The threshold thermal bridge that defeats Passive House envelopes

The Invisible Problem That Defeats Passive House Envelopes

Why aluminum sliding door thresholds create one of the worst thermal bridges in modern residential construction — and what Passive House, PHIUS, and LEED actually require to address it.

A House With a $75,000 Door and Cold Feet

It is January in a Climate Zone 5 city. The homeowner walks toward the sliding door in socks. Halfway across the room, they notice the floor is getting cold. By the time they reach the threshold, the tile is genuinely uncomfortable to stand on.

They look at the aluminum threshold. There is condensation on the interior face. They kneel down and see the beginning of a stain in the engineered wood flooring adjacent to the threshold — a shadow of moisture damage that will become mold within a year.

This is a house with a 75,000-dollar sliding door, specified by a competent architect, installed by a licensed contractor, in a building otherwise built to modern energy code.

And it is failing. Not because anyone did anything obviously wrong. Because the physics of the threshold interface were not addressed at design time.

This is the threshold thermal bridge problem. It is invisible on drawings. It is invisible during inspection. It becomes visible only after the building is occupied — sometimes years after construction, when the flooring damage is already done and the door itself has to be removed to correct the underlying detail.

This article explains why threshold thermal bridges happen, what they cost in energy and comfort, what green building standards actually require, and how to solve the problem at the design and specification stage — before the concrete is poured.

What a Thermal Bridge Actually Is

A thermal bridge is a localized area of a building envelope where heat transfers significantly faster than through the surrounding assembly. If a wall has an R-value of R-20, but a specific area has a much lower effective R-value, that area is a thermal bridge.

Thermal bridges matter for four reasons:

**1. They cause direct heat loss. **Heat that flows through the bridge escapes the building. In heating-dominated climates, that heat has to be replaced by the mechanical system, costing energy.

**2. They create cold interior surfaces. **If the interior surface of a thermal bridge drops below the dew point of interior air, water condenses on it. Water on wall or floor surfaces causes mold growth, material degradation, and indoor air quality problems.

**3. They compromise occupant comfort. **Even without condensation, cold interior surfaces radiate cold to occupants nearby. A cold floor near a door creates a persistent thermal discomfort zone. Occupants respond by raising the thermostat, further increasing energy consumption.

**4. They defeat certification requirements. **Passive House, PHIUS, and increasingly LEED all account for thermal bridges in envelope calculations. A single unmitigated bridge can defeat certification for an entire building.

Thermal bridges are typically classified as linear (measured in ψ, psi, W/m·K) or point (measured in χ, chi, W/K). Door thresholds are linear thermal bridges — a continuous line of heat transfer along the full width of the door opening.

Why Door Thresholds Are One of the Worst

Look at a typical premium sliding door installation and count the thermal bridging opportunities:

**The aluminum threshold itself. **Aluminum has a thermal conductivity of approximately 237 W/m·K. For reference, this is roughly 700 times more conductive than the GFRP used in engineered thermal break spacers, and roughly 100 times more conductive than the wood in typical framing lumber. Aluminum is a spectacularly efficient conductor of heat — which is precisely why it is used in every heat sink and radiator on the planet.

**The concrete curb beneath it. **Concrete has a thermal conductivity of approximately 1.5 W/m·K. That is roughly 40 times more conductive than rigid foam insulation. A continuous concrete slab from exterior grade to interior floor becomes an uninterrupted thermal path from outdoor cold to indoor warmth.

**The typical spacer materials. **Wood shims, aluminum spacer plates, steel plates, concrete blocks, or masonry pieces — the materials traditionally used to level the threshold above the concrete curb — either conduct heat too well (aluminum, steel, concrete) or degrade over time (wood).

**The lack of thermal separation. **Most traditional installations have no dedicated thermal break between the aluminum threshold and the concrete curb. The aluminum sits on shims, and the shims sit on concrete. Heat flows unimpeded.

The result is a linear thermal bridge with a ψ value typically in the range of 0.5 to 1.5 W/m·K for unmitigated installations. For context, a Passive House-compliant thermal bridge at the same location should be below 0.10 W/m·K — five to fifteen times lower than the typical construction.

The threshold thermal bridge is worse than most wall-to-window connections, worse than most balcony slab penetrations, and worse than most roof-to-wall interfaces. In a premium residence with high-performance windows and well-detailed walls, the door threshold may be the single largest thermal defect in the entire envelope.

The Physics: Quantifying the Heat Loss

Let us calculate the actual heat loss through an unmitigated aluminum threshold on a typical premium sliding door installation.

