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Energy and Comfort Performance of Solid Wood Buildings:
Literature Review
For the New Zealand Pine Manufacturers Association Solid Wood Buildings Initiative
by
Larry A. Bellamy1 and Donald W. Mackenzie2
September 2006
1. Ensys Ltd, 590 Trents Road, Prebbleton 7604, New Zealand
2. Natural Resources Engineering Group, PO Box 84, Lincoln University 7647, New Zealand
1.
Introduction
Two recent developments in the building energy and indoor environment area are of potential importance
to the New Zealand timber industry. The first is solid wood building envelopes that exchange moisture
with the indoor air. It is claimed that moisture exchange moderates variations of indoor humidity, resulting
in improved air quality and thermal comfort [1, 2, 3]. It is also claimed that moisture exchange reduces
building energy use [1].
The second development is the emergence of computer programs that model heat and moisture transfer
within the building envelope. These models can be used to demonstrate the comfort and energy benefits
of solid wood envelopes that exchange moisture with the indoor air. In fact, claims about the benefits of
moisture exchange are largely based on simulation studies using these models.
This literature review is mainly focused on the effect of moisture exchange between solid wood building
envelopes and the indoor air on the thermal mass and energy use of buildings. The need for research to
determine the energy benefits of solid wood envelopes in New Zealand houses is identified, and the
suitability of two models for this purpose, WUFI-ORNL/IBP1 [4] and BSim2 [5], are considered.
Thermal mass
Test building experiments in the US [18] show that log houses may use 2.5% to 45+% less energy than
their timber frame counterparts, depending on climate and building design, due to solid wood’s superior
thermal mass, i.e. its superior ability to absorb, store and later release of heat. This level of saving is
similar to that found for concrete houses [17, 19]. This is surprising because concrete appears to be a
better thermal mass material than wood 3. Moisture exchange may explain wood’s better than expected
energy performance.
Wood is a hygroscopic material, i.e. dry wood will adsorb water vapour provided it is not sealed with an
impermeable coating. Adsorbed water vapour is stored as liquid water, adding latent heat (Figure 1) to
wood as it condenses [2]. In the reverse process, latent heat is removed from wood during desorption.
Therefore moisture exchange increases the ability of wood to absorb and release heat, i.e. increases its
thermal mass. Simulation studies comparing solid wood with concrete, such as [17], account for heat
conduction within the envelope but not moisture exchange, and so may underestimate the thermal mass
benefits of solid wood buildings.
Moisture storage and transport in wood [2, 4, 11]
The moisture content of wood in equilibrium with its surroundings depends on the relative humidity of
adjacent air, whether equilibrium is reached by sorption (wetting) or desorption (drying), and on
temperature. A typical sorption isotherm for wood is shown in Figure 2.
Moisture transport in dry wood is predominantly by vapour diffusion, driven by gradients in water vapour
pressure. In moist wood, where capillaries and cavities are coated with a film of bound water, but are not
1
2
3
Available from the Oak Ridge National Laboratory, USA.
Available from the Danish Building and Urban Research, Denmark.
Typical values for thermal conductivity and volumetric heat capacity are 0.12 W/mK and 0.7 MJ/m 3K respectively
for wood, and 1.6 W/mK and 2.1 MJ/m3K respectively for concrete.
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filled with water, moisture is transported by vapour and surface diffusion. Surface diffusion transports
liquid moisture from wetter to drier regions of wood and is driven by gradients in wood water content. In
wet wood, moisture transport is predominantly by capillary flow and is driven by the difference in relative
humidity at the ends of the capillary. While all three moisture transport mechanisms may occur
simultaneously, typically only vapour and surface diffusion occur in a solid wood envelope protected from
extreme conditions, such as rain penetration.
Water vapour in a solid wood envelope is typically transported towards the outdoors (Figure 3) but it will
flow towards the indoors when the vapour pressure within the envelope exceeds that of the indoor air.
