This post will cover the key design considerations when insulating a floor to achieve a good fabric performance.

When it comes to heat loss through the building envelope, floors are fairly low risk.  Only about 15% of heat for an average three bedroom domestic home is lost through floors. This is because heat naturally rises and is lost through other elements such as the roof, walls, doors and windows.  In fact I was reading through some old Part L documents and  up until as recently as 1990, very little insulation was required for ground floors. I could only read this and look amazed as I mentally filed this  information under History.

As alien as it seems that part of a building envelope went with little insulation it highlighted to me a stark contrast in today’s requirements.  The recent changes to Part L 1A for new domestic homes include a second target measuring the energy consumption of a building known as TFEE.  Specifiers are now able to reference a new notional building also introduced for the first time offering guidelines on key areas to focus on to achieve a good fabric performance.  As part of the notional build it requires floors to have the same U-value as a roof 0.13W/m2K.  This establishes the floor as an essential part of the thermal envelope.

There are a number of different floor designs including in-situ ground bearing. This is when the concrete slab is cast directly in contact with sand blinded compacted hardcore. Suspended floors are used when conditions are unsuitable for in-situ methods.  These include suspended beam and block or concrete beams.  There are parts of the building envelope that have habitable rooms over unheated spaces, for example, a concrete soffit or a timber floor.

So, what are the key design considerations when insulating a floor to achieve a good fabric performance?

The perimeter and area ratio

The perimeter and area ratio is not directly linked to energy consumption in the same way as thermal mass and thermal bridging is but it is required when calculating a U-value. This only applies to a ground floor, whether its insitu or suspended. Heat loss can vary considerably from building to building.   As heat is lost mainly around the floor edges the amount lost is related to the size of the floor and the exposed edges. We model this variation with a simple perimeter to area calculation. It means dividing the exposed internal perimeter in linear metres by the floor area in m2 to give the PA. The higher the ratio the more exposed edges a floor has and so the more insulation is required to meet the target U-value.

Thermal mass and energy consumption

This can be understood as fabric energy storage or the ability of the internal floor surfaces to absorb and store energy.  This applies to insitu ground floor slabs. Celotex FR5000 can be positioned both above and below the slab.  The position of the insulation influences the potential fabric energy storage or thermal mass of the floor.

When the insulation is positioned below the slab it means the slab itself is coupled with the internal room temperature increasing fabric energy storage or high thermal mass. This has a higher demand on energy as the surface temperature of the slab takes longer to warm through.  This means the room takes longer to feel warm because when the heating is switched on it has a slow response time. The advantage is when the heating is turned off the slab releases all the stored energy back into the room and this gives for a more constant or even room temperature maintaining feelings of thermal comfort.

Alternatively, Celotex FR5000 can also be installed above the slab.  Celotex will require a 65mm thickness of screed to provide a stable floor substrate for finished floor surfaces. Another option instead of screed is an 18mm – 22mm tongue and groove floating chipboard floor.

This gives for a quicker response time as floors designed this way have a reduced fabric energy storage or low thermal mass demanding less energy or heating. The floor surface heats up quickly and so the room will warm up more quickly too. It also also means the room cools down quickly when the heating is switched off. This is more suited to a house with intermittent heating.

Thermal junctions

The following statement has been taken directly out of Part L 1A

”Thermal Bridges 3.9:  The building fabric should be constructed so that there are no reasonably avoidable thermal bridges in the insulation layers caused by gaps within the various elements, at the joints between elements, and at the edges of elements, such as those around window and door openings.”

Heat loss through thermal bridges and junctions ultimately leads to the internal temperature dropping and a demand of energy as the heating system responds.  Careful design around junctions will reduce paths of heat loss and reducing the demand on heating and energy use leading to a favourable impact on compliance with TFEE.

Design guidance can be found on how to detail floor and wall junctions on the Planning Portal Website Accredited Construction Details.  When following guidance offered by the ACD’s a default psi value can be used within SAP which results in a Y value better than the default value stated in Part L1A.  Again this is favourable when complying with Part L1A

Alternatively a specific psi value can be achieved if the junction design is modelled individually.  This gives an even better psi value for each junction allowing for the flexibility of other metrics required to achieve compliance.  Celotex Energy Assessments are able to model junctions for an individual psi value and indeed offer guidance on how to compliance with Part L1A.

Airtightness and Condensation

Airtightness is another metric included in the Part L1A notional building.  A lack of airtightness presents paths through which heat can be lost or cold air drafts. The causes the internal room temperature to drop and creates a demand on the heating system. It makes sense to where possible limit uncontrolled paths of ventilation.

A timber floor may potentially be subject to increased air leakage and heat loss. Celotex FR5000 is usually cut and friction fitted between timber joists in both a suspended floor and a floor over an unheated space.  A good standard of air tightness is met when Celotex FR5000 is cut and installed flush with the top of the joists and achieves a tight, friction fit between the joists thus minimising drafts and air leakage.

Concrete and beam and block floors are different, air tightness is detailed around the edges by sealing all the gaps around the skirting boards so  there is no air leakage into any small cavities behind the plasterboard.

Concrete and beam and block floors are instead at risk of interstitial condensation more specifically when they are isolated from the internal temperatures by Celotex. This means the surface temperature of the floor structure becomes cold increasing the risk of condensation forming.  This can be prevented by installing a separating layer over Celotex FR5000 before the screed is poured or chipboard floor covering laid.  A separating layer is simply a 500 gauge sheet of polythene.

Its primary purpose is to minimise the risk of interstitial condensation forming on the cold floor structure below Celotex, but it also has two other purposes.  When laid with edges overlapping and sealed it prevents liquid screed migrating between the Celotex board joints contacting the cold slab below to form a path of heat loss or thermal bridge.  It also prevents the chemicals in the wet screed reacting with the Celotex foil facer to form air bubbles which may settle within the screed causing it to weaken.

A separating layer under a T&G floor also has the added benefit of acting as a slip membrane and helps to avoid long term squeaks and creaks.

This month saw the Zero Carbon Hub publish a report on The Performance Gap.  It highlights a number of issues at all points through the build process which impact the actual build having the same energy performance as what has been designed. One of the challenges is knowing how to achieve comfortable living spaces with a low energy demand by simple design, focusing on key areas that impact heat loss through the building envelope the most.

Celotex Specification Support

Our Celotex Technical Centre are able to give advice on how to detail Celotex PIR insulation as part of the thermal envelope including tricky junctions, how to incorporate building services and pipes without compromising the thermal envelope and how different construction materials impact thermal mass are to name but a few common topics discussed.  It all helps to contribute to a building with a good fabric performance which ensures a comfortable living space with consistent room temperatures.

Find out more about how Celotex FR5000 can be used to provide maximum thermal insulation for your flooring project.