Door constructions

A choice of highly insulating door constructions is available in the market. Of particular importance for high-performance housing is that the doors remain air tight over time (no warping of the door, durability of the weather stripping and air tightness reinforced by the hardware). Various door constructions meet these demands.

Wooden sandwich doors

Sandwich doors are built in a multi-layered construction. The visible outer faces are typically plywood with a high quality outer veneer or pattern treatment. The core of the door is a wooden frame construction reinforced with an aluminium profile or diaphragm to prevent warping. A plastic film may be added to ensure air tightness. The void space is filled with insulation with a very good lambda value. To achieve the required insulation levels, these doors must be comparatively thick, typically between 85 mm to 110 mm. The door frame is a thermal weakness in the system. Accordingly, the frame is also an important factor in selection criteria for a door.

Table 9.3.1 Properties of a wooden sandwich door

Thermal performance U-value: 0.71 W/m2K

Air tightness: V = 2.25 m3/hour linear m by 100 Pa Ecology Wood as a CO2 neutral material is a favourable choice, except if the facings are of tropical woods. A typical value for the grey energy of a wooden sandwich door is 125 kWh.

Figure 9.3.1 Entry door

Source: Alex Hastings

Figure 9.3.1 Entry door

Source: VARIOTEC Sandwichelemente GmbH & Co,

Figure 9.3.2 Wooden sandwich door

Metal sandwich doors

The outer faces of sheet aluminium are stiffened with aluminium angles and the core is filled with a sprayed-in rigid insulation. The overall thickness is 80 mm. This type of door is very break-in resistant.

Source: Biffar GmbH, Edenkoben,

Figure 9.3.3 Aluminium sandwich door

Table 9.3.2 Properties of a metal sandwich door

Thermal performance Ecology

Air tightness: V = 1.3 m3/hour linear m by 100 Pa

Aluminium is produced with a high percentage of renewable energy (hydro- or geothermal electric generation) and is easily recycled.

Vacuum insulation doors

Using a vacuum insulation panel instead of conventional insulation allows the door to be much thinner and still achieve the required U-value. A vacuum (0.1 to 20 mbar) eliminates heat transport by convection across the gap, which is filled with a micro-porous material to maintain the separation. As a result, vacuum panels achieve, with very little thickness, very high insulation levels. A vacuum panel at 1 mbar can have a Lambda-value of 4.8 mW/mK compared to closed-cell polyurethane, with 19 to 35 or expandable polystyrene with 36 mW/mK.

Source: VARIOTEC Sandwichelemente GmbH & Co,

Figure 9.3.4 Example of a vacuum-insulated door

Table 9.3.3 Properties of a door with vacuum panel insulation

Thermal performance U = 0.3-0.8 W/mK depending on glass area if there is an integrated viewing window

Ecology The vacuum panel core consists typically of either an open-cell polyurethane

(ICI) or polystyrene (Dow) foam, or carbon/silica aerogels. Neither is more environmentally detrimental than conventional insulation products, and the amount of material is very small compared to conventional insulation volumes.

Glass doors

Thermal glazing has been optimized to such an extent that a glass door offers one of the best solutions regarding insulation value for its thickness. The opaque part of the door and frame construction can be the same as that used for high-performance windows. Glass doors are offered in triple glazing. The weak point of the door is the threshold.

Table 9.3.4 Properties of a window as a door

Thermal performance Uf = 0.6973 W/mK

Ecology The coatings of thermal glazings are so thin that they pose no environmental hazard when the door is recycled

Source: VARIOTEC Sandwichelemente GmbH & Co,

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