The short answer
- New buildings other than dwellings must hit area-weighted average U-values of 0.16 W/m²K for pitched roofs, 0.20 for flat roofs, 0.21 for walls and 0.21 for ground floors (TGD L 2022, Table 1).
- Material alterations to existing buildings use Table 10: 0.16 for a pitched roof insulated at ceiling level, 0.25 insulated on the slope, 0.25 for a flat roof and 0.35 for other walls.
- Airtightness: TGD L 2022 treats 5 m³/(h·m²) at 50 Pa as the reasonable upper limit, confirmed by a test on completion.
- Spray foam helps on both fronts. It adds insulation without fixings through the sheet, and it seals the laps, gaps and junctions where commercial buildings leak air. As a rough guide, about 86 mm of Purios H closed-cell foam gets a sloped metal roof to 0.25.
What TGD L 2022 covers
Part L of the Building Regulations deals with the conservation of fuel and energy. The Department of Housing publishes the practical guidance in two Technical Guidance Documents, one for dwellings and one for buildings other than dwellings. This guide covers the second one, TGD L 2022, which came into force on 25 October 2022. It applies to offices, warehouses, factories, shops, schools, sports halls, hotels, farm buildings that are heated or conditioned, and most other non-domestic buildings.
TGD L is guidance, not law. The legal requirement sits in Part L of the Building Regulations. If you follow the guidance you are treated as meeting the requirement, and you can meet it another way if you can show that way works as well. In practice designers, assigned certifiers and building control authorities work from the TGD figures, so they are the numbers that matter when you plan insulation work.
For a new building, TGD L works on two levels:
- Whole-building energy and carbon performance. The design is modelled in SEAI's Non-domestic Energy Assessment Procedure (NEAP), normally using SBEMie software. Its calculated energy and carbon performance is compared with a reference building, and a share of the energy has to come from renewable sources.
- Fabric backstops. Whatever the model says, each type of element must not fall below a limiting U-value. The envelope also has to be airtight, and thermal bridging and condensation risk have to be dealt with.
For an existing building, the questions are narrower. Is the work a material alteration, a major renovation or an extension? If it is, which elements have to be brought up to standard? The rest of this guide goes through each case.
Limiting U-values for new commercial buildings
Table 1 of TGD L 2022 sets the maximum area-weighted average U-value (Um) for each type of plane element in a new building other than a dwelling:
| Element | Maximum area-weighted average U-value (W/m²K) |
|---|---|
| Pitched roof | 0.16 |
| Flat roof | 0.20 |
| Walls | 0.21 |
| Ground floors | 0.21 |
Source: TGD L 2022 Buildings other than Dwellings, Table 1. Opening, rooflight and door values are set separately in the document.
Three points trip people up.
They are backstops, not targets
Meeting Table 1 does not make a building compliant on its own. The NEAP calculation for the whole building has to pass as well. Designers often specify better fabric than Table 1 because it is the cheapest way to pass the energy and carbon checks, especially for heated warehouses and industrial units with large roof areas.
"Area-weighted" means averaged across all similar elements
A small area of poorer roof can be balanced by a better roof elsewhere. That does not allow a patchy installation, though. Condensation risk and thermal bridging still apply at every point, and a thin spot in insulation over a cold steel purlin is exactly where mould and drips start.
Profiled metal roofs need careful calculation
The U-value of a built-up metal roof depends on the insulation thickness, the sheet profile, the spacers or brackets and how often they bridge the insulation. When spray foam is applied directly to the underside of the sheet, there are no brackets to bridge it. The fixings and purlins still count, so the designer calculates the finished element to I.S. EN ISO 6946 rather than simply dividing thickness by lambda.
Existing buildings: material alterations, major renovations and Table 10
Most insulation projects in Ireland are on existing buildings: an older warehouse that is being heated for the first time, a factory with a failing roof, a unit being fitted out for a new tenant. TGD L 2022 handles these through several triggers.
