Download Indoor Air Quality in a Wooden House Summary 1. Introduction

Indoor Air Quality in a Wooden House
Mikael Salonvaara*, MSc
Research Scientist
[email protected]
Tuomo Ojanen, MSc
Senior Research Scientist
*
VTT Building and Transport
PB 1806, FIN 02044 VTT
Mikael’s expertise is in heat,
air, moisture and pollutant
transport in building structures
and in whole buildings. He
has developed and used
simulation models to analyse
the hygrothermal performance
of building envelope parts in
interaction with indoor
climate.
Carey Simonson, PhD
Associate Professor
University of Saskatchewan,
Saskatoon, Canada
Summary
This paper contains a short summary of laboratory, field and numerical studies of the indoor
temperature, humidity, comfort and indoor air quality in wooden houses. Wood and wood-based
materials have hygroscopic properties that are favourable for producing good indoor climate. The
results show that the conditions in a room improve significantly when the hygroscopic materials
replace non-hygroscopic materials. Furthermore, the diffusion of gases through a permeable
building envelope can significantly increase the effective ventilation rate for poorly ventilated
rooms. Hygroscopic buffering may also result in energy savings. International activities are on
going to produce standardized properties of materials and products and design principles.
Keywords: Indoor air quality, humidity, mass transfer, sorption, hygroscopic materials, tracer gas,
ventilation, air change rate
1.
Introduction
Low levels of indoor pollutants (volatile organic compounds (VOC’s), radon, dust, bacteria, mould,
odors, etc.) and acceptable temperature and humidity are important for indoor air quality (IAQ) and
occupant health. Since occupants and buildings produce carbon dioxide (CO2), moisture, VOC's
and other gases, which affect the quality of indoor air, outdoor ventilation is necessary. Typically
outdoor ventilation rates are set with the assumption that the building envelope and furniture are
sources of contaminants. However, for carefully selected components, it is possible that the
building envelope and furniture can act as contaminant sinks and actually improve the indoor air
quality [1]. If a building envelope is made from permeable materials, the diffusion of pollutant
gases through the envelope can reduce the indoor concentration of pollutants. Therefore, it may be
possible to provide a comparable indoor environment with a lower outdoor ventilation rate when a
permeable and hygroscopic building envelope is applied.
Also, since the perception of IAQ is closely linked to the humidity of indoor air, moisture transfer
between the indoor air and building structures could reduce the needed ventilation rate. Reducing
the ventilation rate could have a significant impact on energy consumption because up to 50% of
the energy consumed in buildings is used to condition ventilation air [2].
Some research on the interactions between the indoor air and the building envelope has been carried
out already decades ago but the phenomena has received increased attention in the past few years.
Workshop 10 that was arranged at the Healthy Buildings 2000 conference in Helsinki dealt with the
effect of wood based materials on indoor air quality and climate. Experts from many different
countries agreed that the moderating effect of wood based materials on the indoor climate is likely
to improve human comfort conditions compared to non hygroscopic and vapour retarding envelope
materials. It was felt that more research is necessary in order to quantify and optimise the passive
indoor climate control. Quite recently new research projects have been started on these issues. The
new Nordtest project “Moisture buffering of building materials” is to provide a standardized
method to determine dynamic moisture properties of materials and products. The IEA’s
(International Energy Agency) new Annex 41 “Whole building hygrothermal performance”
involves participants from approximately 20 countries. Meanwhile research on indoor climate of
wooden buildings has been ongoing in Finland. The following chapter summarises some of the
latest results.
2.
Summary of past and current research
2.1
Past research
2.1.1 Tapanila Ecological House [3-8]
This research on a single-family house (gross floor area of 237 m2) was carried out at the Technical
Research Centre of Finland and sponsored by the Finnish Ministry of the Environment. The wood
frame house has no plastic vapour retarder and is insulated with 250 mm and 425 mm of wood fibre
insulation in the walls and roof respectively. The measurements conducted in the house from 1999
to 2000 demonstrate that it is possible to design and construct a low-energy house with an airtight
and vapour permeable envelope. To realize a moisture physically safe structure, the vapour
permeable envelope must be airtight (e.g., 3 ach at 50 Pa) and the water vapour diffusion resistance
must be greater on the warm side of the insulation (e.g., 5 times) than on the cold side.
Measurement and simulation results show that moisture transfer between indoor air and a porous
building envelope can reduce the maximum indoor humidity in the summer by about 20% RH and
increase the minimum indoor humidity in the winter by about 10% RH when the ventilation rate is
near design (0.5 ach). According to the literature, decreasing the humidity by 20% RH could
possibly double the number of occupants satisfied with the indoor thermal comfort and IAQ
conditions.
2.1.2 Comfortable Wooden Buildings – Phase I [9-12]
This numerical study was conducted at the VTT Building and Transport using the LATENITE
simulation program for Wood Focus Oy. The project investigated the effect of wood based
materials on indoor climate. The results showed that it is possible to improve indoor humidity
conditions when appropriately applying hygroscopic wood based materials. This is important
because the literature shows that indoor humidity has a significant effect on occupant comfort,
perceived air quality, occupant health, building durability, material emissions and energy
consumption. Therefore, it appears possible to improve the quality of life of occupants and the
energy consumption of buildings when appropriately applying hygroscopic wood based materials.
Meanwhile, the risk of mould growth is low for well-designed structures.
