United Kingdom

GRP windows for UK projects

Pultruded GRP (fiberglass) window frames and complete window units for Part L, Passivhaus and EnerPHit projects, with PHI certificate 2491wi03 (Uw 0.78 W/m²·K), no metal thermal break or steel stiffeners, and DDP quotes that itemize UK duty and VAT.

Office facade with dark window frames in a repeating grid

Why GRP frames suit UK energy targets

UK window specification is being squeezed from two sides. Part L in England limits replacement windows to a U-value of 1.4 W/m²·K and uses 1.2 for the notional new dwelling, and the Future Homes Standard tightens the envelope further. Passivhaus and EnerPHit projects, common in social housing and education, work to the PHI component criterion of 0.80.

Aluminum reaches those numbers only with elaborate thermal-break assemblies, and uPVC sashes carry steel stiffeners that conduct heat. GRP, the British term for pultruded fiberglass, conducts about 0.3 W/m·K against 160 for aluminum and needs neither. Our 90-series window is certified at U_w 0.78 W/m²·K (PHI component certificate 2491wi03, phB class).

The import sums differ for GRP too. It is a glass-fiber composite (HS 3925.20 or 7019), outside the trade remedies on aluminum extrusions and outside the UK CBAM planned from 2027 for iron, steel and aluminum. UK customs duty and 20% import VAT are itemized in the DDP quote, so your quantity surveyor sees the landed cost before ordering.

UK projects choose the supply model: complete factory-glazed GRP window units, or the pultruded profile set (frame, sash, mullion, transom and glazing bead with EPDM gaskets) for a UK fabricator to assemble. GRP is a thermoset, so anthracite and other dark RAL finishes do not soften the frame in strong sun as they can with uPVC.

Part L, Passivhaus and EN 14351-1 documents

What a UK specification usually asks of the window, and the document we supply for each requirement.

Requirement	UK standard or threshold	What F1 provides	Documentation
Replacement windows	Part L: Uw ≤ 1.4 W/m²·K	Series and glazing chosen for the target	U-value calculation on request
New dwellings	Part L notional dwelling: Uw 1.2	Series and glazing chosen for the target	EN ISO 10077 calculation on request
Passivhaus / EnerPHit	PHI component criterion: Uw ≤ 0.80	90-series: 0.78, certificate 2491wi03	PHI certificate (published)
Type testing	EN 14351-1 (air, water, wind)	Type testing per project	Test reports and declaration of performance on request
Product marking	CE or UKCA	Documents for the supplied configuration	CE / UKCA documents on request
Trade costs	UK CBAM from 2027: iron, steel, aluminum	GRP falls outside its scope	HS classification in the quote
Architectural finish	AAMA 2604 / 2605	Powder coating in RAL colors, including anthracite	Coater report on request

Free tool · Passivhaus presetCheck the U-value of a GRP window Opens the U-value calculator with a 90-series passive house build-up. Change the series, size and glazing, compare Uw with Part L (1.4 and 1.2) and the PHI 0.80 criterion, then ask for a DDP UK quote.

From FengDu's plants to your UK site

One quote covers the frames, freight, UK duty and import VAT, so the landed cost is known before the order.

Step 1
RFQ and DDP UK quote
Send a drawing or window schedule, the quantity and the delivery region. We reply within one business day, then quote DDP in GBP or USD with UK duty and 20% import VAT itemized, the HS classification and an estimated delivery date.
Step 2
Production and documents
Window projects take 6–12 weeks in FengDu's production network. The document set includes PHI component certificate 2491wi03, plus EN ISO 10077 calculations, coating reports and EN 14351-1 type-test reports on request for your configuration.
Step 3
Ocean freight and DDP delivery
Ocean transit takes about 30–38 days to Felixstowe, Southampton or London Gateway. Sites in London, the Midlands, the North and Scotland are served by road from the port. The quote gives an estimated delivery date for your site.

Frequently asked questions

What are GRP windows, and are they the same as fiberglass or FRP windows?

Yes. GRP (glass-reinforced plastic, or glass-reinforced polymer) is the usual British term for what North America calls fiberglass or FRP. A GRP window frame is a pultruded profile of continuous glass fiber in a thermoset resin. It conducts about 0.3 W/m·K of heat, against 160 for aluminum, expands at a rate close to that of glass, needs no steel reinforcement and does not corrode. F1 Composite supplies GRP window and door systems in nine series from 50 to 140 mm frame depth, as profile sets for fabricators or as complete factory-glazed window units.

Do GRP windows meet Approved Document Part L and the Future Homes Standard?

They can. Part L in England limits replacement windows in existing dwellings to a U-value of 1.4 W/m²·K and uses 1.2 for the notional new dwelling, and the Future Homes Standard tightens the envelope further. The 90-series holds PHI component certificate 2491wi03 at Uw 0.78 W/m²·K. For the other series the whole-window value depends on size and glazing, so check it in the U-value calculator or ask us for an EN ISO 10077 calculation. The GRP frame needs no metal thermal break and no steel stiffeners.

Are GRP windows suitable for UK Passivhaus and EnerPHit projects?

Yes. F1's 90-series GFRP-PU frame holds PHI component certificate 2491wi03 at Uw 0.78 W/m²·K, below the 0.80 W/m²·K component criterion UK Passivhaus designers work to. Social housing and education lead UK Passivhaus work, and Scotland plans a Passivhaus-equivalent standard for new homes. The same 90-series system was supplied for Qinling Station in Antarctica; the certificate covers the cool-temperate climate zone, so extreme-cold sites need project-specific checks.

What about CE marking, UKCA, and import duties for GRP windows into the UK?

Windows are construction products. EN 14351-1 type testing for air permeability, watertightness and wind resistance supports the declaration of performance, and CE or UKCA documents for the supplied configuration are provided on request; confirm which marking your project needs. GRP is a glass-fiber composite classified under HS 3925.20 or 7019, so it falls outside the trade remedies on aluminum extrusions and outside the UK CBAM planned from 2027, which covers iron, steel and aluminum among other goods. UK customs duty and 20% import VAT still apply and are itemized in the DDP quote.

What are lead times and shipping options to UK sites?

Window projects take 6–12 weeks in production from an approved order, depending on the series, finish and quantity. Ocean transit takes about 30–38 days to Felixstowe, Southampton or London Gateway, with road delivery from the port to London, the Midlands, the North and Scotland. The quote gives an estimated delivery date for your site. Samples can go by air freight at extra cost.

Does F1 supply GRP profiles to UK window fabricators, or only finished windows?

Both. For Passivhaus and other performance-driven projects, we ship complete GRP window units, assembled, glazed and gasketed in the factory, including tilt-and-turn openings. For UK fabricators, we supply pultruded profile sets with EPDM gaskets, corner kits and fabrication drawings for local assembly. AAMA 2604 or 2605 powder coatings are available in RAL colors, including the anthracite and other dark tones UK architects specify. GRP is a thermoset, so dark finishes do not soften the frame in strong sun as they can with uPVC.