Water vapour pressure depends on air temperature and relative humidity. The relative humidity of air in
moist wood (Figure 2) may be significantly greater than that of indoor air, in which case vapour may flow
towards the indoors even when the envelope is slightly cooler than the indoor air.
Figure 1 Latent heat of sorption of wood [2]
2500
Energy Level Relative to Liquid Water
(kJ/kg)
Water vapour
2000
1500
1000
Latent heat of sorption
500
0
Water bound in wood
-500
-1000
-1500
0
5
10
15
20
25
30
35
Moisture Content
(%)
Figure 2 Typical sorption isotherm for wood at different temperature [8]
160
140
Water Content
(kg/m3 )
120
100
Temperature = 0 o C
80
60
Temperature = 25 o C
40
20
0
0
10
20
30
40
50
60
70
80
90
100
Relative Humidity
(%)
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Figure 3 Moisture transport through a building envelope
Typical relative humidity
and moisture content
gradients
Water vapour pressure
gradient
Outdoor
Indoor
Surface diffusion
Liquid film on
capillary surface
Vapour diffusion
Surface diffusion
Note: Flow in a solid wood envelope may be in the opposite direction to that shown above,
as discussed in Sec 1.
Adapted from [4].
2.
Energy advantage of solid wood envelopes
Solid wood envelopes have the potential to buffer variations of indoor humidity by adsorbing moisture
from the indoor air when the humidity of indoor air is relatively high and desorbing moisture to the indoor
air when the humidity is relatively low. The amount of moisture exchanged between a building envelope
and the indoor air depends on [1-3, 6-14]:
(i)
(ii)
(iii)
(iv)
(v)
(vi)
(vii)
(viii)
(ix)
(x)
Water vapour permeability of wood, which depends on the wood species, the direction of the grain
with respect to the direction of moisture flow and the wood moisture content.
Moisture storage capacity of wood.
Water vapour permeability of internal linings and/or coatings applied to the wood.
Area of exposed wood.
Rate at which moisture condenses on windows.
Characteristics of moisture sinks (e.g. furnishings) within the room.
Rate at which moisture is produced within the room (e.g. from people, washing and vegetation).
Moisture content and flow rate of air (from outdoors and other rooms) entering the room.
Room air temperature.
Solar radiation within the room.
So moisture exchanged by an envelope depends on the external climate ((v); (viii)-(x)), building design
((i)-(vi); (viii)-(x)) and the behaviour of occupants ((vi)-(x)).
Moisture may penetrate an exposed solid wood envelope by only a few millimetres on a diurnal basis [2,
9], but experiments [6] and simulation [2] have shown that it can reduce diurnal variations of indoor
humidity by more than 50%. Of greater interest here is the effect of moisture exchange on building
energy use.
The thermal conductivity of wood increases with increasing moisture content. Therefore an increase in
moisture content due to moisture adsorption reduces the thermal resistance of a solid wood envelope.
However, moisture desorption increases the thermal resistance of the building envelope. It is not clear
how these moisture-related variations in an envelope’s thermal resistance affect building energy use.
Moisture exchange increases the thermal mass of a solid wood envelope, which is expected to reduce
energy use, in sunny houses at least [17]. Moisture exchange increases thermal mass in three ways. It
increases the thermal conductivity of wood and therefore the rate at which it can absorb and release
sensible heat. Secondly, it increases its heat storage capacity. Lastly, as discussed in Sec. 1, the latent
heat associated with moisture exchange increases the ability of wood to absorb and release heat. The
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latent heat flows may be of similar magnitude to the sensible heat flows [2], so moisture exchange may
significantly increase the thermal mass of a solid wood envelope. Simulation has confirmed this by
showing that moisture exchange may moderate indoor air temperature variations by up to ±2oC [7].
According to [1] the reduction in indoor humidity by a solid wood envelope may indirectly save energy in a
number of ways. Ventilation may be reduced without sacrificing perceived air quality (air freshness) or
occupant health. This is expected to reduce energy use for heating, and possibly also energy use for
cooling in hot, humid climates. A reduction in humidity also allows higher building cooling temperatures
without sacrificing either thermal comfort or warm respiratory comfort (cooling of respiratory tract). This is
expected to reduce energy use for cooling in air conditioned buildings.
