Spray Foam For New Build Homes

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Spray Foam For New Builds | Introduction

Part L of the Building Regulations focuses heavily on the conservation of fuel and power. This means that insulation targets and airtightness have become an essential part of any building design. With the 2022 amendment of Part L determining lower U-Values, better airtightness and reduced thermal bridging, it is important to consider which insulation materials are likely to achieve compliance and which will add the required durability to avoid expensive future top-ups or replacements. Spray foam for new builds could be a perfect solution to meet thermal and airtightness targets.

What’s On This Page About Spray Foam For New Builds?

  • What is a U-Value?
  • Why is airtightness important?
  • How to build to enable insulation.
  • What insulation materials should I consider?
  • Why is spray foam insulation the right solution?
  • Will I be able to get Building Control sign off with spray foam?
  • Will I be able to sell the new build if it has spray foam?

What Is A U-Value?

A U-value is the thermal resistance of the layers that make up a building element – for example, a roof, wall or floor. It also includes adjustments for any inhomogeneous fixings or air gaps. 

A U-value value is shown in units W/m²K which represents watts per metre squared kelvin. It is the ability of an element to transmit heat from a warm space to a cold space in a building, and vice versa. The lower the U-value, the better insulated the building element is and the less heat will be transmitted through the building assembly.

A building element’s U-value is essential as there are certain standards that should be reached according to Part L of the Building Regulations. Current regulations require the following U-Value targets:

Roof – 0.11/0.15 W/m2K

Walls – 0.18/0.26 W/m2K

There are two targets with the lower value representing the notional and desired target whereas the higher value is the absolute maximum a U-Value can be in a new build property. The result of lower U-Values is increased structural assemblies, for example, where 150mm roof rafter depths may have been sufficient under older standards, the new targets will likely require deeper structural elements to account for a greater thickness of insulation.

The major contributor towards the overall U-Value is the type and thickness of the insulation material used. To understand how to meet the U-Value, the thermal conductivity of the insulation needs to be divided into the thickness. For example, if an insulation material has a thermal conductivity (lambda value) of 0.032 W/mK and it is installed at 200mm thick, the U-Value of the material will be 0.16 W/m2K before inhomogeneous layers are considered.

For different types of insulation material, the thermal conductivity of the product varies and this can make a big difference on how much space is required within the building assembly to account for the insulation thickness required. Although there can be variables between different brands within the same insulation type, the general thermal conductivity of different insulation materials is:

  • Mineral Wool – 0.032 to 0.037 W/mK
  • PIR Board – 0.021 to 0.026 W/mK
  • Multi-Foil – 0.034 to 0.040 W/mK
  • Open Cell Spray Foam – 0.037 to 0.039 W/mK
  • Closed Cell Spray Foam – 0.025 to 0.027 W/mK
It gets even more complicated than this because different insulation materials are installed in different ways. Take PIR Board as an example, it should be cut and fit between the pitched roof rafters, leaving an air gap of at least 25mm from the central drape of the roofing membrane. Assuming the drape removes 15mm of space, the PIR Board and membrane drape would use up a combined 40mm of space within the roof rafters. Now if we take the thermal conductivity of PIR Board at an average of 0.024 W/mK and divide this into an available thickness of 160mm between 200mm roof rafters, the insulation would provide a 0.15 W/m2k U-Value before accounting for the inhomogeneous roof rafter layer.
Closed Cell spray foam insulation works slightly differently. As it is spray-applied directly to the roofing membrane, it forms part of the structure with no air gaps or voids between it where condensation or moisture may form and become trapped. With Nexseal LE Closed Cell spray foam insulation, a flash coat is applied gently to the membrane to retain the downward drape. From here, it is built up in 50-100mm layers to its final thickness. With a thermal conductivity averaging 0.026 W/mK, the insulation will reach a 0.13 W/m2K within 200mm rafters before inhomogeneous layers are considered. This means that Closed Cell spray foam can provide better thermal values per inch when compared to other insulation materials. 

Why Is Airtightness Important?

