Windows and doors for a self-build: frames, triple glazing, ordering and insurance

Windows and external doors are one of the biggest single purchases in a one-off house, and one of the easiest to get wrong. The frame and glass you choose affect your energy rating, the airtightness test, your security and the date the house is weathertight. Decide the specification before the roof goes on and the order shortly after, so the units are ready when the shell is.

What Part L asks of windows and doors

Part L (the energy rule in the Building Regulations) sets a maximum U-value of 1.4 W/m²K for windows, doors and rooflights in a new dwelling. The U-value measures heat lost through the whole unit, frame included. Lower is better.

You do not meet Part L window by window. Your designer’s energy calculation (DEAP) adds up the whole house, so a weaker window can be offset elsewhere, but not past the 1.4 limit. The guidance’s own worked example for a typical 126 m² semi-detached house that complies uses triple-glazed, low-E, argon-filled windows in PVC frames at 0.9 W/m²K, with openings equal to 25% of the floor area. That is an example of one way to comply, not a specification for your house.

Ask your designer for the U-value and the solar transmittance (g-value) they assumed in the calculation, and write both into the order. Windows that are cheaper but worse than the assumed figure can put the BER or compliance at risk. See Part L and nZEB requirements.

Double or triple glazing?

Triple glazing costs more. Windows suppliers’ guides put the extra at roughly 30% to 50% over double glazing, and the installed price list below shows the gap by frame. What you get is a lower U-value, warmer glass surfaces and less condensation. It also makes it easier to meet the 1.4 limit when you want large glazed areas, because big windows lose more heat.

Ask your designer to run the calculation both ways. If double glazing passes with room to spare on a house with modest glazing, you may not need triple. If you want large south-facing openings or a Passive House standard, triple is likely to be needed. See Passive House or nZEB.

Frame types and price ranges

Frame and glazing	Installed, per m²
uPVC, double	€450 to €750
uPVC, triple	€650 to €950
Timber, double	€700 to €1,100
Timber, triple	€900 to €1,400
Aluminium, triple	€900 to €1,400
Alu-clad timber, triple	€1,000 to €1,500

These are indicative installed prices per m² of window, not quotations. Prices quoted by some window makers are much lower, around €450 to €620 per m² for triple-glazed uPVC, aluminium or timber, often without saying whether VAT or fitting is included. Expect a wide spread: get at least three written quotes, ask whether each includes VAT, fitting, delivery and the airtightness tapes, and compare the same specification.

A 150 m² house has about 25 to 35 m² of windows. At 30 m², triple-glazed uPVC works out at about €19,500 to €28,500 on those ranges, and alu-clad at €30,000 to €45,000. Add external doors, sliding or bifold doors and rooflights, which are priced separately.

How the frames differ:

• uPVC. Usually the cheapest. Thicker frames, fewer colour and shape options, and little upkeep beyond cleaning.
• Timber. Warm and slim, and can be very good thermally, but the coating has to be renewed over the years.
• Aluminium. Slim, strong frames suited to large openings, with a thermal break to limit heat loss.
• Alu-clad timber. A timber frame inside, aluminium outside. Dearest of the options, with the least outside upkeep.

When to order and when to measure

Sizes drawn on plans are not always the sizes built. Blockwork, timber frame and insulation tolerances all shift an opening by a few millimetres, and a unit that is too big does not go in. Many self-builders confirm the order against the built openings once the walls are up, and some suppliers supply temporary formers to build to. Ask your supplier which they use, and who takes responsibility for the dimensions.

Lead times vary from a few weeks to several months, and they change with the season and the supplier’s order book. Put the delivery date in the order confirmation, and work back from the date you want the house weathertight. The roof and weathertight shell guide covers where windows sit in the sequence, and the timeline planner lets you test a late delivery.

Before you place the order, check that it states:

• The size, opening type, hinge side and handing of every unit.
• The glass specification, U-value and g-value, frame colour inside and outside, and the safety glass where needed.
• The delivery date and who unloads and stores the units.
• The deposit, the balance and when it is due.
• Warranty terms for frame, coating and glass.

Paying for windows

Suppliers usually ask for a deposit when you order, with the balance before delivery or fitting. Your lender releases money against the building’s stages, not against a supplier’s invoice, so a large deposit in month three can open a cash gap. Add it to the stage payment planner as an upfront cost, and see staged drawdown.

Keep the deposit as small as you can negotiate. Ask what happens to it if the supplier cannot deliver, and pay by a method that leaves a record. If the supplier stops trading, a deposit may be a claim among many others.

Fitting, airtightness and sign-off

How the frame is fixed and sealed matters as much as how good it is. Ask who fits the units and who is responsible for the seal between frame and wall. In an airtight build that is usually tapes or membranes applied inside and outside, so the fitter needs the design details first. A poor seal will show up at the airtightness test. See insulation and the airtightness test.

Check:

• Frames are packed and fixed to the manufacturer’s instructions, not just foamed in.
• Cills, thresholds and cavity closers match the drawings, with a tray under the thresholds.
• Glass is undamaged and carries the safety marks where they are required.
• Your photographs show the fixings and seals before they are covered.

Window installation falls under your build’s inspection plan. Ask your assigned certifier what they inspect and when, and keep the delivery notes, specifications and photographs for the completion certificates. The statutory framework is in the Building Control Regulations as amended in 2014. See commencement notice and BCAR.

Stored units, deposits and cover

Windows and doors spend time in places your insurance may not reach:

• At the supplier. Units made and held there may not be covered until delivered. Ask your broker whether the works policy covers materials before delivery, and who insures them until then.
• In transit. Check who carries the risk between the factory and the site, and who signs for damage on the delivery note.
• On site, before fitting. Frames and glass are tempting and fragile. Insurers often attach security conditions to theft cover; see securing your building site. Agree where units are stored, and fit them in the days after delivery.
• Deposits. A deposit paid to a supplier is not site cover. If the supplier fails, it is a financial loss that works insurance does not cover.
• Fitted and open. Until the doors have working locks, the shell is open. Cover for theft of fixtures may depend on the house being secured.

Tell your broker the delivery date and the value of the units, and ask whether the sum insured reflects them. Use What insurance do I need? for a checklist that matches your stage, and see self-build insurance.

Before you order

• Your designer has given you the U-value and g-value assumed in the energy calculation.
• You have three written quotes for the same specification, stating VAT, fitting and delivery.
• The openings have been measured, or the supplier has accepted the dimensions in writing.
• The delivery date is in the order confirmation, and the programme allows for a delay.
• Your deposit is added to the cash plan.
• Your broker knows when the units will be delivered and where they will be kept.