Simulation results
The potential of moisture exchange to reduce building energy use has been recognised by a number of
researchers, but only Osanyintola and Simonson [1] have performed a simulation analysis to study this
matter, as far as it is known by the authors. They modelled a west-facing bedroom in a number of
European cities. Moisture exchange reduced energy use for heating by 5-10% during the night but had
little effect on total heating energy use. Moisture exchange reduced peak cooling loads by up to 30% but
had little effect on total cooling energy use. These results do not include the savings due to reduced
ventilation or relaxed heating and cooling temperatures, which were estimated to be in the order of 5%.
Osanyintola and Simonson concluded that the most promising energy savings from moisture exchange
are for air conditioned buildings in hot and humid climates. This conclusion may be premature as it is
based on a single room model that did not account for the varying pattern of solar heat gain in adjacent
rooms. Consequently the heating energy required to ‘dry’ the building envelope may have been
overestimated. An analysis based on a whole building model may have yielded different results.
Experimental results
The effect of moisture exchange on building energy use has not been determined experimentally, as far
as it is known by the authors.
Künzel et al [6] measured the heating energy used in side-by-side test buildings at Holzkirchen, Germany,
while investigating the moisture buffering effects of solid wood envelopes. This information has not been
published. This is unfortunate because the test buildings were configured to isolate the effect of moisture
exchange – the buildings were essentially identical, apart from the moisture absorbing characteristics of
the interior lining. One was lined with painted plaster and the other with solid wood. The energy data from
this study should indicate the level of the energy advantage of solid wood envelopes. However, the
climate and building designs in this German study are unlikely to represent conditions in New Zealand.
3.
Modelling moisture in solid wood envelopes
Questions have been asked about the capability of hygrothermal models to simulate the moisture
behaviour of building envelopes [2].
A number of models have been used to research heat and moisture flows in building envelopes, including
WUFI-ORNL/IBP and BSim. The key issue here is whether these models are capable of determining the
effect of moisture exchange between the envelope and indoor air on building energy use.
WUFI-ORNL/IBP
WUFI-ORNL/IBP is claimed to be a state-of-the-art model for determining heat and moisture flows in a
building envelope [4]. In validation studies good agreement has been found between its predictions and
experimental data [4]. However WUFI-ORNL/IBP does not simultaneously determine the heat and
moisture flows throughout the whole building and does not determine the indoor climate (this is specified).
So this model is not capable of determining the effect of moisture exchange on building energy use.
The Fraunhofer Institute für Bauphysik is developing WUFI+ [3], which models the heat and moisture
flows for a whole building. It appears that WUFI+ can determine the effect of moisture exchange on
building energy use, but this model has not been released yet, as far as the authors are aware.
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BSim
BSim models the heat and moisture flows throughout the whole building and has proved to be relatively
accurate at predicting the thermal and moisture conditions of test buildings with [5] and without [15, 16] a
moisture buffering envelope.
BSim takes a simple approach to modelling moisture within the building envelope as it only accounts for
moisture transport by vapour diffusion and does not account for liquid flow. Vapour diffusion is typically
the dominant mode of transport in weather-tight building envelopes [7], which probably explains why
BSim’s simple moisture model has proved acceptable in validation studies. So this model is expected to
be suitable for researching the effect of moisture exchange on the energy used in New Zealand houses.
Two other leading building energy models, esp-r and Energyplus, are capable of determining the effect of
moisture exchange on building energy use. However, the moisture parts of these models do not appear to
have been tested to the same extent as WUFI-ORNL/IBP and BSim.
4.
Conclusions and Recommendations
The ability to adsorb significant amounts of moisture is not unique to solid wood envelopes, but it does set
them apart from conventional envelopes with painted plasterboard internal linings, which have limited
ability to absorb moisture [6].
