Rise of the passive house

Nov 30, 2012
Posted by: admin
Category: General
We appear in Passive House Plus Issue nov 2012

Rise of the passive house


Architect’s own passive house
One thing about passive house that’s frequently debated is the price — how much extra does it cost compared to conventional build to go passive? Figures of between zero and 20% are frequently thrown about, though it’s usually stressed that with the extraordinarily low heating costs, any extra outlay can be recouped.

Trying to make a passive house as affordable as possible, one Mayo-based architect decided to build her own to the passive House standard.

“We’ve got Oak Barn Construction to do the main structural aspect but we’re basically finishing it off ourselves in order to manage the high specification within our budget,” says Cuala McGann of McGann Scahill Architects.

“I’ve been interested in the passive house standard for quite some time, and being a certified passive house designer, I’m seeing this as an opportunity to put it into practice,” she adds.

The stick built timber frame structure
The stick built timber frame structure

McGann says it’s difficult to know how much extra aiming for passive will add to the final cost of the project. “When I finish this exercise I’ll be able to put it in exact numerical form,” she says.

She reckons that if one stays true to the spirit of the passive house standard by not installing a conventional central heating system, going passive shouldn’t cost more than 5% extra.

McGann chose the stick-built timber frame system from local contractor Oak Barn for the project. “We were basically looking to go with a breathable construction, and we wanted to work with Oak Barn Construction,” she says.

“She wanted to have her own house as passive, partly to show clients that you can achieve this,” says Oak Barn’s Willie McDonagh. The company stick-built the timber frame structure with I-beams. “The advantage of that was that we were able to get wider walls [meaning more insulation], and less cold bridging.”

Natural zinc roofing
Natural zinc roofing

The walls of the house feature 160mm of Gutex woodfibre insulation externally, with 300mm of cellulose between the studs and an insulated 100mm service cavity. The roof includes 360mm of cellulose plus 170mm of woodfibre insulation in the service zone. Optiwin triple-glazed Passive House Institute certified windows also feature throughout.

“That’ll be the highest spec that we have worked to,” Willie McDonagh says. “The walls were massive but it wasn’t difficult to work with.”

McGann plans to get the house certified by the Passive House Institute. She says that this project has been “good in terms of professional experience.” She adds: “We’re using it as an opportunity to apply the passive house standard to an architectural design.”

Selected project details

Client: Cuala McGann & Olivier Bauduin
Architect: McGann Scahill Architects
Civil / structural engineer: Structural Design Services
Main contractor: Oak Barn Construction
Wall insulation, roof insulation & airtightness products: Ecological Building Systems
Windows and doors: Optiwin
Mechanical & electrical contractor: Ecowest

Project overview:

Building type
: 204 square metre detached two-storey timber frame house

: Westport, Co Mayo   

Completion date
: under construction 2011

Passive house certification
: pre-submission

Space heating demand (PHPP)
: pending

Heat load (PHPP)
: pending

: max. 0.60 air changes per hour at 50 pascals pressure


Ground floor
: raft foundation insulated with 200mm Kingspan insulation under-slab, 35mm insulation over-slab.
U-value: 0.12 W/m2K

: stick-built I-Joist timber frame: render finish on render carrier board on treated SW battens on Solitex facade membrane, 160mm woodfibre board, 302mm timber I-joists fully filled with cellulose insulation, 18mm taped and sealed OSB, 100mm insulated service cavity and 12mm plaster carrier board internally.
U-value: 0.07 W/m2K

: natural zinc finish on 18mm OSB on 50x35 treated SW battens on Solitex-Mento breathable roofing membrane, on 35mm Gutex-Multiplex woodfibre insulation, on 360mm timber I-joists fully filled with cellulose insulation, 18mm taped & sealed OSB, 170mm woodfibre insulation service zone, plastered finish.
U-value: 0.07 W/m2K

: Optiwin triple-glazed aluminium-clad timber windows, with argon filling and an overall U-value of 0.73 W/m2K

Heating system
: wood insert stove (pending) supplying heating coil on HRV and 300L buffer tank, plus solar vacuum tubes (pending) supplying separate 300L domestic hot water tank
Ventilation: heat recovery ventilation system (pending) - Passive House Institute certified to have heat recovery rate of 92%

Green materials: timber frame, cellulose insulation, woodfibre insulations, 50% GGBS cement, rainwater harvesting system & water conserving fittings (both pending)
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