Airtightness has become an increasingly important consideration for new buildings. The theory of airtightness address penetrations and gaps within the building that may become an exit point for heat loss and an entry point for cold draughts. A building can be well insulated but if there are penetrations where heat and air can escape from, the performance of the insulation will be compromised.

Air leakage is measured as the rate of leakage per metre of the external envelope per hour at an artificial pressure differential through the envelope of 50 Pascals. This is to simulate higher wind pressure and to ensure the airtightness layer is robust. Building Regulations demands a maximum air leakage of 10m3/hr/m2@50Pa, although most projects aim for a lower figure of 5 to get a Standard Assessment Procedure (SAP) pass.

Passivhaus construction is even more onerous on airtightness with an air leakage rate of no more than 0.6 times its total volume in air per hour when tested at a pressure of 50 Pascals. This is because Passivhaus standards focus heavily on energy efficiency as opposed to green energy generation to meet compliance.

Most insulation materials do not achieve airtightness because they are pre-manufactured and require cutting to size and slotting into building assemblies. This usually leads to gaps and air leakage points which means further airtightness materials are needed. Generally, airtightness layers come in the form of paint coatings or membranes which need to be applied monolithically across the building envelope. This can increase labour time and cost. Conversely, spray foam insulation can achieve airtightness in a single application. Keep reading to find out why.

How To Build To Enable Insulation

There’s many different insulation materials that you can choose when building a new home. these range from mineral wools and PIR boards to multi-foil layers and PUR Spray Foam Insulation. Each will have a different thermal conductivity which may promote or restrict their use depending on the space available within the building structure. This means that the building should be constructed using deep enough floor joists, roof rafters and wall studs and it’ll vary substantially depending on the insulation material used.

Historically, for pitched roof insulation, 150mm roof rafters were the popular choice but as we’ve seen U-Value targets become more stringent, reducing from a 0.18 W/m2K minimum to a 0.15 W/m2K following the June 2022 release of the Part L Building Regulations, 150mm is no longer deemed adequate. To put this into perspective, if using PIR board insulation between the roof rafters, a layer of insulation will also be required above or below the rafter which can reduce internal available space. It is recommended that a “sensible” air gap of between 25mm to 50mm is provisioned between the PIR board insulation and the roof membrane so that there is adequate airflow for moisture to escape.

The presence of an air gap can take up valuable space within the roof rafter assembly, therefore, 150mm rafters will likely lead to around 100-125mm of PIR board insulation which will only achieve a 0.22-0.26 W/m2K U-Value (depending on the type and brand of PIR board manufacturer). This will inevitably mean that a further 40-50mm of PIR board is required under the rafter which is usually insulated plasterboard. The trouble is, this takes up internal room space which may not be favoured in habitable spaces. A benefit of insulated plasterboard or the use of PIR board above or below the rafter is the reduction of thermal bridging through the roof rafters, something Part L of the Building Regulations encourages to address.

We recommend that when you are considering building a new home that you obtain U-Value calculations for the different types of insulation you are interested to use. This can help to plot the design of the building nice and early and avoid any unwanted problems once the structure is complete. All too often we see our clients contact us during the latter stages of the project and whilst spray foam insulation can help to save space, invariably, additional thermal layers are required under the rafters or on the warm side of the stud wall frame.

It is worth bearing in mind that airtightness is an essential part of modern buildings which is why the insulation choice isn’t just about the most thermally efficient but also the product or material that can achieve airtightness without the need for expensive membranes, paints and tapes. Spray foam is applied as a liquid that expands within the cavity void, clinging to the structural frame as it expands. This natural air sealing can help you to comply with more stringent airtightness requirements.

What Insulation Materials Should I Consider?

By now, you will have established that different insulation types have varying thermal performance which may make some unsuitable for your build. There’s merit to choosing any type of insulation material but each has its pros and cons. The thermal conductivity is generally a good gauge for which material you should choose. Mineral wools are generally the cheapest but they have a higher thermal conductivity which means a greater thickness of material needs to be used, therefore, taking up valuable space. Furthemore, mineral wools are not airtight and can shrink and degrade over time which may render them less effective in years to come.

