While TGD Part L provides guidance they are by no means considered best practice. To achieve best practice (i.e. Passiv Haus levels), much higher levels of insulation and attention to detail is required.

Table 1 Maximum elemental U-value (W/m2K) for New Dwellings (correct as 2010)

Table 2 Maximum elemental U-value (W/m2K) for Existing Dwellings (correct as 2010)

The following section outlines a brief synopsis of the type and thickness of insulation commonly used to attain the U value’s as set out in Technical Guidance Document Part L.

Providing guidance regarding every possible detail is outside the scope of this article, therefore to begin with, I will provide guidance regarding the most common details used to insulate roofs in new dwellings. Where existing dwellings are been substantially thermally upgraded, the levels of insulation outlined in the next section may be used as guidance.

1) Insulation of pitched roof:

As homeowners seek to maximise the living area within buildings, it has become much more common to inhabit the attics of our homes.

As Table 1 indicates, the U value required for this type of roof in a new dwelling is 0.2W/m2K. The most efficient means of attaining this U value is to combine preferably an external insulation or internal insulation layer with an insulated layer between the rafters as illustrated in figures 1,2 and 3 below. The combination of an internal or external insulation with the insulation between the rafters ensures that the required U value is attained and that repeating thermal bridges through the structural rafters are eliminated.

There are a range of external insulation products now available on the market which may be applied as an external roofing insulation sarking board. One such product is GUTEX Multiplex Top or GUTEX Ultratherm. These boards are both very effective thermal and acoustic insulants as well as been exceptionally vapour permeable. This is an important consideration to offset the risk of condensation occurring between structural rafters over the lifetime of the building.

The typical Irish rafter is approximately 7 Inches or 180mm deep. On such a construction one means by which the required U value can be attained is by combining 22mm of GUTEX Multiplex Top woodfibre softboard externally with 180mm of a fibre insulation with a thermal conductivity of at least 0.038W/mK, such as Thermo Hemp natural insulation.

Figure 1: Insulated pitched roof including external insulation

Figure 2: Insulated pitched roof including internal insulation

Figure 3: Insulated pitched roof including internal insulation

2) Insulation of horizontal roof/attic

Where a homeowner does not intend to live in the attic space it is important to ensure that adequate insulation is fitted between and over ceiling joists.

As table 1 indicates, the U value required for a horizontal ceiling in a new dwelling is 0.16W/m2K. The most efficient means of attaining this U value is to combine insulation above the joists with an insulated layer between the joists as illustrated in figures 4 and 5 below. The combination of an insulation layer between and above the joists ensures that the required U value is attained and that repeating thermal bridges through the structural joists are eliminated (see figure 4 and 5).

Figure 4: Insulation between ceiling joists

Figure 5: Insulation between and over ceiling joists

When insulating at ceiling level, wiring needs to be dealt with safely. Wires should be kept above the insulation but not stretched if they don’t comfortably reach. An electrician will be able to re-route any problematic wiring. If any wires are installed below a sarking/crawling board the position of these wires should be noted for future reference.

Particular care should be taken when installing recessed light fittings. Measures should be taken to offset the risk of insulation having been installed directly on top of recessed lights. This can lead to over heating and damage to both the recessed light and the insulation. Penetrations in the vapour control layer in the ceiling to accommodate recessed lights can also significantly increase the risk of condensation in the attic.

A solution for this detail may be to install a recessed light protector, such as the Optime Downlight Housing unit supplied by Ecological Building Systems.

All pipe work and tanks in the loft space should also be insulated correctly since there will no longer be the same amount of heat escaping into the loft space to protect pipes from freezing.

There are a range of insulation products now available on the market which may be applied above the joists as a sarking insulating board. One such product is GUTEX Thermosafe Homogen. As attic spaces are often used as a storage space this may be worth considering. These boards are both very effective thermally and also feature a high compressive resistance. This is an important consideration to offset the risk of compressing the insulation over the lifetime of the building. It is recommended to apply a supporting layer beneath the Thermosafe Homogen (i.e. 18mm OSB sheathing) as well as a layer of OSB/Ply sheathing above the board to prevent damaging the Thermosafe over the life of the building. Such an application is illustrated in figure 6.

The typical Irish ceiling joists in new buildings are generally 9 Inches or 225mm deep. With such a construction, one means of attaining the required U value is by combining 40mm of GUTEX Thermosafe Homogen woodfibre softboard externally with 225mm of a fibre insulation with a thermal conductivity of at least 0.038W/mK, such as Thermo Hemp natural insulation.

Figure 6: GUTEX Thermosafe Homogen applied above ceiling joists in an attic

Minister Gormley is expected to further improve the energy performance requirements of Part L by 60% relative to current requirements by the end of this year. This will mean that buildings will have to be constructed with even more focus on increasing insulation levels, reducing thermal bridges and high levels of airtightness than that currently required.

It should be remembered that the Technical Guidance Document Part L sets out the minimum requirement for new and existing buildings. With rising energy costs, security of energy supply and the range of benefits insulation provides, why aim to just meet these requirements when with some minor amendments and attention to detail we can easily surpass them.

The follow up to this article will focus on options to insulate external walls in timber frame and masonry constructions.

Niall Crosson
Technical Engineer BTech, MEng Sc, MIEI

Pictures and graphics provided courtesy of Ecological Building Systems – 2010