REN EWABLE
EN ERGY
PERGAMON zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
Renewable Energy IS (1998) 377-382
DYNAMIC
THE APPLICATION
OF
INSULATION
IN BUILDINGS
B J TAYLOR’
and M S IME.4BI
of Architecture, The Robert Gordon Umversity, Aberdeen, ABlO 7QB, U.K.
’ Scott Sutherland School zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
’ Department
of Engineering, Kings College, The University of Aberdeen, Aberdeen, AB24 3UE, U.K.
ABSTRACT
Dynanx
insulation, a form of ‘Breathing Wall’ construction which allows the movement of air and moisture
through the external walls of a building, was seen as one possible method for reducing building envelope heat
A research investigation was conducted
fo provide a firm
losses and achieving high indoor air quality.
scientific understanding
of dynamic msulation.
An important outcome of the work will be the development
of building envelope designs which effectively and economically employ dynamic insulation in cold climates.
This paper presents some general conclusions,
confmnmg
that the energy saving produced
by dynamic
insulation alone 1s small relaave to that obtained in conjunction with conventional air heat recovery methods.
0 1998 Elsevier Science Ltd. All rights reserved.
KEYWORDS
Breathing
wall; buildings;
dynamic Insulation;
PROPERTIES
OF DYNAMIC
heat recovery;
U-value;
ventilation.
INSULATION
Modem buildings have attempted to reduce their energy requirements by improvmg the air tightness of the
envelope and increasing the thickness of insulation. This trend has developed simultaneously with increased
use of synthetic
materials in construction,
furnishings and decorations,
which give off volatile organic
compounds.
Increasing
living standards
have also resulted in higher indoor temperature
and moisture
generation rates wtthin homes. The outcome has been a reduction in indoor air quality which chrectly affects
occupant
health, and increasing problems of dampness in homes, particularly for the poor.
Dynamic
Insulation, a form of ‘Breathing Wall’ construction
which allows the movement of air and moisture through
the external walls of a building. was seen as one possible method for reducing ventilation and building
envelope heat losses and achieving high indoor air quality.
Heat Transfer
Physical insight into
design can be gained
dynamic U-value for
be mcorporated
into
savings. This simple
the heat and mass transfer processes
from a simple 1-D analytical model.
the envelope and the mass transport
an energy and air flow balance for
analysis which can be camed out on
0960-148!/98/$-see
front matter
PII: SO960-1481(98)00190-6
0
In dynamic insulation for any proposed envelope
The model can be used to predict the effecnve or
rate for any gas species. The dynamic U-value can
the whole building to estimate the overall energy
a spreadsheet is ideal for the conceptual design of
1998 Elsevier Science Ltd. All rights reserved
WREC
378
1998
buildings. However, for the detailed design of the air permeable envelope, 2-D models of the heat air and
moisture transport should be used to assess (I) air bypassing the insulation through defects or construction
details, (ii) buoyancy effects in the porous insulation, defects and cavities, and (iii) increased heat losses and
vapour transport due to the above. There are a number of such models in existence but they tend to be
research tools and not available for use by practitioners.
Hens (1996) p rovides an excellent review of heat air
and moisture transport modelling in general and specific computer programs in particular.
This paper will describe the practical results of the 1-D analytical model. It was shown (Taylor et.,
1996)
that the dynamic U-value for a multi-layer envelope can readily be calculated from the total thermal resistance
of the wall (R_) and the air flow through the wall (v)
“, =R,(expg.E:R,,-1,
The dimensionless
group of variables
resemblance to the P&let number
that
controls
the behaviour
of dynamic
(1)
insulation
has a formal
c L
\‘p
Pe = -.-AA-k
(4
Unlike boundary
layer analysis where it is the fluid physical properties
that are employed, the thermal
conductivity, k, in this case refers to the porous material. The density, r, and specific heat, c; are that of the
air. Table 1 illustrates how the material thermal conductivity and the air flow combine to determine the
dynamic U-value for two envelopes, one comprising 200 mm of cellulose insulation and the other 200 mm
thrck porous masonry block such as Pumalite.
Table 1: Dynamic
U-Value versus thermal conductivity
Cellulose (k = 0.035 W/mK)
and air flow rate.