Setup Parameters

• Door width: 16 feet (4.87 meters) — typical four-panel sliding door

• Climate zone: IECC Zone 5A (5,500 heating degree days base 65°F)

• Interior design temperature: 70°F (21°C)

• Exterior winter design temperature: 0°F (-18°C)

• Heating season hours: approximately 4,000 hours per year

Thermal Bridge Coefficient (Ψ)

For a traditional installation with an aluminum threshold, wood shims, and continuous concrete beneath, the linear thermal bridge coefficient is typically 0.60 to 1.0 W/m·K. We will use 0.80 W/m·K as a representative value.

Annual Heat Loss Calculation

Annual heat loss through a linear thermal bridge is calculated as:

Q = Ψ × L × HDD × 24

Where Ψ is the thermal bridge coefficient (W/m·K), L is the length of the bridge (m), HDD is heating degree days in Kelvin, and 24 converts days to hours.

For our example:

Q = 0.80 W/m·K × 4.87 m × 5,500 × (5/9) × 24 = approximately 286 kWh per year

At first glance, these numbers look modest. A homeowner burning $43 per year on one unmitigated threshold is not going to notice on their utility bill. But this calculation captures only the direct conductive heat loss. The full penalty is significantly larger.

The Hidden Multipliers

**HVAC oversizing. **Peak heat loss determines heating system size. If the calculated design heat load is 152 W higher than a properly detailed threshold would produce, the entire HVAC system is oversized to compensate. Oversizing reduces equipment lifespan and cycling efficiency.

**Cold air infiltration. **Cold surfaces adjacent to occupied space induce air movement. Warm interior air contacts the cold aluminum threshold, cools, sinks, and flows across the floor toward interior space. This "cold air spillage" is not captured in ψ calculations but significantly increases perceived draftiness and heat loss.

**Thermostat compensation. **Occupants who feel cold near the door raise the thermostat. A 2°F setpoint increase to compensate for one thermal comfort problem increases whole-house heating load by roughly 6-10%. On a house with a $2,000 annual heating bill, this is $120-200 per year of extra energy use — all traceable to one unmitigated threshold.

**Reduced envelope lifespan. **Condensation on and around the threshold accelerates degradation of adjacent materials — wood flooring, drywall, paint, sealants. Repair costs over decades far exceed the direct energy penalty.

Total annualized cost of an unmitigated threshold on a premium residence, accounting for all factors, is probably in the range of $100 to $250 per year per door — not $39. And that is before considering the eventual repair costs when condensation damage becomes visible.

Condensation: The Invisible Failure Mode

Direct heat loss is expensive but manageable. Condensation is worse because it damages the building.

The Dew Point Math

Interior air at 70°F and 50% relative humidity has a dew point of approximately 51°F (10.6°C). When interior air contacts any surface below 51°F, water condenses on that surface.

Calculating the interior surface temperature of an aluminum threshold on a cold day:

Ti (interior surface temperature) = Tin - (Tin - Tout) × Rsi / (Rsi + R_thermal_bridge)

Where Tin is interior air temperature, Tout is exterior air temperature, Rsi is interior surface film resistance (typically 0.13 m²·K/W for still air), and R_thermal_bridge is the effective R-value of the assembly through the bridge.

For an aluminum threshold on concrete with no meaningful thermal break, R_thermal_bridge is very low — perhaps 0.02 to 0.05 m²·K/W. Running the math:

• Tin = 21°C, Tout = -10°C

• Interior surface temperature: approximately 6-8°C

• Dew point at typical interior conditions: 10.6°C

The interior surface of an unmitigated aluminum threshold on a cold day is 3-5°C below the dew point of interior air. This is not a marginal case. This is guaranteed condensation for any period when exterior temperatures fall below approximately 25°F, in any building with normal indoor humidity levels.

Condensation forms. Water pools on the interior threshold. It wicks into adjacent flooring — engineered wood, natural stone with polymer setting compounds, tile grout, LVT adhesive. Over months and years, this water damage accumulates.

The visible symptoms show up two to five years after occupancy:

• Discoloration and warping of wood flooring near the door

• Efflorescence on tile grout near the threshold

• Mold growth in the flooring underlayment

• Delamination of engineered wood layers

• Corrosion of any steel components in the threshold assembly

By the time these symptoms are visible, remediation requires removing the door assembly, addressing the underlying detail, and reinstalling. On a premium door, this is a $10,000-$30,000 repair for a component that cost less than $50 to install correctly during original construction.

