This document summarizes the construction of an eco-friendly house in Brittany, France built with I-beams and cellulose fiber insulation. The owners wanted a comfortable, wheelchair accessible home with natural materials. They hired a contractor experienced in building with I-beams, which provide structure while allowing thick insulation. Combined with cellulose fiber, the walls achieved high thermal resistance. The build faced some cost and technical challenges but ultimately was very energy efficient, meeting passive house standards.

1. 76
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
Rennes
Bréhan
Bréhan
Overview - Photo © Franck Robidou - Positivement Bois

2. www.libnam.eu 2
Context
As 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.
Theywereattractedbythetechnique
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

3. French-British collective Libnam3
Building overview
The 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

4. www.libnam.eu 4
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

5. French-British collective Libnam5
Technical focus :
I beam
An 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

6. www.libnam.eu 6
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.
Wood stove - Photo © Servane Guihaire - Construc-
tys Bretagne

7. 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

8. 8
Stakeholders
Main contractors : a couple wishing to keep
their identity private
Architect : 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 tax
Project manager : £3,922 including tax
General contractor : £90,020 including tax
Flooring : £6,276 including tax
Plumbing- heating : £6,276 including tax
Electricity : £4,707 including tax
Roofing : £14,121 including tax
Total 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
Contact
Constructys Bretagne
www.constructys-bretagne.fr