What does a buyer check before specifying windows or doors?
They check the whole-window thermal value rather than the glass alone, the air and water tightness classifications, and the security rating where required. Whole-unit performance is what regulations reference, and a glass-only figure will not satisfy them.
A housebuilder is specifying windows that must pass both a regulatory calculation and a site air tightness test.
Reviewed 4 August 2026
Scope and specifications
- Product type and material
- Aluminium, timber, steel and composite frames differ in every property.
- Whole-unit thermal transmittance
- The value regulations reference, not the glass value.
- Air permeability classification
- Directly affects the building's air tightness test.
- Water tightness and wind resistance
- Determines suitability for exposure and height.
- Security rating
- Required by regulation and insurers in defined applications.
- Maximum sizes and configurations
- The design constraint.
- Acoustic performance
- Frequently required near roads and railways.
Documents buyers may need
| Document | What it proves | Where it belongs |
|---|---|---|
| Test report for the window systemAsked at: specification — the primary evidence | Air, water and wind performance under the system standard | Publish it |
| Declaration of PerformanceAsked at: regulatory compliance | Regulated declaration of the product's performance values | Publish it |
| Thermal performance calculationAsked at: building compliance calculation | Whole-unit thermal value for the configuration specified | Publish it |
| Security certificationAsked at: where regulation or insurance demands it | Testing against the security standard the application requires | Publish it |
| Acoustic test reportAsked at: noise-exposed sites | Sound reduction under a named method | Name it, share on request |
| Installation guidanceAsked at: installation — the performance depends on it | How the tested performance is achieved in the opening | Publish it |
| Warranty termsAsked at: purchase | Coverage for frame, hardware, finish and sealed units | Publish it |
Details for a useful enquiry
- Building type and application
- Determines the requirements.
- Performance targets
- Thermal, acoustic and security values required.
- Sizes, configurations and quantities
- The basis of the quote.
- Exposure and building height
- Determines wind and water requirements.
- Finish and colour requirements
- Affects lead time and cost.
- Programme dates
- Windows sit on the critical path of a build.
Questions suppliers ask
Why does whole-unit performance matter more than glass performance?
Because the frame is where heat is lost and where regulations focus. Whole-unit values are what compliance calculations use, and a manufacturer quoting glass values is either unaware of the requirement or avoiding it. Buyers can tell which.
How should installation guidance be presented?
As part of the performance claim rather than as a separate document. Air and water performance depend on how the unit is fixed and sealed into the opening, and the manufacturer who explains it clearly is protecting their own product's reputation on site.
What convinces a large housebuilder?
Consistency evidence and supply capability alongside performance: test reports for the systems actually offered, production capacity, and the ability to hold a delivery programme across a site. Performance gets you considered; supply reliability gets you the contract.
Check which documents and standards your buyer needs; requirements vary by market, customer and contract.
Plact Research. Plact's editorial desk for buyer-evidence research. Industry standards, documents and buyer questions are researched and drafted with Plact's AI tools, then reviewed by Praveen Kavuri before publication. Every claim cites the standard, regulation or public source it rests on.
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