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An unorthodox nearly zero energy building

While Susan McNamara’s home initially fell short of Ireland’s proposed standard for nearly zero energy buildings (nZEBs) — which all new dwellings must meet from 2021 — an updated assessment obtained by Passive House Plus puts the house within the nZEB criteria.

The initial building energy rating (BER) calculation, performed by BM Energy Consultants, gave the house an A3 BER (71.74 kWh/m2/yr) and an energy performance co-efficient (EPC) of 0.327. Dwellings must achieve an EPC lower than 0.3 to be deemed nZEBs according to Ireland’s proposed standard – with 0.3 meaning 30% of the energy demand of a home built to the 2005 building regulations.

However, this initial calculation used a default thermal bridging factor of 0.08 W/ m2K. A fresh thermal bridging calculation obtained by Cooney Architects from the consultancy Passivate, based on the dwelling’s as-built details, reduced the thermal bridging factor to 0.0215 W/m2K. This in turn pushed the EPC down to 0.285, within the nZEB standard, with the primary energy demand dropping to 62.17 kWh/m2/yr — still an A3 BER, but a much better one.

If the house had been built to exactly meet the 0.3 EPC target for nZEBs, its primary energy target would have been a mid A3 BER of 65.44 kWh/m2/yr – a surprising finding, given the Irish government has indicated that the nZEB standard should equate to approximately 45 kWh/m2/yr primary energy consumption for a typical dwelling.

The reason is fairly simple: it’s the form factor. Susan McNamara’s home has an architecturally-expressive design, featuring double height spaces and an L-shaped floor plan, adding up to a large amount of surface area relative to the floor area – surfaces through which heat can escape. This result also suggests that at least for some dwelling types, achieving nZEB may not be as difficult as previously thought.

Simple construction, executed well

Thermal bridging expert Andy Lundberg of Passivate praised the thermal bridging details designed by Cooney Architects for Susan McNamara’s home. In correspondence with Passive House Plus, he explained:

“The junctions themselves are quite simple, albeit robust, in that it’s cavity wall construction with full-fill bonded bead, plus internal insulation, so there’s nothing new there. There is a warm roof construction and the floor build-up would also be considered standard. There is use of some Quinn Lite thermal blocks but that’s it, there are no other special thermal breaks used. “So that’s what’s best about it — simple construction executed well, with good thicknesses of insulation carried across junctions, and careful airtightness detailing. All this means the performance gap [the difference between designed and real-world performance] is likely to be close to zero in those areas — which is far more important than designing buildings and junctions with incredible performance that are, however, difficult to execute on site.

“Looking at the building envelope, one might argue the form of the building is less than ideal from an energy efficiency point of view, and this is true. However, as individual blocks the forms are relatively efficient. “Also, purely from a y-factor [thermal bridging factor] point of view, the fact the building has a high surface area to volume ratio means heat losses from thermal bridging area are weighted over a larger area, and therefore the y-factor reduces. So interestingly, the most efficient built form does not necessarily lend itself to a low y-factor. Indeed, the opposite can sometimes be the case.”

Energy costs explained

Susan McNamara’s total energy bills have averaged a parsimonious sum of €80 per month – totaling €955 per year for all uses – heating, hot water, lighting and all household electricity use. But how do the figures in the Deap calculations compare to real usage for this nZEB?

“Looking at the building envelope, one A comparison is made tricky by the fact Deap ignores all so-called unregulated energy use – namely the plug loads for white goods, TVs, phone & laptop chargers, etc. But the figures in Deap appear fairly close to actual usage.

“Looking at the building envelope, one Assuming 50/50 use of a day and night rate meter, and competitive tariffs of 0.1680c day rate and 0.0830c night rate, the house’s total regulated usage would come out at roughly €380/yr – including €78/yr for the tiny calculated demand of 618 kWh/yr for the primary space heating system. McNamara’s free wood supply means savings of roughly €50/yr on fuel for the calculated 490 kWh of heat from the wood stove. The remainder is made up of a calculated €210 for hot water, €65 for lighting and €30 for pumps and fans. A standing charge of €344 would bring the total bill up to €915/yr. If those figures are reflected in reality, it leaves €228 for all plug loads, suggesting a fairly low usage of less than 2,000 kWh/yr.

Selected project details

Client: Susan McNamara

Architect: Cooney Architects

Main contractor: Kinnear Construction

Thermal bridging analysis: Passivate

BER: BM Energy Consultants

Ventilation: Aereco

Airtightness test: BERCerts.ie

Mineral wool insulation: Isover

Rigid insulation: Xtratherm

Airtightness products: Alfa, via Topline Hardware

Breather membrane: Tyvek

Windows & doors: Velfac

Thermal blocks: Quinn Building Products

Heat pump: Samsung, via Joule

In detail

Building type: 109 sqm detached cavity wall house

Location: Kildalkey, Co Meath

Completion date: February 2017

Budget: Under 4,500 kWh of metered electricity use per annum.

Energy performance coefficient (EPC): 0.285

Carbon performance coefficient (CPC): 0.249

BER: A3 (62.17 kWh/m2/yr)

Measured energy consumption: N/A

Airtightness (@ 50 Pascals pressure): 0.52 air changes per house / 1.492 m3/hr/m2

Thermal bridging: Quinn Lite block to inner leaf where cavity wall meets the foundation, low thermal conductivity wall ties

Energy bills (measured or estimated): Average of €80 per month for electricity (covering primary space heating, hot water and all electrical use) according to correspondence with the client. Free supply of wood for the stove from fallen trees.

Ground floor: 75mm screed incorporating underfloor heating, over 150mm Xtratherm Xt/ UF insulating board, over 150mm concrete slab. U-value: 0.12 W/m2K. 50mm insulation to perimeter.

Walls: Sand & cement render externally on 150mm external leaf concrete blockwork, on 150mm cavity fully filled with bonded-bead insulation, on 100mm internal leaf blockwork, with 82.5mm insulated plasterboard internally. U-value 0.12 W/m2K

Roof: Fibre cement roof slates, on 50mm x 50mm treated timber battens, on Tyvek Supro breather membrane, on 100mm Xtratherm Thin-R board over rafters, on 200mm rafters insulated with Isover Metac quilt insulation, with Alfra Rufol Vario vapour control membrane to underside. U-value: 0.11 W/m2K

Windows: Velfac 200 Energy triple glazed timber alu-clad windows. U-values: 0.88 to 0.95 W/m2K. Velfac timber alu-clad door with overall U-value of 0.76 W/m2K

Heating system: Samsung 12kW air-to-water heat pump supplying underfloor heating, via Joule Smart Plumb 340L cylinder integrating buffer tank and hot water cylinder. Dik Geurts Ivar 8 wood-burning stove with heat output up to 10kW.

Ventilation: Aereco humidity-sensitive demand-controlled ventilation system.