Skip to main content Skip to footer

Unventilated Roof Tile Underlayments

May 15, 2007

Proceedings of the RCI 22nd International Convention Brandt, Hansen, and Bunch-Nielsen – 11
Unventilated Roof Tile Underlayments
Erik Brandt and Morten Hjorslev Hansen
Danish Building Research Institute (SBi)
Tommy Bunch-Nielsen
Danish Roofing Advisory Board (TOR)
Doorways to the Future
In recent years, it has become increasingly popular to use new, breathable products
for unventilated roof tile underlayments (also called under-roofs/underslating felt,
underliner, etc.) in steep, tiled roofs as an extra barrier against water penetration. The
introduction of the new materials/constructions was looked upon with great concern
due to a number of fast-appearing, expensive failures resulting in water penetration
and mold growth. A research project was carried out in order to 1) find the most
important properties of the products and 2) study the influence of these properties in
full scale in a test house. In addition, the behavior of the roof tile underlayments was
studied by computer simulations and in laboratory tests.
The paper presents the experiences gained in Denmark during the last 10+ years.
This includes the outcome of the research project, later experiences from in-situ
investigations of roofs in practice, and a new classification scheme used to ensure
that the products used have the necessary properties.
ERIK BRANDT is a senior researcher at the Danish Building Research Institute. He
has worked for many years with building technology problems in general and problems
dealing with moisture, service life, building surveys, roofs, floors, and bathrooms
in particular. Erik is a member of a number of national as well as international
working groups, standardization committees, etc., and is the author of numerous scientific
papers and publications about building problems.
Brandt, Hansen, and Bunch-Nielsen – 12 Proceedings of the RCI 22nd International Convention
Proceedings of the RCI 22nd International Convention Brandt, Hansen, and Bunch-Nielsen – 13
For quite some years, it has
been common practice in Denmark
to use underlayments in
connection with sloping roofs,
especially in constructions where
the ceiling and the roof covering
are parallel. The purpose of the
underlayment is to act as an extra
barrier against penetration of
water and drifting snow from the
exterior. Traditionally, roofs with
underlayments have been ventilated,
i.e. with a ventilation gap
between the insulation and the
roof underlayment and with openings
to the surroundings (see Figure
1, left). The ventilation removes
moisture which has penetrated
from the interior of the
building by diffusion or convection.
Some 10+ years ago, a new
type of material was introduced to
the market. These products were
used in a different way than previously,
as they were placed as a
roof tile underlayment directly on
the insulation. The missing ventilation
gap means that moisture
from the interior must be removed
in another way than ventilation,
namely by diffusion. In order to
achieve this, the new products
need not only to be watertight, but
at the same time to be very open
to water vapor diffusion, similar
to the way Goretex acts in clothing.
The materials are mainly thin
membranes that can be produced
in a variety of ways and are often
laminated and produced with a
“carrier” material to reinforce and
protect the membrane itself.
Other types of materials act in the
same way – e.g., gypsum board,
bitumen-impregnated plywood
(only in wood-based roof elements),
and oil-impregnated or
coated fiberboard. Oil-impregnated
fiberboard has been used in
Denmark as a roof tile underlayment
for the last 30+ years.
Shortly after the introduction
of the new materials, a project
was launched in order to investigate
the reasons for the problems,
including identification and
assessment of the properties necessary
to ensure a satisfactory
performance. The project was primarily
focused on properties
related to moisture transport in
the roofs, as this is normally considered
to be crucial to the performance
and durability.
Analysis of performance properties
An analysis was conducted in
order to identify the properties
necessary for a satisfactory performance
of a roof underlayment.
This was based on the functions
the underlayment shall fulfill and
all the agents anticipated to act
on it as described in CIB
Publication 64 [1]. It should be
Unventilated Roof Tile Underlayments
Figure 1 – Cross section of ventilated roof tile underlayment
with a ventilation gap between insulation and underlayment
(left) and unventilated roof tile underlayment with the underlayment
directly on the substrate/insulation (right).
Brandt, Hansen, and Bunch-Nielsen – 14 Proceedings of the RCI 22nd International Convention
noted that some of the functions
and acting agents are only relevant
for some materials and/or
constructions; and similarly,
some properties are only relevant
for certain materials and/or in
certain periods of the service life,
e.g., the construction period.
For this particular purpose,
only properties related to moisture
transportation and build-up
have been investigated. The most
important properties are found to
Tightness against precipitation