Pumalite
(k = 0.3 W/mK)
1
10
1
10
1.91
19.1
0.224
2.24
U, / US
0.33
9.5 E-8
0.89
0.27
U, (W/m%
0.058
1.7 E-8
1.34
0.4
v (m/hr)
Pe
--_
The masonry wall requires an air flow approximately
ten times that of cellulose to achieve a comparable
improvement
(U,/UJ
in U-value.
However, to achieve the same insulation value the air flow through a
PumaJite wall would have to be about 100 times that for cellulose. Consideration
of the pressure drop across
the wall (280 Pa) at a flow rate of 100 m/h leads to the conclusion that this would not be practical.
Thus dynamic insulation works best with materials that are inherently good insulators.
However, the thermal
capacity of the masonry can be combined with the insulating properties
of the cellulose to produce a
composite permeable wall with a low U-value and high thermal capacity.
Another reason why this is the case is that the analytical theory assumes that the air and the solid matrix of
the porous insulation are in local thermal equilibrium. This assumption is valid for low air flows. Calculating
the air flow at which the equilibrium theory is not applicable in terms of the physical properties of the porous
medium is one of the useful results to be obtained from a non-equilibrium
theory of dynamic insulation which
is under development.
It is sometimes suggested that with dynamic insulation less insulation material may be
used in the wall.
From Table 2 it can be seen that to get a significant
insulation high air flows are again required.
reduction
in U-value
for a wall with only 40 mm of
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Table 2: Dynamic
U-Value versus insulation
Cellulose (L= 200 mm)
v (m/hr)
1
10
319
1998
thickness.
-_-
Cellulose (L= 40 mm)
..--..-_-._.-_-..10
1
Pe
1.91
19.1
0.382
3.82
U, 1 U,
0.33
9.5 E-8
0.82
0.085
U, (W/m%
0.058
1.7 E-8
1.23
0.13
.
Another feature of dynamic insulation is that as the air flow increases the inner surface temperature
decreases
(Taylor and Imbabi, 1997). This is because more heat has to be put into the inner surface of the wall to heat
the increasing amount of air which in turn increases the ‘temperature drop across the air film thermal
resistance. The temperature drop is about 0.5 “C for a flow of 1 m/h through a wall with 200 mm of cellulose
Even at low air flows this temperature
depression will
insulation increasing to over 5 “C at 10 m/h.
significantly alter the radiant heat exchange within a room.
Mass Transfer
Diffusive insulation is a special case of dynamic insulation where the air flow is zero. In other words its
thermal behaviour is no different from a conventional wall. Indeed diffusive insulation is merely a wall which
suc h a s polythene
or metal foil. Such wall
does not include a vapour retarder with a high vapour resistance zyxwvutsrqponmlkjihgfedcbaZYXWVU
constructions
are acceptable
in certain circumstances
and BS 5250 (BSI, 1989) quotes a useful but not
infallible rule of thumb that the vapour resistance on the warm side of the insulation be at least five times
greater than that on the cold side. Diffusion can be stopped if the air is flowing in the opposite direcnon to
the diffusion process.
The critical air velocity V~ required to do this is dependent only on the ratio of the
concentrations
of the gas (inner concentration
C, assumed to be greater than the outer concentration
C,) and
the total diffusion resistance of the multi-layer wall R,, (Taylor d.,
1996):
This explains how dynamic insulation can act as a vapour barrier. If the air velocity is greater than v, then
water vapour will be carried from outside to inside despite there being a higher water vapour concentration
on
the inside. For a typical timber frame insulated wall construction
with total thermal resistance of 6.434
m’K/W (200 mm cellulose insulation) and the indoor and outdoor temperature and humidity conditions of 15
“C, 85% RH and 5 “C, 95% RH respectively as specified in BS 5250, this critical air velocity is very low at
0.0063 m’/m’h.
This is very much lower than the recommended
air flows of 0.5 to 1.5 m’/m’h (Dalehaug,
1993). The partial vapour pressure difference corresponding
to the standard internal and external conditions,
stated above, is 621 Pa.
The authors have measured the air permeability of a variety of commonly used insulating materials. The air
permeance of 200 mm of cellulose is found to be 1.5 m’/m’hPa.
and that for 12 mm thick fibreboard was
0.116 m’/m*hPa.
The controlling resistance to air flow in a wall construction
compnsing of wood wool board
(air permeance too high to measure), 200 mm cellulose, and 12 mm fibreboard is that of the fibreboard.
