by Anthony Lymath
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. 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.
The concluding part of this two-part series (see part one, if you missed it) examines the insulation options in more detail, with a focus on Building Regulations performance.
Gable walls and dormer cheeks
For gable walls that are faced in masonry with a timber frame to the inner leaf, the available types of insulation solution can be determined by reference to Diagram 12 and Table 4 of Approved Document C of the Building Regulations. Diagram 12 gives a ‘driving rain exposure zone rating’ for each part of the British Isles, while Table 4 lists the maximum ratings for different types of wall construction.
Dormer cheeks are typically faced in a lightweight cladding such as timber weatherboarding, or lead (on a plywood sheathing), which is fixed back to a timber framework. Gable walls can also be faced in a similar way, with tile hanging being another common treatment. For these types of construction, reference should be made to paragraph 5.17 of Approved Document C of the Building Regulations. This says that a framed external wall needs its cladding to be separated from the insulation (or sheathing) by a ventilated and drained cavity, with a vapour-permeable membrane. Full-fill insulation products (such as injected insulants) are therefore generally unsuitable as a retrofit solution, if there is no such separating cavity and the cladding is not being removed. Under these circumstances, the options available are:
- ‘Dry line’ the interior of the rooms, by fixing an insulation-backed plasterboard to the inside of the existing plasterboard (on the ‘outside’ walls only). This will reduce the size of the room by typically 50mm to 75mm on each side. Radiators will need to be re-hung; window cills will need to be extended; redecoration will be needed
- Remove the existing plasterboard (from the ‘outside’ walls) to retrofit insulation between the timber studs – in which case the available thickness of insulation will be limited to the stud depth minus 50mm, to maintain the requisite minimum air gap (on the ‘cold’ side of the insulation) for ventilation. Use foil-faced (sometimes known as foil-backed) plasterboard, to provide an internal vapour barrier/vapour control layer
- Remove the external weatherboarding to retro fit, as above – in which case insulation can fill to the full stud depth. Then fix a breathable membrane on the cold side of the insulation to act as breather membrane/ wind and weather barrier, and 50mm thick counter battens for fixing new weatherboarding (PVC-U may be considered in lieu of timber for longevity and reduced maintenance), which will provide a drained and ventilated cavity behind the boarding
- Over-clad the weatherboarding with EIFS insulation and a thin-coat render system, but this will need adequate ventilation holes, and may also need planning permission.
However, the advantages of removing the external weatherboarding, for fitting new insulation, are that:
- The condition of the existing structural timbers can be inspected and treated
- Fitting new cladding will remove the need for maintenance for a number of years – particularly if PVC-U cladding is used, as mentioned above. The same argument applies to dormer roofs, which are discussed below.
Lay 250mm minimum of insulation quilt over flat ceilings in the roof voids, with one layer between the joists and two more layers over the top, perpendicular to the direction of the joists. Similarly, where upper floor rooms have walls open into the roof voids, fix rigid insulation boards between the wall studs, with more insulation fixed over the top to counteract cold bridging from the studs themselves.
Flat roofsThis is a similar situation to the walls, as described above, with the options being:
- Remove the existing plasterboard to insulate from below (but only to a reduced depth, in order to maintain the 50mm air gap above)
- Line the underside of the roof/ dormer ceiling with an insulation-backed plasterboard, as described previously
- Remove the roof covering (three-layer felt has a maximum life-expectancy of 15 years, and is commonly overdue replacement in renovation projects) and timber boarding, then fill the gaps between the rafters with insulation. Fix 50mm counter battens over the top of a breather membrane, then 12mm thick ‘WBP’ or ‘marine quality’ plywood, and finally fix a new roof covering. GRP is a durable solution and has the advantage over flexible membranes of being seamless (since joints are the most likely locations for defective workmanship, or product failure). Specify a pale colour, to reflect solar heat effectively (refer to part one of this article for a discussion on summertime overheating).
These are similar in treatment to dormer cheeks, as described above. The options include:
- ‘Dry line’ the interior of the rooms, by fixing an insulation-backed plasterboard to the inside of the existing plasterboard (on the ‘outside’ walls only). This will reduce the size of the room by typically 50mm to 75mm on the slope. Redecoration will be needed
- Remove the existing plasterboard (from the ‘outside’ walls) to retrofit insulation between the timber rafters – in which case the available thickness of insulation will be limited to the rafter depth minus 50mm, to maintain the requisite minimum air gap (on the ‘cold’ side of the insulation) for ventilation. Use foil-faced plasterboard, as before
- Remove the roof tiles (or slates) to retro fit, as above – in which case insulation can fill to the full rafter depth. Then fix a breathable membrane on the cold side of the insulation to act as breather membrane/ wind and weather barrier, and 50mm thick counter battens for fixing new (or salvaged) tiles or slates.
