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Cellulose fibre insulation and I beams : a marriage made in

heaven for a bioclimatic house.

Key dates Planning permission : December 2012. Work : March 2013 to october 2013.

A couple built a detached eco house in the Morbihan area with «new generation» beams. Combined with recycled newspaper insulation (cellulose fibre), these I-beams provide fast and efficient insulation to the house.

Introduction

Saint Brieuc

Quimper

Vannes

RennesBréhan

Bréhan

Overview - Photo © Franck Robidou - Positivement Bois

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ContextAs they wanted to live in a comfortable house, the owners imagined a light, single level (wheelchair-friendly) house with plenty of storage and a low pitched roof. Their ambition was also the optimal orientation of the house and to use as many natural materials as possible.By visiting business fairs with the idea of finding a building method compatible with their project, the main contractors came across the «Positivement Bois» company to whom they awarded the contract. They were attracted by the technique of building with I beams combined with cellulose fibre for insulation.This company put them in touch

with an architect who came up with drawings adapted to their 927 sq m plot situated in the rural village of Bréhan. The plot was ideal as the 103 sq m house occupies a flat site, fully exposed apart from 2 oak trees that shed their leaves in winter.For the design, the architect followed the bioclimatism golden rules (south facing living areas with large bay openings and north facing service areas). He split the bedrooms on either side of the main living area and designed them as projections to the main building space, defining the outside terrace.

Plan © Yannick Leroy

French-British collective Libnam3

Building overviewThe building is made of I-beams (of larger size for the frame), insulated with cellulose fibre of different density : 60 kg/cubic metre for walls and parts of the roof where it was blown in. However, the density is only 35 kg/cubic metre in other parts of the roof where access was difficult. The cellulose fibre was blown onto a dry and horizontal open surface.The outer shell is made of untreated Douglas fir cladding – which means it will naturally turn grey with time, something the owners acknowledged.The structure lies on a polystyrene-insulated concrete slab with raised edges to avoid thermal bridges.In the building, the internal walls are made of class 3 Douglas fir sitting on wood fibre strips (Phaltex) which improves the acoustic properties. To complete the sound insulation of the partition walls, wood wool boarded

with plasterboards has been used, except in wet rooms where gypsum (Fermacell) was chosen for its resistance to dampness.Double-glazing (K-line) was used instead of triple-glazing because of costs and loss of light, but also because it is relatively low impact in Brittany’s mild weather and on a south facing façade: according to the glazing company triple glazing is only useful for passive houses. In this case, 4/16/4 Argon-filled glass panes with warm edges (for thermal bridges) were fitted.The heating is provided by a wood burner (3.6 kW) and an electric towel drier. To avoid cooling the room, the burner is airtight and the airflow comes from the outside through a pipe under the slab.The airflow is of hydro A type with air vents suited to the Breton climate. Due to a constant high humidity rate outside, this will avoid under-ventilation and the risk of air depression, according to Franck Robidou, director of «Positivement

I-beam - Photo © Franck Robidou - Positivement Bois

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Bois».Hot water comes from a thermo-dynamic water heater : its small heat pump extracts calories from the air from the garage. To maximise the building resources, the owners went for a rain-water tank for toilets and watering the garden. They also chose a LED lighting system and wall mounted electric sockets with switches.Roof structure : EPDM for water proofing – CTBH water resistant wooden board – 300 mm I-beam and cellulose fibre – Isocell vapour barrier (SD : 1500 mm with no air gap) – Douglas fir batten - plaster board or gypsum.

Wall structure : Douglas fir cladding – batten – SIGA rain barrier- 10 mm OSB – 240 mm I-beam and cellulose fibre – vapour barrier (SD 18m) – Douglas fir batten – plasterboard or gypsum.Internal wall structure: plasterboard or gypsum – 45x45 Douglas fir batten – Phaltex wood fibre strips – 45 mm wood wool – plaster board or gypsum.Floor structure: 50 cm bedded rock- polyethylene – 100 mm polystyrene with perimeter edge strip – 12 cm concrete slab – 6 cm concrete topping – tiling or floorboard.

