Matthew Evans at Kingspan Insulation GB explains why getting the insulation right is crucial for loft conversions.

Loft conversions are a great way to unlock extra living space, potentially raising the value of your home by over 20%. When planning a conversion, one of the key early conversations to have with your contractor or architect is around insulation. Factors such as the performance and thickness of the insulation, where it is installed, and the quality of the workmanship can all impact how spacious and comfortable the finished room is and on your long-term energy bills.

Why Insulation Matters For Loft Conversion

The insulation in your home serves a fundamental purpose: slowing the speed at which heat can travel through the floors, walls or roof. In cooler months this helps to keep warm air inside your home more effectively, while in summer it can keep your rooms cooler than the outdoor air for longer (in combination with good practices like shutting windows and closing curtains and blinds during the daytime).

The impact of insulation is particularly noticeable for loft conversions as your entire roof is exposed to the outside, providing a large surface area for heat to either escape or enter your home. Your roof will also typically receive direct sunlight throughout the day, meaning that without good insulation it can quickly heat up.

Insulation can obviously also impact your energy bills, reducing the amount of heating or cooling that is needed to maintain comfortable internal temperatures. This is increasingly important as we switch to low carbon heating systems (such as heat pumps) which perform most efficiently at lower flow temperatures than gas boilers. If heat is allowed to escape, it can mean these systems have to work much harder and could significantly raise bills.

Key Points To Consider

When insulating lofts, the insulation will need to be fitted either between and above rafters (only possible if your tiles are being removed or replaced) or between and below them. This is to ensure there is a continuous layer of insulation which is vital as any gaps in the insulation layer can allow heat to bypass it (known as a thermal bridge). This not only reduces its effectiveness but can also allow cold spots to form which can lead to condensation and damp issues.

When it comes to insulating performance, the key metric is the U-value. This measures how easily heat can travel through a full construction. The lower the U-value, the better the roof/wall is at slowing heat transfer. Building Regulations in England, Scotland and Wales set improved/maximum U-values which your builder should typically either meet or improve on for your loft conversion. These values may vary depending on whether you are retaining the existing tiles (renovating) or replacing them. In some cases, it may be possible to relax these values where meeting them is technically impractical.

The thickness of insulation needed to meet these values will depend on the thermal conductivity of the material being installed. Again, the lower this value is, the more effective the insulation is at stopping heat from passing through. This can allow a thinner layer of insulation to be installed than would be possible with a worse performing product. This could help unlock extra headroom if your insulation is being fitted between and below rafters.

Phenolic boards are some of the thinnest commonly used insulation solutions for pitched roofs with a thermal conductivity as low as 0.019 W/mK. Phenolic insulated plasterboard products are also available which combine insulation, drylining and vapour control in a single layer for below rafter applications. It is important to check the thermal conductivity of the product being installed matches the designs for your home as there are a number of insulation products which look similar but can have very different thermal conductivities.

Your contractor should also consider how the construction will handle condensation risk. In homes, warm moist air will rise into the loft. If this is not effectively released it can condense on cool surfaces such as sarking boards and timbers in your roof, leading to potential damp problems.

In a ventilated construction, this moist air is removed by maintaining a 50mm clear cavity between the insulation and the sarking/tiles. This cavity should be fitted to allow air to flow through the eaves and out through the ridge, carrying moist air away. It is important to check that this 50mm cavity is included in your contractor’s plans, where relevant.

Alternatively, contractors can use a breathable sarking material which allows water vapour to pass through and out of your roof. This avoids the need for an air gap meaning a greater thickness of insulation can be fitted between your rafters, allowing the target U-value to be achieved with a thinner thickness of below rafter insulation – maximising headroom.

Whatever approach is taken, your contractor should take care to eliminate any gaps in the insulation layer during installation. For phenolic boards, this should include carefully measuring and cutting the boards to fit the exact space between rafters, filling any small gaps with expanding foam and using flexible insulation to fill any unwanted gaps at the eaves. Careful installation can prevent potential issues in the future and some manufacturers can provide specific best practice guidance to support this.

Better Results

Getting the insulation right in your loft conversion needs to be a key consideration and not an afterthought. With good planning, the right insulation solutions and careful installation, it should be possible to create a loft
which can be kept comfortable at an affordable cost across the year and has plenty of headroom.

Matthew Evans is director of technical & regulatory affairs at Kingspan Insulation GB