Moisture exchange between a solid wood envelope and the indoor air depends on many factors but the
vapour permeability of any coating applied to the internal surface of the envelope has a major effect on
moisture flow.
There is a growing body of knowledge about the moisture buffering capability of solid wood envelopes
and the comfort and health improvements resulting from reduced variations in indoor humidity [7]. Much
less is known about the energy advantage of solid wood envelopes, especially those that exchange
moisture with the indoor air. It can be confidently stated that moisture exchange significantly increases
the flow of heat to and from the envelope, but insufficient evidence has been found in the scientific
literature to support or oppose the hypothesis that moisture exchange significantly reduces building
energy use. We are not alone in this conclusion [20].
Overseas research shows that solid wood buildings can provide significant thermal mass benefits
including savings in building energy use [18]. However, research needs to be undertaken in order to
determine the energy advantage of solid wood envelopes in New Zealand houses. A simulation study is
the only feasible way of systematically evaluating the many factors that affect moisture exchange within
these envelopes. A simulation study is recommended as it will help to build a knowledge base that
supports product development and marketing initiatives by the New Zealand timber industry. It will also
support the position of solid wood buildings during future changes to the energy efficiency provisions of
the building code.
A simulation study could stand alone but a complementary experimental program is recommended. The
main purpose of an experimental program would be to demonstrate the veracity of the simulation results.
However, the experimental buildings would clearly be designed to showcase the energy advantage of
solid wood envelopes. Model validation could be built into an experimental program, but this is not seen
to be essential as there is already a high level of confidence in these models.
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References
[1]
O.F. Osanyintola and C.J. Simonson, Moisture buffering capacity of hygroscopic building materials:
Experimental facilities and energy impact, Energy and Buildings 38 (2006) 1270-1282.
[2]
S. Hameury, Moisture buffering capacity of heavy timber structures directly exposed to an indoor
climate: a numerical study, Building and Environment 40 (2005) 1400-1412.
[3]
H.M. Künzel, A. Holm, D. Zirkelbach and A.N. Karagiozis, Simulation of indoor temperature and
humidity conditions including hygrothermal interactions with the building envelope, Solar Energy 78
(2005) 554-561.
[4]
A. Karagiozis, H. Künzel and A Holm, WUFI-ORNL/IBP – A North American hygrothermal model,
Proceedings of Performance of Exterior Envelopes of Whole Buildings VIII, December 2-7, 2001.
ASHRAE, Atlanta, US
[5]
C. Rode, K. Grau and T. Mitamura, Model and experiments for hygrothermal conditions of the
envelope and indoor air of buildings, Proceedings of Performance of Exterior Envelopes of Whole
Buildings VIII, December 2-7, 2001. ASHRAE, Atlanta.
[6]
H.M. Künzel, A. Holm, K. Sedlbauer, F. Antretter and M. Ellinger, Moisture buffering effects of
interior linings made from wood or wood based products, IBP Report HTB-04/2004/e, 2004,
Fraunhofer Institut für Buaphysik, Germany.
[7]
C.J. Simonson, M. Salonvaara and T. Ojanen, Improving indoor climate and comfort with wooden
structures, VTT Publications 431, 2001, Technical Research Institute of Finland, Finland.
[8]
C. Rode (Ed.), Moisture buffering of building materials, Report BYG∙DTU R-126, 2005, Department
of Civil Engineering, Technical University of Denmark, Denmark.
[9]
S. Hameury and T Lundström, Contribution of indoor exposed massive wood to a good indoor
climate: in situ measurement campaign, Energy and Buildings 36 (2004) 281-292.
[10] K.K. Hansen, T Padfield, C. Rode, F. Kristiansen and E.J. de Place Hansen, Experimental
investigation of the hygrothermal performance of insulation materials, Proceedings of Performance
of Exterior Envelopes of Whole Buildings VIII, December 2-7, 2001. ASHRAE, Atlanta.