PIR boards and PUR spray foams are the most stable insulation products available as they do not tend to shrink, degrade or reduce in thermal performance over time. With most insulation materials being covered over once installed, it is essential to choose the product that will yield the longest lifespan which makes PIR and PUR foam the favoured options in new buildings. Material wastage, fixing, airtightness and overall thermal performance should also be considered.

Although PIR board insulation has a slightly lower thermal conductivity than PUR closed cell spray foam, the thermal benefits are marginal compared to the air gap requirements with PIR versus the direct to membrane approach of PUR spray foam. What you’ll gain on marginally better thermal performance is likely to be lost by the time an air gap of at least 25mm is provisioned for PIR boards. Additionally, PIR boards come in 2.88m2 slabs which require cutting to the size of the structural assembly and then slotting into place. This can promote wastage of materials and generate mess on site.

Spray foam insulation offers little in the way of wastage and is manufactured in-situ by the installer. It is adaptable and flexible to the requirements of the building and can be used to seal and insulate the most difficult of spaces. With modern buildings coming in all shapes and sizes, spray foam can be particularly beneficial where there are 

Why is Spray Foam Insulation a great alternative.

Spray foam insulation is popular worldwide, and in North America, it accounts for 17% of the entire insulation market. This means that for every $6 spent on insulation, $1 relates to spray foam. In Europe and Ireland, spray foam is becoming increasingly popular and is widely used and accepted. In the UK, we are a little more reserved when it comes to modern construction materials; therefore, some building professionals are unaware of spray foam or reluctant to use it. 

Spray foam insulation works exceptionally well with the stringent requirements of the modern Building Regulations. The increased demand for airtightness of the structural envelope through better insulation and reduced thermal bridging is offset by robust ventilation strategies that ensure moisture and interstitial condensation are handled at the source.

Sprayed in-situ as a liquid which rises and expands to seal the cavity or void into which it’s applied

Will I get Building Control Sign Off With Spray Foam?

When using a BBA or KIWA-certified spray foam brand, the Building Control Officer should accept it. Regulation 7 of the Building Regulations, Clause 1.7 states, “If the declared performance of a product is suitable for its intended use, the building control body should seek to prohibit or impede the use of the product”. Therefore, provided the product certification is relevant to the scenario where it’s to be used, and the insulation design and installation is compliant with the requirements of Part L of the Building Regulations, Building Control should sign it off with no issue.

A Building Control Officer is responsible for ensuring that new buildings, alterations, installations, and extensions meet the regulatory standards of safety, sustainability, accessibility, and design. They usually analyse the proposed type of materials used within the project and cross-reference their correct use, proper design, and implementation against the product/materials Agrément certification. If the system is suitable for the building and is installed correctly, there should be no reason why a Building Control Officer would not sign off on that element of the project as appropriate.

Will I Be Able To Sell The New Build If It Has Spray Foam?

There’s been a lot of stigma and discussion about spray foam insulation in recent years, and this shouldn’t discourage you from using it as part of a new building project. The concerns over spray foam insulation relate to its historic use on older homes with a lack of ventilation and non-breathable roofing membranes. Additionally, closed-cell foams had been used to stabilise roofs where a roofing membrane was absent, raising question marks over the correct use case for spray foam insulation.

The fact is that modern methods of construction promote the use of airtight insulation materials thanks to robust ventilation strategies and generally low-resistance to moisture roofing membranes. The recent Government HSE study into the use of spray foam insulation in pitched roofs determined its use to be suitable when applied directly to low-resistance roof membranes and/or when vapour control layers are incorporated as part of the overall roof assembly.

As with any product or material, it should always be installed in accordance with BBA or KIWA certification. When it comes to insulation, particularly at the pitched roof level, it is essential that the roof assembly is free from moisture and that a condensation risk analysis accompanies the installation to show that the design is compliant with BS5250:2021. Provided the correct paperwork and evidence are in place, the project is signed off by Building Control, and a structural warranty is issued against the building, there should be no problems selling the property in the future.

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