Occupant Comfort: The Persistent Complaint

Even without condensation, cold interior surfaces near occupants degrade thermal comfort. ASHRAE 55, the standard governing thermal comfort in occupied buildings, defines acceptable operative temperature ranges that account for both air temperature and mean radiant temperature from surrounding surfaces.

A cold interior threshold surface has three specific comfort effects:

**Radiant asymmetry. **ASHRAE 55 limits radiant temperature asymmetry to specific values — 10°C for warm walls, 5°C for cold walls, 4°C for cold ceilings, and 14°C for warm floors. A cold threshold does not trigger the wall or floor limits directly, but the localized asymmetry near the door affects occupants who spend time in that zone.

**Cold floor sensation. **A cold zone of flooring adjacent to a threshold radiates cold to occupants standing or seated nearby. The perception is "cold feet" and general discomfort, even when air temperature meets setpoint.

**Draft perception. **The cold air movement induced by convection off the cold threshold reads to occupants as a draft. Occupants often perceive drafts in air-tight buildings — and blame the door for "not sealing properly" — when the actual cause is thermal bridging inducing cold air movement.

The comfort penalty of a thermal bridge is difficult to quantify economically, but it is the reason most homeowners eventually notice and complain. Energy bills are abstract. A cold floor near the sliding door on a January evening is not.

What Passive House Requires

The Passive House Institute (PHI) in Darmstadt, Germany, and the Passive House Institute US (PHIUS) both establish rigorous thermal performance requirements for building envelopes. Neither certification specifies a maximum ψ value for door thresholds directly. Both work through whole-building energy budgets that thermal bridges must fit within.

Passive House (PHI) Requirements

• Annual heating demand: ≤15 kWh/m²·year

• Total primary energy demand: ≤120 kWh/m²·year

• Airtightness: ≤0.6 ACH50

• Thermal bridge accounting: mandatory via PHPP (Passive House Planning Package)

In PHPP calculations, every thermal bridge in the envelope is enumerated with its ψ value and length. The sum contribution of thermal bridges affects the whole-building heat loss calculation. A single door threshold with ψ = 0.80 W/m·K over 4.87 m contributes approximately 3.9 W/K to the whole-building heat loss coefficient. If the building has 250 m² of floor area, this represents about 0.4% of the total heat loss budget from a single component — a significant slice for one linear meter of building envelope.

Passive House projects typically target ψ values below 0.10 W/m·K at critical thermal bridge locations, including door thresholds. Achieving this requires deliberate design intervention: continuous insulation across the threshold zone, thermally-broken components at the aluminum-to-concrete interface, and materials with thermal conductivity comparable to insulation rather than metal.

PHIUS 2021 Requirements

PHIUS certification uses climate-specific performance targets calibrated to local climate zones. The 2021 standard introduced explicit thermal bridge accounting through the WUFI Passive tool.

For a typical Climate Zone 4-5 residence targeting PHIUS+ certification:

• Annual heating demand: 4-8 kWh/m²·year (climate-dependent)

• Peak heating load: ≤10 W/m² (climate-dependent)

• Thermal bridge factors incorporated into WUFI Passive envelope model

The critical implication: a 152 W peak load contribution from a single threshold thermal bridge is 15% of the entire peak heating load budget for a 100 m² residence. Unmitigated door thresholds make PHIUS certification difficult or impossible to achieve for premium residences that include large sliding doors.

The Design Response

Passive House and PHIUS practitioners handle door thresholds through three interventions:

• Thermal break block between aluminum threshold and concrete curb

• Continuous rigid foam insulation surrounding the thermal break block

• Verification of ψ value through THERM or equivalent 2D heat transfer modeling

The block material specification is critical. Wood, aluminum, steel, PVC, and standard concrete blocks all fail to achieve the required thermal performance. Purpose-engineered composite materials — pultruded GFRP, rigid polyurethane composites like Purenit, or compressed EPS composites like Compacfoam — are the materials that Passive House projects specify for this application.

What LEED and Energy Codes Consider

LEED v4.1

LEED does not require thermal bridge calculations at the same level of detail as Passive House. However, several LEED credits are affected by threshold thermal performance:

**Energy and Atmosphere: Optimize Energy Performance. **Building energy modeling (ASHRAE 90.1 Appendix G) captures envelope performance. Significant thermal bridges reduce modeled energy savings and therefore reduce points earned.

**Energy and Atmosphere: Enhanced Commissioning. **Envelope commissioning requirements at higher tiers include verification of thermal bridge details. Contractors submitting for enhanced commissioning may need to document threshold detail thermal performance.

**Indoor Environmental Quality: Thermal Comfort. **Cold interior surfaces contribute to thermal comfort complaints. Buildings with unmitigated thermal bridges may fail post-occupancy thermal comfort surveys required for this credit.