The numerical investigation concentrated on a bedroom in a wooden building located in 4 European
countries (Finland, Belgium, Germany and Italy). When a well-ventilated bedroom (0.5 ach) is
occupied by two adults for 9 hours each night, the indoor humidity is close to the outdoor humidity
in the evening and increases during the night. Figure 1 shows the hourly temperature and humidity
values for the bedroom located in Espoo. The increase in absolute humidity during the night is quite
independent of the climate. However, the level of indoor temperature and humidity are very
dependent on the climate. Passive methods of controlling the indoor climate are more successful in
moderate climates than in hot and humid climates, even though they provide benefits in all
climates.
The results show that moisture transfer between indoor air and hygroscopic materials significantly
reduces the peak indoor humidity (up to 35% RH) and increases the minimum indoor humidity (up
to 15% RH). Based on correlations from the literature, which quantify the effect of temperature and
humidity on comfort and perceived air quality for sedentary adults, hygroscopic materials can
improve indoor comfort and air quality as well. According to the numerical results, it is expected
that people will be more satisfied in a room with hygroscopic structures than in a room with nonhygroscopic structures, especially at the end of occupation (i.e., in the morning for a bedroom). In
the morning, an average of 6 more people (out of 100) will be satisfied with the air quality in the
hygroscopic bedroom than in the non-hygroscopic bedroom studied in this research. During certain
times of the year (mainly summer), as many as 25% more people will be satisfied in the
hygroscopic bedroom.
hygroscopic
non-hygroscopic
16
14
0%
10
RH
10
8
80%
6
6 0%
4 0%
4
2 0%
2
W (g/kg)
12
0
10
15
20
25
30
T (°C)
Figure 1. Simulated hourly values of indoor temperature and humidity during the entire year in the
hygroscopic and non-hygroscopic case in Espoo (Finland).
This research also shows the connection between the moisture removal capacity of ventilation and
hygroscopic materials. For example, the increase in humidity during the night was the same in the
hygroscopic bedroom with a ventilation rate of 0.1 ach as in the non-hygroscopic bedroom with a
ventilation rate of 0.9 ach. Since the indoor conditions at the beginning of occupation are often the
same regardless of the night-time ventilation rate (due to airing or air exchange with the rest of the
house during the day), a room with a permeable and hygroscopic structure will have a similar
moisture performance at a significantly lower ventilation rate than a room with an impermeable or
non-hygroscopic structure. This and other results suggest that the ventilation rate could be
decreased slightly in a room with hygroscopic materials without degrading the indoor humidity,
comfort and air quality conditions. The possible decrease typically ranges from 20% to 50%
depending on the variables and criteria chosen.
2.1.3 Comfortable Wooden Buildings – Phase II [13-16]
The second phase of the project focused on verifying the results found in phase I. In Phase II VTT,
Helsinki University of Technology and Fraunhofer Institute for Building Physics (Holzkirchen,
Germany) worked together carrying out detailed material property testing, small- and full-scale
laboratory tests, some field tests and numerical analyses. The results of the second phase in the
project supported the findings of phase I.
Material testing in the laboratory proved the importance of surface treatment of materials on the
ability of the material to interact with the indoor climate. Surface treatments or paints that are more
permeable that those used today would be needed in order to effectively exploit the phenomena.
New simulations show that the moisture safety of building structures can be improved by adding
hygroscopic capacity in contact with indoor air. The hygroscopic buffer lowers the maximum
humidity in indoor air, which reduces the risk of condensation on or immediately below the surface
materials of the structures.
Different surface materials were tested in Holzkirchen with constant ventilation rate in the room
and the results were compared to those of the reference room. The reference room had plastered and
painted surfaces. Wood as a surface material performed even better than expected by the
simulations as a moisture buffer dampening the variations in indoor humidity efficiently. The
numerical simulations compared well with the experiments and thus allows us to assume that the
results of phase I simulations are valid.
Preliminary results from field measurements in Germany show that wooden houses with
hygroscopic mass or moisture capacity have less varying and more constant indoor humidity
throughout the year than houses with less active moisture capacity (e.g., stone houses with painted
plaster walls).
3.
Conclusions
Moisture transfer between indoor air and structures is important and several wood based materials
are appropriate. One of the most important findings is that the vapour resistance of the interior
coating and the active area are very important and can be used to compensate each other. This
means that in new and retrofit buildings it may be possible to apply surface texturing to increase the
active area or small but highly active modules. These modules could employ natural or forced
convection and could take up a small fraction of the internal surface area of the room, but have an
internal surface area that would be comparable to the entire surface area of the room. Since the
local airflow and mixing will have a large impact on the performance of very small modules, large
rooms with poor mixing may need several modules distributed throughout the room.
Moisture is an important comfort and indoor air quality parameter and the comfort and perceived
indoor air quality can be improved when applying permeable and hygroscopic materials. It appears
possible to provide similar indoor climate and perceived air quality conditions with a smaller
ventilation rate when permeable and hygroscopic materials are correctly applied. To quantify the
amount that ventilation can be reduced when applying wood based materials in real buildings
would be a long and difficult task, but future research in the area of indoor climate and air quality
could focus on defining appropriate ways to quantify the effect of humidity on IAQ. It is expected
that health affects would create additional arguments for the application of hygroscopic wood based
materials.
It is known that the durability of building materials and the risk of mould growth are affected by
moisture. Results show that it is possible to design a permeable envelope with good moisture
performance. In fact a permeable envelope made of wood based materials is less susceptible to
condensation and mould growth at the internal surface of thermal bridges because the peak indoor
humidity is lower when applying hygroscopic wood based materials.
The commercial application and exploitation of the moisture buffering effect requires the
introduction of the phenomenon with correct argumentation and solid design methods.
Demonstration of the designed performance in real and occupied buildings is still needed.
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