U-values and thermal breaks for roof hatches, smoke vents and rooflights: how to compare like-for-like

The reason is nearly always the same. Different suppliers (and sometimes different documents from the same supplier) use “U-value” to refer to different things. Unless the scope is stated, you can end up comparing lid-only, glass-only, and whole-unit figures in the same tender spreadsheet.

This guide shows how to compare like-for-like on roof hatches, AOV smoke vents, and glazed roof access products, using a simple checklist you can copy into enquiries and tender clarifications.

The quick reality check: what people usually mean by “U-value”

In real enquiries, wording varies, but it typically points to one of these:

• “Hatch thermally broken” (is there a thermal break in the lid/upstand/frame?)
• “Thermally broken and insulated” (thermal break and insulation being treated as the same thing)
• “Lid and upstand insulation U-value” (a component figure, not whole-unit performance)
• “Glazed unit has a U-value” (glass-only performance being assumed to represent the whole product)
• “Still have the same U-value?” (a valid challenge, because size and frame proportion can matter)

Practical takeaway: before you compare numbers, force the document to state what the U-value actually refers to.

Step 1: Be explicit about what U-value you are asking for

For roof access products, “U-value” may refer to any of the following:

1. Insulated lid/leaf infill (common on solid roof hatches and insulated vents)
2. Glazing performance (for glazed rooflights and glazed access hatches)
3. Whole unit (assembly) including frame and edge conditions
4. Installed performance including assumptions about upstands, roof build-up, and insulation continuity

If you are doing Part L modelling or an energy consultant is asking, you almost always want the whole unit basis, not a single component value.

Example: component U-values are useful, but they are not whole-unit figures

To illustrate the difference:

• The SRHP75 Standard Roof Hatch highlights 75mm PIR insulation board and quotes 0.26 W/m²K as the insulation core value. This is helpful, but it is not automatically the whole-unit U-value.
• The thermally broken AOV datasheet for SRH-PAOV references 50mm PIR insulation and quotes 0.43 W/m²K as the insulation core value. Again, useful for understanding the build-up, but you should still confirm the basis for any like-for-like comparison.

If you are comparing suppliers, ask them to state clearly whether the declared value is:

• U-value of the insulation core / infill
• Ug (centre-pane glass only)
• Uw (whole unit including frame)
• Installed U-value based on a defined roof build-up and upstand detail

Step 2: What “thermally broken” means (and what it does not)

A thermal break is a design feature intended to interrupt a conductive path, typically through the frame and upstand zone. It can reduce thermal bridging at the perimeter.

However:

• Thermally broken does not automatically mean insulated. You still need to confirm insulation thickness, material, and continuity.
• Thermally broken does not tell you the whole-unit U-value. It describes construction, not the declared performance basis.
• Location matters. A thermal break in one part of the build-up does not automatically mean the entire installed interface is thermally continuous.

If your tender question is “is it thermally broken?”, add the follow-up: “Please provide a typical section detail showing where the thermal break sits through the lid, upstand, and frame.”

Step 3: Rooflights and glazed access hatches add a common twist: Ug (glass) vs Uw (whole unit)

With glazed rooflights and glazed roof access hatches, the most common trap is not the units used. It is the scope of what is being quoted.

• Ug (centre-pane): thermal performance of the glass only. It excludes the frame and edge effects.
• Uw (whole unit): thermal performance of the entire assembly, including frame and spacers. This is the number you need when comparing rooflight products for compliance and energy modelling.

The trap: a supplier headline may focus on “glass performance” or “low U-values”, while the whole-unit performance is higher once the frame is included.

Specifier takeaway: never accept a U-value for a glazed unit without asking: “Is this Ug (glass only) or Uw (whole unit including frame)?”

If you are specifying glazed access products, you may also want to review the Glazed Roof Access Hatches & Rooflights range so you are comparing equivalent product types.

The like-for-like checklist (copy/paste into tender RFIs)

Use this table to standardise supplier responses and avoid lid-only vs whole-unit comparisons.

Do not ignore the installation detail (it can create the thermal bridge)

Even a well-insulated, thermally broken unit can underperform if the roof interface is not detailed and installed correctly.

Surespan SRH/SRHE fitting guidance states that roof hatches are manufactured with an integral insulated upstand incorporating an external fixing flange, and that the foot of the fixing flange should be insulated after fixing the unit before the roof weathering membrane is dressed up the sides and sealed under the weathering lip.

If you are comparing like-for-like, ask suppliers to confirm what their declared value assumes at the flange, upstand, and roof insulation interface.

For downloadable installation guides and typical details, use:

• Roof Hatch Resources
• Smoke Vent Resources

Where U-values tend to matter most for roof access products

• Heated stair cores and conditioned top-floor enclosures
• Plant rooms or areas where the hatch/vent sits directly over a conditioned space
• Glazed access hatches where Ug and Uw can be confused
• Projects with condensation risk sensitivity at cold bridges (frame and perimeter interfaces)

Request a “Thermal Data Pack” (template email you can copy)

If you want to avoid multiple rounds of clarification, ask for a single pack up front:

Please provide a Thermal Data Pack for the following roof access item(s):

- Product type: roof hatch / AOV smoke vent / glazed roof access
- Size(s): structural opening and overall frame (if known)
- Application: over conditioned space / unconditioned roof void
- Required basis: whole-unit Uw preferred (or confirm if value is Ug glass-only / lid-only)
- Upstand/curb: manufacturer-supplied / builder’s upstand (timber / concrete / steel)

Please include:
1) the stated U-value basis and reference size
2) a typical section detail showing the thermal break location
3) the supporting thermal calculation / report basis and inclusions/exclusions
4) any installation notes that affect insulation continuity at the flange/upstand interface

Next step: if you send the above with your project details, our technical team can point you to the correct product data and typical details.

Contact Surespan

Relevant product links (for specification and comparison)

• Roof Access Hatches
• Standard Roof Hatch (SRHP75)
• AOV Smoke Vents
• Smoke Vent with Access
• Glazed Roof Access Hatches & Rooflights

FAQs

Are roof hatches thermally broken?

Some models are described as thermally broken in the lid and upstand construction. For like-for-like comparison, request a typical section detail showing where the thermal break sits through the lid, upstand, and frame.

Is the U-value for the lid or the whole unit?

It depends what the supplier is declaring. Many documents quote insulation core performance (lid/kerb infill) rather than the whole unit. Always ask the supplier to state whether the figure is a component value or whole-unit Uw.

What does “thermally broken” actually mean?

It means a conductive path has been interrupted (typically through the frame/upstand zone). It reduces thermal bridging, but it is not the same as insulation thickness and it does not automatically equal a whole-unit U-value.

For glazed roof access, what should I ask for?

Ask whether the U-value is Ug (centre-pane glass only) or Uw (whole unit including frame). Uw is the number you need for like-for-like product comparison.

Does the U-value stay the same at all sizes?

Not always. Size and frame proportion can change results. Ask for the reference size used for the declared figure and whether bespoke sizes vary.

Does installation affect thermal performance?

Yes. Interfaces at the fixing flange, upstand, and roof insulation continuity can create thermal bridges. Always request the installation assumptions and typical detail alongside declared values.