The
pressure drop across this wall at the critical air flow corresponds
to a difference in air pressure of only 0.054
Pa Thus water vapour cannot flow from inside to out through a wall operating in contra-flux @eat and mass
flow in opposite direction) mode.
There LS then a conflict between the air flow requirements to minimise heat iosses and that necessary to
maximise the removal of water vapour or other indoor pollutants.
On the other hand provided one can
ensure that air is flowing inwards through the envelope at all time then there should, in general, be no
problem of interstitial condensation.
However, if the outer wall cladding is saturated by wind-driven
rain
380
WREC
followed by heating by the sun then the
and condensation
could occur in the
completed IEA Annex 24 on Heat Air
on air permeability, vapour permeability
1998
temperature and relative humidity in the cavity could rise very quickly
A useful outcome
of the recently
still relatively cool insulation.
and Moisture transport in buildings has been the compilation of data
and hygroscopicity
for many building materials (Kumaran, 1996).
SYSTEMS ANALYSIS
Equation (1) can be readily incorporated
into an air tlow and energy balance for a whole house to calculate the
heat loss through the air permeable parts of the envelope (Taylor and Imbabi, 1996). A fact that is often
overlooked by the proponents
of dynamic insulation is that whilst the heat loss to the outside is reduced more
heat needs to be put into the interior surface of the wall in order to warm the incoming air than would be the
case without air flow. Therefore, if the air coming through the wall is merely vented to atmosphere without
heat recovery little is gained. With an air-to-air heat recovery scheme as shown in Figure 1 the ventilation
requirements
are supphed partially through the wall, my and partially through the heat exchanger, m,. The
model also allows for air leakage through doors and windows, m,. The heat input to the building Q (Partly
supplied by incidental gains) compensates
for the heat lost through the porous envelope Q,, the non-porous
part of the envelope, Q” and the ventilation loss.
The only way a building can be reliably de-pressunsed
in the mild and variable UK climate is by using fans. In
northern Scandinavia with a 40 “C temperature difference between indoors and out in winter a reliable and
significant stack affect may be obtained. The de-pressurisation
must be no greater than 5 to 10 Pa otherwise
the occupants
will have difficulty opening doors and windows (BSI, 1989).
This restriction
on depressurisation
could be relaxed if the opening and closing of windows and doors were mechanically assisted.
Since the pressure drop through an air-to-air heat exchanger and associated ductwork is in the region of 50 to
100 Pa both a supply and an extract fan are required. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFED
mL
Fig. 1: hlodel of a dynamically
insulated house with air to air heat recovery
The results of analysis of such a scheme are shown in Figure 2. The ordinate plots the reduction in energy
consumption
over a conventional
envelope construction
of the same static U-value for the same air change
rate to maintain an indoor temperature of 20 “C when it is 0 ‘C outside. The curves show how a dynamically
insulated building and conventional
envelope compare when both use air-to-air heat recovery.
At low air
change rates the conventional
building performs better than the dynamically insulated building.
The bigger
2nd better the heat exchanger the higher IS the air change rate before it becomes worth while to think about
dynamic insulation.
Both schemes show a maximum saving at around 1.5 to 2 sch. This level of ventilation
in a conventional
house could be achieved merely by opening the windows.
W REC‘ 1998
Reduction
Fig. 2: Variation
m Power
in power consumption
with convenoonal
and dynamic
envelopes
,4n air-to-water
heat exchanger operating at 2.0 ach would require a connnuous steady flow of water of the
order of 1550 kg/day in a home. This flow rate is much larger than the domestic requirements
for bathing,
showermg and laundering.
Also, the temperature constraints imposed by the exhaust air flow and the heat
exchanger mean the water temperature will nse only from about 5 ‘C to 15 “C. If the warm air were used
instead to melt snow the water it would provide at, say, 10 “C is a more manageable 170 kg/day.
However,
snow is not a r&able heat smk in most areas of the UK.
With the simple tools developed so far the designer can explore, for example, how the proportion
of nonpermeable surfaces (such as glazing) to permeable surfaces and how the size of the building affect thermal
To make the most effective use of dynamic
performance.
The results are much as one might ekpect.
insulation as great a proportion
of the external envelope as is practical should be air permeable.
This has
obvious implications
for the use of incident solar radiation for lighting and heating.
It also means that a
detached house IS a more suitable candidate for dynamic insulation than a small apartment with only one or at
most two external surfaces.