Any ‘renovation’ of a ‘thermal element’ needs to comply with section 5 (paragraph 3.3d) of Approved Document L1B of the Building Regulations. This includes replacement weatherboarding, or replacement plasterboard, or adding insulation, to part of the ‘thermal envelope’ of the house; in other words the habitable, heated rooms. This work is notifiable by law, either by submitting a ‘Building Notice’ to the Local Authority Building Control, or through an Approved Inspector. However, it is NOT notifiable if the work is carried out by a ‘competent person’ as registered under an approved government scheme (note that loft insulation, namely quilt over the horizontal ceiling joists, is exempt, but only if done on its own, i.e. in isolation, and voluntarily rather than in compliance with any regulations).
The thermal performance of the building fabric needs to be considered. At the time of writing, the U-values for ‘upgraded’ thermal elements, carried out through ‘renovation’ (which includes overcladding or drylining) are:
- 0.55 W/m2K for a wall, complete with insulation in the cavity
- 0.30 W/m2K for a wall, complete with insulation on either the inside or the outside of the wall structure
- 0.16 for a pitched roof, complete with insulation between the horizontal ceiling joists
- 0.18 for a pitched roof, complete with insulation between the sloping rafters
- 0.18 for a flat roof.
Note that these are for the entire element, and not the insulation itself. The following examples give an indication of the types of work, and types of insulation, that would give the best or worst performance, for the differing remedial work options described earlier in this article:
Assume 12.5mm plasterboard/100mm cavity/25mm softwood weatherboard: U=1.56 W/m2K.
- Remove plasterboard, add 50mm mineral (rock) wool or glasswool: U=0.54 W/m2K
- Remove plasterboard, add 50mm urea formaldehyde foam: U=0.54 W/m2K
- Remove plasterboard, add 50mm rigid EPS (expanded polystyrene): U=0.51 W/m2K
- Remove plasterboard, add 50mm rigid polyurethane foam: U=0.39 W/m2K
- Dryline internally with 12.5/37.5mm rigid polyurethane foam-backed plasterboard: U=0.45 W/m2K
- Remove weatherboarding, add 100mm rigid polyurethane foam, breather membrane, 50mm counterbattens, PVC-U weatherboarding: U=0.23 W/m2K
Existing dormer flat roof
Assume 12.5mm plasterboard/100mm cavity (roof joists)/12mm plywood/3-layer roofing felt: U=2.06 W/m2K.
- Remove plasterboard, add 50mm rigid polyurethane foam: U=0.40 W/m2K
- Dryline internally with 12.5/37.5mm rigid polyurethane foam-backed plasterboard: U=0.48 W/m2K
- Remove felt and plywood, add 50mm rigid polyurethane foam, breather membrane, 50mm counterbattens, 12mm plywood, GRP roofing (for longevity): U=0.22 W/m2K
Existing pitched roof to loft voids
Assume 100mm rafters, roofing felt, concrete tiles: U=3.13 W/m2K.
- Add 50mm rigid polyurethane foam between rafters (you’ll need to maintain a 50mm airgap above, behind the existing roofing felt, which won’t be ‘breathable’): U=0.43 W/m2K
- Add 50mm rigid polyurethane foam under rafters, 50mm rigid polyurethane foam between rafters (maintain 50m airgap above, behind the felt): U=0.23 W/m2K
As can be seen, the choice of insulation material can radically affect the U-value that can be achieved, so although more thermally-efficient insulants will be more expensive to purchase, they will reduce heating bills more effectively.
When deciding on the type of insulant, and nature of remedial work to be done, have a ‘condensation risk analysis’ carried out by the insulation manufacturer. This will indicate if interstitial condensation could be a risk, and can hence be designed-out by either changing the insulation type, or increasing ventilation. This is especially important in pitched roof voids, where old roofing felt won’t be breathable – hence adequate eaves and ridge ventilators will be needed, to maintain air flow.
Retrofitting insulation into an older timber-framed property is inevitably going to be a costly and intrusive exercise, but the benefits are a more durable and thermally efficient dwelling. As has been seen, financial assistance may have been difficult to obtain under the outgoing Green Deal and ECO schemes. At the time of writing this article, no replacement schemes have so far been announced. However, the DECC have provided the following statement to the author:
“[the] Government is committed to creating a more stable, more coherent and more affordable policy framework for home energy in the long term. They will establish that framework, working closely with partners so as to understand their ideas, based on evidence of what works, and the Government will set out their thinking in due course. To help with this, the Government has commissioned an independent review led by Peter Bonfield to look at consumer protection, advice, standards, and enforcement for UK housing energy efficiency. This aims to review the framework that underpins household energy efficiency policy.”
Previous: Retrofit insulation for timber frame homes (Part one)
- The Building Regulations (England, Wales, Scotland and Northern Ireland)
- DECC the Department of Energy & Climate Change
- 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 Agrément
- BRE Information Paper 25/82 Formaldehyde vapour from urea-formaldehyde foam insulation, 1982