**Materials and Resources: Building Life Cycle Impact Reduction. **Threshold assemblies that require replacement or remediation over the building life cycle reduce Life Cycle Assessment (LCA) points.

IECC 2021 and 2024

The International Energy Conservation Code (IECC), which forms the basis for state and local energy codes, includes thermal bridge provisions at increasing stringency:

IECC 2021 introduced mandatory continuous insulation requirements at slab edges in cold climate zones. IECC 2024 further tightens envelope requirements and explicitly references thermal bridge calculation methods for compliance paths.

While IECC does not currently mandate ψ value calculations for individual thermal bridges, the trend is clearly toward more rigorous thermal bridge accounting. Buildings designed today to meet minimum code will likely need retrofit within a decade if thermal bridges are ignored.

ASHRAE 90.1-2022

ASHRAE 90.1, the reference energy standard for commercial buildings and increasingly for residential projects, includes envelope performance requirements that account for thermal bridging in Appendix A. Design teams following the performance path must account for major thermal bridges including slab edges and fenestration perimeters.

Premium residential projects targeting Net Zero, LEED Platinum, or ASHRAE 90.1 Appendix G modeling must account for door threshold thermal bridges in their energy modeling — or accept significantly reduced envelope performance credits.

Why Most Installations Ignore This

Given how significant the thermal bridge problem is, why do the vast majority of sliding door installations ignore it?

**It is invisible on the drawings. **Architectural drawings show the door in plan and elevation. Section details rarely include a thermal analysis or a specific thermal break specification at the threshold. The problem is not documented, so it is not addressed.

**It is not a code violation. **IECC and most local codes do not currently require thermal bridge calculations at door thresholds. A building can pass energy code inspection with a severely thermally bridged threshold.

**The door manufacturer does not address it. **Alumil, Reynaers, Schuco, Sky-Frame, Solarlux, Vitrocsa — all provide excellent thermally-broken threshold profiles. But their installation instructions typically show the threshold sitting on a generic "structural support" without specifying what that support should be made of. The connection between the manufacturer's thermally-broken threshold and the substrate is the installer's problem to solve.

**Installers use what they have. **On a cold morning at a jobsite, an installer whose truck contains pressure-treated wood shims and shims from previous jobs is not going to specify a thermal break block. They use what they have. What they have is thermally conductive.

**The consequences are delayed. **Direct energy penalty accumulates slowly. Condensation damage takes 2-5 years to become visible. By the time anyone connects the failure back to the original installation detail, the installer is long gone and the specifier has moved on to other projects.

**There has not been a good solution. **Until recently, the market lacked a purpose-engineered thermal break block designed specifically for concrete-embedded threshold applications. Improvised solutions (Purenit or Compacfoam blocks cut down from wall applications, wood shims wrapped in plastic, custom-fabricated composite pieces) required substantial site labor and produced inconsistent results.

The result is that thermal bridges at door thresholds have become an accepted defect in premium residential construction — a problem everyone knows about, no one solves, and no one is held accountable for.

The Solution Architecture

The correct thermal break architecture at a sliding door threshold has three components:

1. A Structural Thermal Break Block

Between the aluminum threshold and the concrete curb, a load-bearing block with low thermal conductivity carries the door weight while breaking the thermal path. This block must:

• Have thermal conductivity comparable to insulation (target: <0.5 W/m·K, ideally <0.2 W/m·K)

• Support the full structural load of the door with substantial safety margin

• Maintain dimensional stability over decades of thermal and moisture cycling

• Resist alkaline degradation from concrete pore water (pH 12.5+)

• Be dimensioned precisely for consistent threshold elevation

Purpose-engineered GFRP (Glass Fiber Reinforced Polymer) composites are the material specification that meets all these requirements simultaneously. GFRP with ECR-glass reinforcement and vinyl ester resin — the specification called for in ACI 440 for FRP in concrete — provides 50+ year service life in the buried environment while maintaining thermal conductivity approximately 700 times better than aluminum.

2. Continuous Insulation Around the Block

The structural block alone does not eliminate the concrete thermal bridge. The concrete curb still connects exterior grade to interior floor through the substrate beneath and around the block. Continuous rigid foam insulation surrounding the thermal break zone extends the thermal break into the concrete assembly.

Typical materials:

• Rigid EPS foam board (R-4 per inch, cost-effective)

• Rigid XPS foam board (R-5 per inch, better moisture resistance)

• Polyisocyanurate rigid foam (R-6 per inch, best thermal performance)

The insulation is placed in the load-free zone beneath and around the structural block, bonded together with polyurethane expansion foam at seams to create a monolithic insulation layer. Because the structural block carries the door load, the insulation never sees compressive stress and can be optimized purely for thermal performance.