Material alterations and works to thermal elements
When you carry out a material alteration, or replace or renovate part of the thermal envelope, the elements involved should reach the values in Table 10:
| Element | U-value to achieve (W/m²K) |
|---|---|
| Pitched roof, insulation at ceiling level | 0.16 |
| Pitched roof, insulation on the slope | 0.25 |
| Flat roof | 0.25 |
| Other walls | 0.35 |
Source: TGD L 2022 Buildings other than Dwellings, Table 10 (selected rows).
The "on the slope" row is the one that matters for most spray foam work. Nearly every industrial and agricultural building has no ceiling. Insulation goes to the underside of the roof sheets, so the target is 0.25 rather than 0.16. That gap is worth several tens of millimetres of insulation.
Major renovation
When more than 25% of the surface area of the building envelope undergoes renovation, it counts as a major renovation. The whole building's energy performance should then be brought up to the cost-optimal level, as far as that is technically, functionally and economically feasible. Re-roofing a large single-storey building can pass the 25% line on its own, because the roof is often the biggest part of the envelope. Raise it with your designer early. It can change the scope from "insulate the roof" to "upgrade the building".
Change of use and extensions
A change of use, such as converting a store into offices or a showroom, and a new extension each bring their own fabric requirements. The guidance on each is in TGD L 2022, and your assigned certifier or building control authority will confirm which applies. We will not guess at a classification for your project. The survey report records the question so your designer can answer it.
Practical tip: ask your designer, in writing, which U-value the insulation must achieve before you ask anyone for a price. A price for "100 mm of foam" means nothing if the element needs 0.25 and 100 mm of that product only gets you to 0.30.
Know your target U-value?
Send it with a few photos of the roof or walls. We will arrange a free survey and give you a fixed price for reaching it.
Air permeability: the 5 m³/(h·m²) figure
Insulation only works if warm air stays inside the building. TGD L 2022 (paragraph 1.3.4.3) gives 5 m³/(h·m²) at 50 Pa as the reasonable upper limit for air permeability in buildings other than dwellings. It is demonstrated by a pressure test on completion, carried out to the ATTMA TSL2 method.
The figure sounds abstract, so here is what it means on site. A pressure test blows air into or out of the building and measures how much escapes through the envelope at a fixed pressure difference. The result is divided by the total envelope area. Warehouses and industrial units usually fail in the same places:
- side laps and end laps on profiled roof and wall sheets
- the eaves, where the roof sheet meets the wall cladding and the profile leaves a gap
- ridge closures and verge flashings
- around roller shutter doors, rooflights and service penetrations
- the junction between the cladding and the ground-floor slab or blockwork
Why spray foam helps with airtightness
Spray foam is applied as a liquid that expands and self-bonds to the surface. On the underside of a profiled sheet it follows the corrugations, fills the laps and seals around fixings. At an eaves or ridge it fills gaps that are hard to close with board or quilt. Insulation and air sealing happen in one layer. With board or quilt, the airtightness line is a separate job of tapes, membranes and closures, and each of those can be missed or damaged.
Spray foam is not a substitute for detailing. Doors, shutters and rooflights still need proper seals, and the test result depends on the whole envelope. If the building is being tested, tell the installer. The survey can then pick out the leakage paths the foam can close and the ones that need other trades.
How spray foam meets the TGD L 2022 U-values
The thickness you need depends on the target U-value and the product's thermal conductivity (lambda, λ). Lower lambda means less thickness for the same result. Duratite Ireland supplies two closed-cell products that are commonly used on commercial roofs and walls:
- Purios H: closed-cell spray foam, λ 0.0224 W/m·K, 35–40 kg/m³, compressive strength of at least 200 kPa, CE marked with a published Declaration of Performance. It uses a new-generation foaming agent with zero ODP and low GWP.
- Enverge Nexseal LE: low-emission closed-cell spray foam, typical published λ 0.027 W/m·K (at 125 mm, EN 14315-1), water vapour resistance factor μ 62.
