There are five key Certified Passive House design principles;
1
Air Tightness
Elimination of drafts.
2
Super-insulated & thermal bridge-free envelope
Retain heat energy.
3
Heat recovery ventilation (MVHR) Recycles heat energy & provides fresh air.
4
Water tightness
Essential for building durability.
5
High-performance joinery
Building enclosure
4. Water tightness
Water tightness in a passive home not only contributes to the health of the building enclosure (you can read more about this here, and air tightness (you can read more about this here), it also makes the building more durable and less prone to long-term damage caused by moisture.
The exterior of the building enclosure includes a weather-resistant barrier designed to prevent the penetration of wind-driven rain. A continuous damp-proof membrane (DPM) sits all over the outside of the building, under the floor, up the walls and across the roof.
For concrete floors, after the DPM is installed, a layer of polystyrene insulation is added – a type of rigid foam insulation that provides effective insulation against heat transfer through the building envelope. Then, concrete and steel elements are added to form the structural components of the building.
A DPM is designed to prevent rising dampness and moisture infiltration from the ground into the building’s walls, floors, and foundation. As it seeps in, ground moisture can reduce the effectiveness of the insulation layer and cause damage to steel and timber building components.
The same type of membrane will be used at the exterior of the foundation walls and to encapsulate the basement or under-house area to prevent moisture infiltration to the living spaces. The underslab DPM connects to the weather-resistant barrier (WRB) applied to the walls and roof. Even without the roof and wall cladding, the structure has a weather-resistant barrier all over the outside.
Think of it like a plastic bag under the foundation that connects to the weatherproof membrane enclosing the structure.
The building paper on regular construction just overlaps at joints in the underlay and often doesn’t connect roof to wall junctions. the underfloor DPM doesn’t have to be brought up to meet the WRB on the walls. Similar house wraps, building paper, and other sealants might be used, but a passive house will have the membrane completely encapsulate the structure and taped at all the joints for continuity of weather tightness.
Our passive house designs incorporate ventilation behind the exterior cladding to create a buffer zone that helps prevent moisture from reaching the enclosure’s encapsulated layer. This ventilation space allows air movement for faster drying and promotes the evaporation of moisture that may have penetrated the cladding.
Proper vapour control layers throughout the structure with humidity barriers and retardants prevent moisture from diffusing through the building materials. Flashings and seals are used around openings, joints, and connections, preventing water from entering.
Around the outside of the house, we ensure proper drainage with sloping surfaces, generous gutters, downpipes, and overhangs – designed to direct water away from the building and foundation, reducing the chances of water pooling nearby.
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