[11] M. Salonvaara, A. Karagiozis and A. Holm, Stochastic building envelope modelling – the influence of
material properties, Proceedings of Performance of Exterior Envelopes of Whole Buildings VIII,
December 2-7, 2001. ASHRAE, Atlanta.
[12] C.J. Simonson, M. Salonvaara and T. Ojanen, Moderating indoor conditions with hygroscopic
building materials and outdoor ventilation, ASHRAE Transactions 2004: 804-819.
[13] O.F. Osanyintola, P. Talukdar and C.J. Simonson, Effect of initial conditions and thickness on the
moisture buffering capacity of spruce plywood, Energy and Buildings 38 (2006): 1283-1292.
[14] C. Rode, N. Mendes and K. Grau, Evaluation of moisture buffer effects by performing whole-building
simulations, ASHRAE Transactions 2004: 783-794.
[15] K.J. Lomas, K. Eppel, C.J. Martin and D.P. Bloomfield, Empirical validation of building energy
simulation programs, Energy and Buildings 26 (1997): 253-275.
[16] L.A. Bellamy and D.W. Mackenzie, Thermal performance of buildings with heavy walls, BRANZ
Study Report No 108, 2001, Building Research Organisation of New Zealand, Porirua.
[17] L.A. Bellamy and D.W. Mackenzie, The thermal performance of concrete homes in New Zealand: A
review of research projects sponsored by the CCANZ, Technical Report TR28, New Zealand
Concrete Society Conference 2003, New Zealand Concrete Society, Auckland.
[18] R. Prickett (Ed.), The energy performance of log homes, 2005, Log Homes Council, USA.
[19] J. Kosny, P. Childs, T. Petrie, A. Desjarlais, D. Garwin and J. Christian, Energy benefits of
application of massive walls in residential buildings. Proceedings of Performance of Exterior
Envelopes of Whole Buildings VIII, December 2-7, 2001. ASHRAE, Atlanta.
[20] S. Hameury, Personal communication, August 2006.
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Appendix: Research Organisations
1.
Fraunhofer Institut für Bauphysik, Holzkirchen, Germany
Key contact:
Activities:
2.
VTT Building and Transport, Technical Research Centre of Finland, Espoo, Finland,
Key contact:
Activities:
3.
Hartwig Künzel
Development and validation of models for heat and moisture transfer in building
envelopes, including WUFI-ORNL/IBP, in collaboration with Oak Ridge National
Laboratory [4].
Simulation of the moisture buffering effects of solid wood building envelopes [3].
Testing the moisture buffering capacity of solid wood building envelopes, including the
use of side-by-side test buildings [6].
Mikael Salonvaara
Simulation of the moisture buffering effects of solid wood building envelopes [7, 11].
Royal Institute of Technology (KTH), Stockholm, Sweden
Key contact:
Activities:
Stéphane Hameury
Development of models for heat and moisture transfer in building envelopes, including a
modified version of the IDA ICE model [2].
Simulation of the moisture buffering effects of solid wood building envelopes [2].
Assessment of the comfort and energy performance of residential buildings constructed
with solid wood envelopes [9].
4.
Technical University of Denmark (DTU), Lyngby, Denmark
Key contact:
Activities:
5.
University of Saskatchewan, Saskatoon, Canada
Key contact:
Activities:
6.
Carsten Rode
Development of standard methods for testing and characterising the moisture buffering
capacity of building materials [8, 10].
Development of models to account for heat and moisture transfer in building envelopes,
in collaboration with Danish Building and Urban Research [5].
Carey Simonson
Simulation of the moisture buffering effects of solid wood building envelopes [1, 12] in
collaboration with VTT Building and Transport.
Development of standard methods for testing and characterising the moisture buffering
capacity of building materials [1, 13].
Oak Ridge National Laboratory, Tennessee, USA
Key contact:
Activities:
Achilles Karagiozis
Evaluation of the hygrothermal performance of building materials.
Development of models for heat and moisture transfer in building envelopes, including
WUFI-ORNL/IBP, in collaboration with Fraunhofer Institut Bauphysik [4].
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