How Do You Actually Build a High Performance Timber Frame? The Forrest Passive House, Layer by Layer.
Most builders will tell you their insulation levels exceed the minimum. They have no idea what their walls actually achieve. Here is every assembly in the Forrest Passive House in Spotswood, every U-value, every material layer, every heat loss figure, and exactly why each decision was made from a building physics and durability standpoint.
A high performance home is not defined by what insulation you put in. It is defined by how every layer of the building envelope works together as a system, from the crawl space membrane to the roof truss depth, all of it modelled in PHPP, all of it tested after construction, and all of it detailed to manage both heat and moisture.
The Forrest Passive House achieved a certified average envelope U-value of 0.283 W/(m²K) across 580.81 m² of total thermal envelope area.
Why do most builders not know the U-value of their own walls?
U-value is the rate at which heat passes through a building element, measured in W/(m²K). The lower the number, the better the thermal resistance. It is the single most important number describing the thermal performance of any wall, roof, or floor assembly, and most builders in Australia could not tell you what their standard wall achieves.
The reason is that NatHERS, the compliance tool most builders use, does not require them to know. The software calculates an approximate whole-building energy rating from simplified inputs. Individual assembly U-values are not reported, not verified, and not tested. You can build a wall and have a reasonable idea it is performing to something close to what you specified. You can also build a wall with compressed batts, poorly detailed junctions, and a framing fraction that nobody accounted for, and the NatHERS certificate will not change.
PHPP is different. Every assembly is modelled individually. The framing fraction is explicitly accounted for. The thermal conductivity of every material layer is stated. The composite U-value of each assembly is calculated from those inputs and used directly in the heating and cooling demand calculation. If the U-value is wrong, the demand figure is wrong, and the certification fails. Every number has to be defensible.
What follows is every assembly in the Forrest Passive House, pulled directly from the certified PHPP post-construction file, with the building physics and durability rationale for each decision.
The whole building envelope as a system.
Before going layer by layer, it helps to understand the total picture. The PHPP models the Forrest Passive House as a series of area groups, each with its own U-value and area. The total thermal envelope is 580.81 m² with an average U-value of 0.283 W/(m²K) across every surface.
The heat loss breakdown for the heating season:
External walls: 1,240 kWh Floor through ground: 728 kWh Roof and ceiling: 510 kWh Windows: 1,157 kWh Ventilation: 320 kWh Total heat losses: 3,955 kWh Solar and internal heat gains offsetting those losses: 1,944 kWh Net annual heating demand: 2,040 kWh — or 13.1 kWh/(m²a)
Everything below feeds into those numbers.
The subfloor. What sits between your feet and the ground.
Suspended floor over a ventilated subfloor, 0.55m below ground level.
The floor sits over a ventilated subfloor. Without insulation, the ground beneath acts like a permanent heat sink, pulling warmth out of the building every day of winter. Two cross-installed layers of R2.5 batts in the floor joists slow that process down dramatically, achieving a U-value of 0.259 W/(m²K), between three and six times more thermally resistant than an uninsulated suspended floor.
The moisture side matters as much as the thermal side. The ventilation openings in the crawl space wall allow air to move through and dry out any moisture below the floor. The membrane at the base of the floor assembly stops ground moisture vapour from migrating up into the insulation. In a tight, well-insulated building, moisture that enters the subfloor has nowhere to escape. Get this detail wrong and you create conditions for damage that takes years to show itself.
The external walls.
The wall is a 140mm timber frame with R4.0 insulation batts. What makes it different is everything around the frame.
The service cavity is the detail most builders skip. It is a dedicated 45mm zone inside the airtight membrane where all cables and pipes live. The Pro Clima membrane runs continuously behind the primary frame. Every penetration through that membrane is a potential air and moisture leak. The service cavity means the membrane is never touched after installation. Cables run inside it on the warm side, unable to cause any damage to the airtight layer.
The Roof
There are five different roof assemblies because the geometry of the roof is not uniform. The key distinction is between raked ceilings and truss sections.
A raked ceiling follows the pitch of the roof. Insulation has to fit within the rafter depth — 290mm here — so there is a physical limit to how much you can get in. A truss roof gives more room. R2.5 in the bottom chord plus R7 in the truss depth delivers performance a rafter section simply cannot match without dramatically deeper rafters.
The best-performing assembly at 0.105 W/(m²K) is roughly five times better than a standard Melbourne roof built to NCC minimum with R3.5 batts. For every unit of heat that escapes through this roof, five units would escape through a code-minimum roof of the same area.
On moisture: all roof assemblies use a breathable membrane on the warm side of the insulation, allowing any moisture in the roof cavity to move outward and dry when conditions allow. Paired with the Proclima airtight membrane (intello) at ceiling level below, the roof is protected from moisture drive in both directions. A roof that can only dry one way is vulnerable. This one is not.