As the volume of the building increases, the ratio of volume to surface area
increases and so the relative importance of the ventilation heat loss to envelope loss increases.
In general,
where energy conservation
is the main objective, dynamic insulation would appear to be appropriate
only for
small detached buildings.
FURTHER
ASPECTS
OF DYNAMIC
INSULATION
It has been theoretically established that a dynamically insulated wall will inherently act as a tilter (Taylor et.,
1997). Studies of porous ceilings in barns where the ventilaaon rate can be as high as 80 m’/m’h have shown
that over a span of 20 years the pressure increase due to dust accumulating
in mineral wool insulation is
insignificant (Wlvik, 1989). In homes, the ventilation rate will be an order of magnitude smaller and the rate
of dust accumulation
in the walls will be correspondingly
slower.
However, insulation materials such as
cellulose and mineral wool will not remove chemical pollutants in the way that activated charcoal filters would.
Cellulose insulation fibre is treated with borax to prevent fungal growth and infestation
by insects and
rodents. Bacteria cannot survive in the air on their own: they require dust particles to sustain small colonies.
When such dust (and other) particles are trapped in the insulation, bacteria hving on them may multiply
unaffected by the borax in the cellulose. The microbes and or toxins they produce could then subsequently
be disseminated into the living space. It is also known that certain types of bacteria provide the nutrients
requrred by moulds and fungi to grow (Singh, 1994). This potential health hazard requires investigauon
in
order to identify the circumstances
under which dynamic insulation may act as an amplifier and disseminator
for bacteria, fungal spores and viruses.
In view of the risks attached
to mechanical
ventilation systems, which may be overcome
by proper
maintenance,
the hybrid scheme (Fig 2) offers no health advantages over a purely mechanical ventilation
382
W REC 1998
system.
Contaminants
released within the building, such as Volatile Organic Compounds
(V’OC’s), body
odours, cooking smells, spores from moulds are best dealt with by extracting them at source and venting
directly to outside.
Dynamic insulation could be able to contribute
to their dilution and removal by
permitting higher ventilation rates or preventing their spread by plug tlow of fresh air from a wall or ceiling.
THE FUTURE
OF DYNAMIC
INSULATION
RESEARCH
Dynamic insulation ~111, as demonstrated,
reduce conductive heat loss through the wall. Operating in contraflux mode, it will also prevent water vapour getting into the wall from the interior. This had not previously
been fully appreciated.
However, these benefits are not easy to achieve given that (i) the rest of the building
needs to be exceptionally air tight and air flow through the walls needs to be reasonably uniform and (ii) it is
difficult to ensure air flows inward through the wall under all internal and external climate conditions.
Furthermore,
(iii) energy must be recovered from the ventilation air, (iv) under certain conditions
(e.g. sun
shining on wet timber cladding) one may get interstitial condensation
with air tlowing inwards, and (1.) the
building occupants need to understand how the building works and to behave accordingly.
In short, the design effort and quality control during manufacmre
and erection of a dynamically insulated
building is greater than that required for a building with conventional
air tight, well insulated envelopes in
order to ensure acceptable hygro-thermal
performance.
The acceptance of dynamic insulation will ultimately
hinge on the development
of designs that are safe for humans, durable and cost effective.
The filtration
properties of dynamic insulation, as a means of controlling the harmful effects of particulate air pollution
within the built environment
could have a profound
effect on future building design, and will be fully
investigated.
ACKNOWLEDGEMENTS
This study was funded by the Engineering and Physical Sciences Research Council (EPSRC), Grant Reference
GR/K23461.
The authors’ are grateful to Mr C Weidermann of Camphill Architects, Beildside, Aberdeen for
supplying drawings and data for their dynamically insulated house.
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BSI (1989). BS 5250: Control of Condensation
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Pomus Insrhtion in fYuL!r. Research Repoti No 53. Hokkaido
Prefectural
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Dalehaug, A. (1993).
Housing and Urban Research Institute.
Hens, H. (1996). LEA- Annex 24 on Heat, Air andM oistureTmnsportin H&b4 Insuhted Envelopes.Final Report L’ol zyxwvutsr
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Task I: M ode&g. Leuven.
Kumaran, M K. (1996). lEA- Annex 24 on Heat, Air and M oisturnTranq~oriin High4 huhted Envelopes, FinaL
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