What insulation should you use to insulate a room in the roof?
Choosing the right insulation to install on a loft conversion is important, as not only are there regulations to meet, but it can have a huge impact on how much living space and head room there will be once it is complete.
The Building Regulations in England state that pitched roofs insulated at rafter level must achieve a minimum U-value of 0.18 W/m²K. Where possible, it makes sense to target an even lower U-value as this will provide additional energy bill savings over time. In addition to factors such as room height and rafter size, the thickness of insulation you will need to meet these targets is affected by the type of insulation you use and its thermal conductivity. The lower the thermal conductivity of an insulation material, the more effective it is at stopping heat escaping. This means you can fit slimmer thicknesses with no loss of insulating performance.
Suitable for insulating all parts of a room in the roof, from the roof pitches and flat ceiling to the dwarf walls, our Eco-Versal
insulation achieves a thermal conductivity of 0.022 W/mK which is much lower than other options such as glass mineral fibre rolls. This allows you to maximise floor to ceiling heights without compromising on U-values.
To help you decide what thickness of Eco-Versal boards to use, take a look at our U-Value calculator
. This no-jargon tool will help you to quickly determine the build-up you need to achieve the project’s targeted U-value.
What to check before fitting room in the roof insulation
1. Does the design meet the requirements for a room in the roof?
To be classed as a room in the roof, it must be accessible by a permanent staircase which you can safely walk down facing forwards (i.e. no loft ladders). Additionally, the height of 50% of the common walls (those that are a continuation of the external wall of the storey below but excluding gable ends) should not be over 1.8 m tall, otherwise it is classed as adding another storey.
2. Make sure the roof structure is sound and there are no signs of damp
Damp and mould can cause serious problems, both to the structure of a roof and to the health of the people who live underneath it. Thoroughly check the rafters, joists, and walls for any sign of moisture such as damp patches, condensation or soft, flaking, crumbling, or rotting wood. The roof and sarking felt should also be inspected for any holes or weak points such as missing tiles.
These issues will need to be fixed before installation begins.
Any electrical connections housed in the loft should also be checked for safety. Also be on the lookout for any signs of wildlife, especially bats or other legally protected species. If present, then you must get some advice from the relevant nature conservation agency before starting any work.
3. Check the age of the property and any planning restrictions
Obviously, it’s important to ensure the owner has planning permission before carrying out any work. We would also recommend that any insulation retrofit is carried out following the process in PAS 2035, including appointing a TrustMark Retrofit Coordinator
4. Choose between cold and warm roof approaches
When installing insulation as part of a room in the roof, you have two options:
• fit insulation between and above the rafters (warm roof); or
• fit insulation between and below the rafters (cold roof).
The between and above approach is known as a ‘warm roof’ as the insulation layers help to keep the structure warm. This makes detailing more straightforward and reducing the risk of thermal bridges which can allow condensation to form. As the insulation layer is fitted above the rafters, however, this approach can only be used if the existing roof covering is being stripped back.
Whilst a cold roof approach requires additional care, particularly in terms of ventilation and thermal bridging (as we discuss in the next two points) it can be completed without having to replace the existing roof covering. As a result, it is usually the more cost-effective option.
In the rest of this blog, we will be focusing on the cold roof approach.
5. Assess ventilation
Installing any insulation in the roof space will restrict the natural flow of air through the roof space. So, it is important to make sure there is enough ventilation to prevent humid air building up inside the room/s and causing condensation. For rooms in the roof, this is done by providing a path for the air to enter at the eaves, flow over the insulation, and out through the top of the ridge. To do this with a cold roof construction, you must:
• leave a continuous 25 mm gap along the length of the eaves. Make sure there’s no material that might block the air flow;
• leave a 50 mm gap between the insulation and the roofing felt. To guarantee enough space is left, it may be necessary to add battens underneath the roof joists; and
• leave a 5 mm wide opening along the length of the eaves. If you are creating a flat ceiling on the underside of the ridge, you can ventilate this void using vent tiles.
Habitable rooms will also need other ventilation strategies, such as opening windows with trickle vents or, if creating a damp environment such as a bathroom, a fan vent. If you need, you can also get air-leakage tests carried out.
6. Take steps to reduce thermal bridging
Any cold spots left in the roof can also lead to condensation and mould. This means it is important to pay attention to detailing around elements such as windows and skylights and at junctions between the wall and roof to avoid any gaps in the insulation layer. It is also important to prevent heat losses through the regular timber bridging of insulation between the rafters, which is why this approach includes a second unbroken layer of insulation to the underside of the rafters.
How to install insulation board between and underneath the rafters (Cold Roof)
1. Measure the gap between the rafters and cut the Eco-Versal boards to the right dimensions using a fine-toothed saw. It is important to measure each gap as these can often vary slightly and the insulation needs to fit snugly. Any small gaps can be filled with expanding urethane sealant.
2. For cold roof constructions, the insulation layer should run continuously out to the eaves (rather than insulating at dwarf walls).
3. The boards should then be pushed in until they are flush with the bottom of the rafters. The ends of the boards should also be tightly butted with no gaps. Make sure you leave a 50 mm ventilation cavity above the insulation and below the sarking felt. The gap should run out to the eaves allowing any moisture that may form on the cold roof structure to be evacuated out of the construction before it condenses. An eaves ventilator should be fitted to ensure the channel remains open.
4. Once this first layer is completed, to prevent thermal bridges, then either
a. Fix a secondary thinner layer of Eco-Versal insulation to the underside of the rafter and cover with 12.5 mm plasterboard; or
b. Fix a layer of our insulated plasterboard, EcoTherm Eco-Liner
5. In both cases, the boards must be laid at right angles to the rafters and supported on all four edges. Noggings should be used across the rafters to provide this support.
6. The boards should be fitted with drywall screws at 150 mm centre. The screws should penetrate the timber by 25 mm deep and be at least 10 mm from the edge of the board. Screw heads should also be driven just below the surface taking care not to over-drive them.
How to install insulation at gable ends
1. Check that the gable end structure is clean, dry and in good condition.
2. Fix pre-treated 50 mm wide x 25 mm deep timber battens at maximum 600mm centres. Horizontal battens should also be fitted at the ceiling and just above floor level.
3. Install Eco-Liner using drywall screws as described above