The table below shows indicative foam thicknesses for common TGD L targets. It uses standard surface resistances and counts the foam layer only. The sheet, purlins, fixings and thermal bridging are left out, so the designer's full calculation will differ. Use it to understand the scale of the job, not to specify it.
| Target (W/m²K) | Where it applies | Purios H (λ 0.0224) | Nexseal LE (λ 0.027) |
|---|---|---|---|
| 0.16 | New pitched roof (Table 1) | about 137 mm | about 165 mm |
| 0.20 | New flat roof (Table 1) | about 109 mm | about 131 mm |
| 0.21 | New walls (Table 1) | about 103 mm | about 124 mm |
| 0.25 | Existing roof, insulation on the slope, or flat roof (Table 10) | about 86 mm | about 104 mm |
| 0.35 | Existing walls (Table 10) | about 60 mm | about 73 mm |
Indicative only: foam layer with Rsi + Rso of 0.14 (roofs) or 0.17 (walls). Not a U-value calculation for a specific building.
Why closed cell for most commercial work
Closed-cell foam is rigid, has a high resistance to water vapour and bonds directly to steel, fibre cement and timber. On a single-skin metal roof it insulates, adds stiffness to the sheet, stops condensation forming on the underside and seals the laps, all in one application. Open-cell foam has its place, mainly in lined, internal and acoustic applications. It is compared in detail in our open cell vs closed cell guide.
Build it up in passes
Closed-cell foam is sprayed in controlled passes rather than one thick layer. The Purios H data sheet gives 10–25 mm per pass. A 100 mm build-up is therefore several passes, each allowed to cure before the next. It is one reason the installer's experience and the site conditions matter as much as the product.
Fire, condensation and the rest of the regulations
Part L is not the only regulation an insulation project has to satisfy.
Fire safety (TGD B)
Every insulation material has a reaction-to-fire classification, declared to I.S. EN 13501-1. TGD B – Fire Safety sets out where materials of each class may be used, and when an insulant must be protected by a thermal barrier or lining. Spray foam is an organic, combustible insulant. Whether it can be left exposed depends on the building's use, its size, where the foam is and how the building is occupied. Your designer or fire engineer makes that call. We supply the product's declared classification and data sheet to support it. We will not tell you foam can be left exposed without that review.
Condensation risk
Adding insulation moves the point where warm, moist air cools down. Put the vapour control in the wrong place and you can create interstitial condensation. Closed-cell foam applied directly to the underside of a metal sheet puts a low-permeability layer on the warm side of the sheet, which is why it controls condensation well on single-skin roofs. Lined roofs, walls with an internal finish and buildings with high internal humidity, such as food processing, pools or livestock, still need a condensation risk assessment using the EN ISO 13788 method.
Ventilation
A tighter building needs planned ventilation. As the envelope gets tighter, the designer should confirm that the ventilation strategy still delivers enough fresh air for occupants, processes and any combustion appliances.
From survey to sign-off: how a compliant job runs
- Tell us the target. Send the U-value your designer, certifier or BER assessor needs, if you have one. If you do not, the survey report notes which TGD L table is likely to apply so you can confirm it.
- Free survey. An approved installer covering your county inspects the substrate, profile, condition, access, fixings, existing insulation and airtightness weak points. On asbestos cement sheets the survey follows the asbestos process first. See our asbestos cement roofs guide.
- Specification and fixed price. You receive a specification naming the product and thickness for each element, the area derivation (including profile allowances) and a fixed price. Nothing is estimated "per square metre, subject to site".
- Installation. Surfaces are cleaned and dried and the area is masked. Foam is built up in passes to the specified thickness, and the thickness is checked as the work goes on.
- Handover. You receive the installation record, the product data sheets and Declarations of Performance, and the manufacturer-backed product warranty documents for your certifier and BER assessor.
Duratite Ireland does not have a branch in every town. We supply the materials and route survey requests to approved installers who cover your county. That keeps the specification consistent wherever the building is, whether it is on the edge of Dublin or in rural Mayo.