How to specify folding doors for a Passive House project

Folding doors are no longer the weak point of a low-energy building envelope. With thermally broken aluminium frames, triple glazing and carefully detailed thresholds, a well-specified folding door system can hold its own in a Passive House project — opening up whole walls to the garden while keeping heat loss to a minimum.

But specifying folding doors for a Passive House project is different from specifying them for a standard extension. The margins are tighter, the targets are stricter, and small detailing errors can undo the performance of the entire envelope. This guide walks through the specification steps that matter, in the order they should be considered.

What a Passive House actually demands from an opening

Before choosing a product, it helps to be clear about the performance targets a folding door has to meet in a certified Passive House project:

• U-value: The Passive House Institute (PHI) certification for transparent components typically requires an installed U-value of 0.80 W/m²K or lower, with the glazing itself at 0.85 W/m²K or below. Even where full certification is not the goal, aiming for an installed U-value around 1.0 W/m²K or lower keeps the door from becoming a thermal weak point.
• Airtightness: The building envelope must achieve 0.6 air changes per hour at 50 Pa. Large opening elements are a common source of leakage, so the door’s sealing system — gaskets, compression seals and the threshold — must be specified to a high standard.
• Thermal-bridge-free design: Passive House construction aims to eliminate thermal bridges. The threshold, the sill and the frame-to-wall junction all need to be detailed so that heat cannot bypass the insulation layer.

These targets shape every decision that follows, from the frame profile to the glazing to the hardware.

Step 1: Set the thermal performance target first

Start with the numbers, not the aesthetics. Decide the target U-value for the whole installed door — not just the glass — and write it into the specification. For a Passive House project, an installed U-value of 1.0 W/m²K or lower is a realistic starting point, with the glazing centre-of-glass U-value typically between 0.5 and 0.7 W/m²K.

This matters because a folding door is a large area of glazing. If the frame is poorly insulated, a modest-looking difference in U-value multiplied across several square metres of opening adds up to real heat loss over a heating season.

Step 2: Choose a thermally broken aluminium frame

Aluminium is the natural choice for folding doors — it is strong enough to carry large panels, slim enough to keep sightlines clean, and it does not warp or swell like timber in wet conditions. But bare aluminium is an excellent conductor of heat, so the frame must be thermally broken.

The quality of the thermal break is what separates a Passive House-grade folding door from an ordinary one. Look for:

• Multi-chamber thermal break profiles with polyamide insulation strips — PA66GF25 is a common high-performance specification
• Adequate profile depth — deeper frames allow more room for insulation and stronger hardware
• Reinforced profiles where panels are large, with wall thickness sufficient to carry the panel weight without deflection

A manufacturer that already builds passive-grade windows to tight tolerances is a strong signal that its folding door systems will be detailed to the same standard. For example, ALPES manufactures its system windows and doors from 6060-T66 aluminium profiles with PA66GF25 multi-chamber thermal break strips, and its AW90 passive window achieves a K-value of 1.3 — the same engineering discipline carries across its product range.

Step 3: Specify the glazing

For a Passive House project, single glazing is out of the question and even standard double glazing is rarely enough. Specify:

• Triple glazing with two Low-E coatings and a warm-edge spacer
• Argon or krypton gas fill to reduce heat transfer between the panes
• A centre-of-glass U-value of 0.5–0.7 W/m²K
• A g-value matched to orientation — south-facing openings can benefit from useful solar gain in winter, while west-facing glazing may need a lower g-value to avoid overheating

Remember that the glazing unit is only as good as its edge seal. Warm-edge spacers and a well-sealed unit reduce condensation and keep the whole unit performing close to its centre-of-glass value.

Step 4: Detail the threshold — the most common failure point

The threshold is where most folding door installations leak heat and air. In a Passive House project, the threshold must be:

• Thermally broken, so the floor slab does not conduct heat to the outside
• Sloped and drained, so water runs away from the building rather than pooling at the seal
• Capillary-broken, to stop water being drawn up between the threshold and the floor
• Coordinated with the insulation layer, so the building’s insulation continues to the edge of the opening without a gap

A flush threshold is attractive for accessibility, but it demands careful detailing to remain airtight and watertight. If the project requires step-free access, budget for the extra detailing work — it is not a place to cut corners.

Step 5: Specify the sealing and locking

Airtightness at 50 Pa is a demanding target, and a folding door has many more moving joints than a fixed window. Every panel-to-panel joint, every hinge edge and every gasket must work together. Specify:

• Continuous compression gaskets around every panel, not just the outer frame
• Multi-point locking with anti-pry lock points, so the panels are pulled firmly against their seals when closed
• A proven hardware system — for aluminium systems, German hardware such as ROTO, Winkhaus, CMECH or Wehag is a reliable benchmark for smooth operation and long service life

The hardware also carries the panels. Check the load rating of the rollers and hinges against the panel size and weight — an under-specified roller will wear quickly and let the door sag, breaking the seal over time.

Step 6: Plan the installation and testing

Even the best folding door fails if it is installed badly. In a Passive House project, the frame-to-wall junction is part of the airtight layer, so:

• Coordinate with the structural engineer early — large openings need lintels or steelwork to carry the load above the door
• Set the frame plumb, level and square — a racked frame will bind and leak
• Tape and seal the frame-to-wall junction as part of the airtightness strategy, not as an afterthought
• Blower-door test the completed installation to confirm the whole envelope, including the door, meets the 0.6 ACH target

Common specification mistakes to avoid

• Specifying the glass U-value but ignoring the frame — the installed performance is what counts
• Choosing a flush threshold without detailing drainage — water ingress follows
• Under-specifying hardware for large panels — sagging panels break the seal and the airtightness
• Forgetting the stack space — folded panels need room to stack, inside or outside, and this must be planned into the layout
• Skipping the blower-door test — a leaky installation is invisible until it is measured

A final checklist for your specification

• Installed U-value of 1.0 W/m²K or lower, with glazing centre-of-glass at 0.5–0.7 W/m²K
• Thermally broken aluminium frame with a multi-chamber polyamide thermal break
• Triple glazing with Low-E coatings and inert gas fill
• Thermally broken, sloped, drained and capillary-broken threshold
• Continuous compression gaskets and multi-point anti-pry locking
• Hardware rated for the panel weight, from a proven manufacturer
• Structural coordination, airtight frame-to-wall detailing and a blower-door test

Working with a manufacturer that understands Passive House

Specifying folding doors for a Passive House project is as much about the manufacturer as the product. A supplier that already produces passive-grade systems to tight tolerances — with 6060-T66 profiles, PA66GF25 thermal breaks and German hardware — will bring that discipline to your folding doors. As a folding doors manufacturer with a 100,000 m² production base in Foshan and Zhaoqing, ALPES builds custom aluminium system windows and doors to international standards, with a full range of colour and glass options. Whether your project needs folding patio doors for a villa in the Middle East or folding doors interior for a high-end apartment, the specification principles above — thermal break, glazing, threshold, sealing and installation — are the same. Get them right, and your folding doors will earn their place in the building envelope rather than compromise it.

For project-specific advice and passive-grade specifications, contact the ALPES team directly.