3. Cavity Elimination

Any air cavity in the threshold zone becomes a location for moisture accumulation, air movement, and thermal short-circuiting. The complete solution eliminates all air cavities through:

• Foam injection into the interior cavity of the structural block

• Foam sealing around the block-to-concrete interface

• Foam sealing at the threshold-to-block interface

• Sealant continuity at all exterior weather boundaries

The result is a fully sealed, thermally-broken, structurally-adequate threshold assembly with no cold spots, no moisture traps, and no long-term degradation vectors.

How to Verify a Threshold Has Been Designed Correctly

For architects and specifiers reviewing sliding door installation details, six specific verifications separate a properly-designed threshold from a thermal bridge disaster:

**1. Section detail shows a dedicated thermal break block. **Look for a specified component between the aluminum threshold and the concrete curb. If the section shows the threshold sitting directly on concrete, or on unspecified "shims," the design is deficient.

**2. Block material is specified. **The thermal break block should be identified by material — GFRP, purpose-engineered polyurethane composite, or compressed EPS composite. "Wood shims" or "aluminum plate" is not acceptable.

**3. Continuous insulation is shown. **Rigid foam insulation should be shown beneath and around the thermal break block, connecting to the wall or slab insulation system. Discontinuous or missing insulation defeats the thermal break block.

**4. Cavity elimination is called out. **The section should indicate foam infill or equivalent air cavity elimination at all internal spaces. Open cavities are moisture traps.

**5. Ψ value is calculated or specified. **For Passive House or PHIUS projects, the ψ value at the threshold should be calculated through THERM, PHPP, or WUFI Passive and documented in the energy model. The calculated value should be below 0.10 W/m·K for high-performance targets.

**6. Material properties are documented. **The specified thermal break block material should have documented thermal conductivity, compressive strength, alkali resistance, and long-term dimensional stability. Materials without published test data should not be accepted.

If any of these six items is missing from the design documents, the specifier should require the design team to address the deficiency before construction begins.

The Bottom Line

The thermal bridge at a sliding door threshold is:

• Larger than most other bridges in the envelope

• Consistently ignored in traditional installations

• Expensive in direct energy cost

• More expensive in HVAC oversizing and comfort compensation

• Damaging in condensation-driven material degradation

• Disqualifying for Passive House, PHIUS, and net-zero targets

• Solvable with purpose-engineered thermal break components

• Almost never addressed in current construction practice

For a homeowner building or renovating a premium residence, the specific question to ask the architect is: what is the specified thermal break at the sliding door threshold? If the answer is "wood shims" or "we'll figure that out during installation," the design is not ready to build.

For an architect specifying a sliding door for a high-performance envelope, the specific requirement to include is: a dedicated thermal break block with documented thermal conductivity below 0.5 W/m·K, ECR-glass reinforced vinyl ester composite construction, ACI 440 conforming material specification, and post-installation cavity elimination through foam infill.

For a builder or installer executing a premium sliding door installation, the specific practice to adopt is: never use wood, aluminum, steel, or improvised shims at the threshold interface. Specify a purpose-engineered thermal break block. Verify continuous insulation around it. Eliminate all cavities. Document the assembly.

The threshold thermal bridge is one of the most impactful envelope details in a modern residence, and one of the easiest to solve correctly at the design stage. The consequences of ignoring it are visible for decades. The consequences of addressing it are invisible for decades — which is exactly what "correctly detailed" looks like in a high-performance building.

About This Article

This article is published by WINDO, a specialty premium sliding door dealer and manufacturer of the KORSYS engineered threshold assembly system.

KORSYS is a purpose-engineered thermal break block designed specifically for permanent embedment beneath premium sliding door thresholds. Manufactured from pultruded GFRP composite with ECR-glass fiber reinforcement and vinyl ester resin, KORSYS provides structural load-bearing capacity with thermal conductivity approximately 700 times better than aluminum spacer alternatives. Material properties are documented in CMA/CNAS certified testing (Report LH250612050101E).

For technical specifications, project consultation, or to request KORSYS samples for evaluation, visit korsys.build or contact WINDO engineering.

Related articles:

• The Zero-Threshold Problem: Why Millimeter-Precision Sliding Doors Need a Millimeter-Precision Base

• Why GFRP? A Materials Analysis for Threshold Spacer Blocks

• KORSYS Technical Properties (specification document)