Cladding fitting - Photo © Servane Guihaire - Constructys Bretagne

French-British collective Libnam5

Technical focus : I beamAn I beam consists of wood fibre between 2 flanges. Unlike solid wood beams, the I-beam has the same solidity but uses fewer resources, reduces thermal bridges (heat loss) and gives room for a thick layer of insulation.The thermal resistance for an I-beam wall is of 6.15 sq Km/W against 5.75 sq Km/W for a solid wood beam wall. Furthermore, a wooden frame would not go beyond a 22 cm thickness. With engineered I beams, you can reach 90 cm, which makes the use of thick insulation possible in one go. These types of beams (Steico) are certified PassivHaus by the « Dr. Wolfgang Feist » German Passive House Institute. Cost-wise, for walls with thermal resistance higher than 6 sq Km/W, I-beams are more competitive than a classic wooden frame. I-beam

Framing - Photo © Franck Robidou - Positivement Bois

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The beams can be delivered 13 meters long. The carpenter can then cut them to size, which creates less thermal bridges than pre-manufactured beams, according to him, since more wood may be added to ensure the rigidity of the structure, which in turn would create heat loss. However, the traditional timber-frame remains popular as it is simpler to engineer. In the meantime, I-beams are 3 times lighter than classic beams. Therefore no cranes were needed on site for the build. I-beams are also useful for roofing or floors thanks to the length available (9m), so no supporting wall are needed.

I-beams are made of softwood (pine) which requires treating (Class 2 for this build); also the wood is rarely local (except the Trica beams) or even French, but more likely to be European. Compared to solid wood though, the I-beams use less natural resources (39 x65 mm beams).

Like any wooden frame, I-beams structures lack thermal mass. This is why cellulose fibre gives a perfect combination in eco-construction. This material, thanks to its good thermal qualities (much higher than rock wool) is more efficient against the summer heat.

I-beam

Wood stove - Photo © Servane Guihaire - Construc-tys Bretagne

French-British collective Libnam7

Barriers and solutions«Positivement Bois» is not specifically specialised in residential building and works more as a general contractor . The company was used to manage the project. To facilitate communication between trades and improve the quality of the work, the director used his network. During the design phase, it was easier to adjust costs with an architect they knew already.The cost was the main obstacle for this project as the owners’ budget was limited. They had to make concessions: reduce the floor size and do the cladding themselves. Some technical aspects required

a particular focus: for example, in order to anticipate the necessary bracing and the fitting of the airtight insulation, the kitchen designer and fitter had to finalise the design well in advance of fitting the kitchen. Also, the air-tight door leading to the garage was not sufficiently air-tight.On the other hand, some technical aspects became successful: for instance, the use of rolling blinds accessible from the outside and covering the water tight membrane, thus avoiding thermal bridges.Trades were following CPD. The main company attended an airtightness course with SIGA and Energy-saving in the building industry (FEEBat).

Inside- Photo © Servane Guihaire - Constructys Bretagne

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StakeholdersMain contractors : a couple wishing to keep their identity privateArchitect : Yannick Leroy (Baud)General contractor : Positivement Bois (Lanester)Roofing : CEI56 (Plougoumelen)Electricity : JS Elec (Pluvigner)Plumbing : Robert Thermie (Locmiquelic)Tiling : Le Huitouze Dominique (Languidic)

Costs Plot and connection to services : £7,845 including taxProject manager : £3,922 including taxGeneral contractor : £90,020 including taxFlooring : £6,276 including taxPlumbing- heating : £6,276 including taxElectricity : £4,707 including taxRoofing : £14,121 including taxTotal construction cost : £133,167, or £1,215/sq m including tax

The company « Positivement Bois » relied on show houses to evaluate their building system and be able to work out the dew point or the air tightness. The results show a superior performance to low consumption building standards, especially for airtightness between 0.14 and 0.30 against a 0.60 required. The electricity bill is about €46 per month and the heating bills do not exceed 10 kWh/ sq m per year.

Performance

West facade © Yannick Leroy

ContactConstructys Bretagnewww.constructys-bretagne.fr