– This property is especially
important during the construction
period until the primary roof
has been laid. (In Denmark, the
underlayment often serves as a
temporary roof during the construction
Tightness against water –
This property covers standing
water as well as water running on
the surface. When well constructed,
no ponding should occur on
the underlayment, i.e., it should
have a well defined slope.
No tent effect – It has been
found that in several cases, water
penetration occurs for underlayments
laid directly on wood or
insulation. In Denmark, this is
called “tent effect,” with reference
to the well-known fact that touching
of the inside of a tent during
rain may cause penetration of
water. For roof underlayments, no
tent effect should occur, as it
would impair the watertightness.
Water vapor permeability –
For unventilated constructions, it
is evident that moisture from the
interior of the building can only
escape through the underlayment
by diffusion. Consequently, the
material shall be very permeable
to diffusion of water vapor.
Moi s ture-accumulat ing
properties – These can be supplementary
assets if all other
requirements to the roof are fulfilled.
It allows the take-up and
accumulation of moisture during
periods with high exposure. The
moisture is allowed to be removed
during other periods.
Dimensional stability against
changes in RH – This is an
important property, but is not
considered a problem with current
Based on the findings of the
analysis, it was decided to per-
Figure 2 – Mounting of roof elements on the test house.
Figure 3 – Moisture sensors are embedded in the wood near the
bottom and in the top of the sides of the element.
Proceedings of the RCI 22nd International Convention Brandt, Hansen, and Bunch-Nielsen – 15
form exposure tests in a fullscale,
test-house, computer simulation
of the moisture conditions
in unventilated roofs and with a
number of laboratory tests. The
investigations dealt with the new
types of materials as well as the
changed requirements to the entire
roof construction which this
might lead to.
Full-Scale Test House
To investigate the behavior of
unventilated roofs, a new roof
construction was made on the
test house at The Danish Building
Research Institute (SBi). The roof
has a slope of 40° (1:1.2) and consists
of 11 pairs of elements, each
pair with an element oriented
towards north and south respectively.
Each element is 1m wide and
has a height of 240mm. Ten pairs
are unventilated and the last one
– acting as a reference – is ventilated.
The elements are made with
two timber members as sides and
with a gypsum board as interior
surface. There is no vapor barrier
but the gypsum board is painted
to achieve a desired water vapor
permeability. The elements are
totally filled with mineral wool as
thermal insulation. On the outside,
the underlayment is placed
directly on top of the insulation
material. The underlayment is
fastened to the rafters and a distance
batten of 22mm is attached
to the rafters over the underlayment.
The roof is finished with
battens and roof tiles.
The climate in the test house
is controlled in wintertime to 23°C
(73°F) and 60% RH, a very high
humidity level compared to the
expected 30% RH or less under
winter conditions.
Measurements in the test house
The test house was equipped
with sensors and datatakers to
monitor temperature, humidity,
etc., according to the following.
The roof elements
are supplied
with humidity
in the timber
members so humidity
can be
recorded at 1)
the bottom (inside),
2) just
below the und
e r l a yme n t ,
and 3) in the
counter (or distance)
Some of the
humidity sensors
are supplemented
t emp e r a t u r e
sensors. A
number of the
elements are
supplied with
humidity sensors
glued to
the underside
of the underlayment.
sensors, made
by Wetcor, were
developed to
measure time of
wetness and
are herein used
to measure whether
occurs on
the inside of the
In two pairs of elements, sensors
are mounted to monitor the
amount of water running on the
underlayment in use. Similarly,
for two pairs of elements, the temperature
on the inside of the tiles
is measured with sensors mounted
on the surface of the tiles.
Supplementary to the measurements
of the roof elements,
the temperature inside and outside
the house, the relative
humidity inside and outside, the
number of hours of sunshine, the
wind velocity, and the wind direction
are monitored.
Results of the exposure in the
test house
The humidity level in the roof
construction showed, as expected,
a variation over the year with
a rather high level in winter and
drying out to a lower level during
the summer. Even though the
humidity level in winter is high, it
is not assessed to be alarming,
considering the use conditions,
with 23°C and 60% RH in the test
Besides, it should be remembered
that the construction has
no vapor barrier but only a surface
treatment providing a water
Figure 4 – Moisture in the top of two roof elements
as measured with the sensors. Note that
the properties for the underlayments are not
the same.
Brandt, Hansen, and Bunch-Nielsen – 16 Proceedings of the RCI 22nd International Convention
vapor resistance of approximately
17 GPa s m2/kg (1.0 perm) in contrast
to at least 25-30 (0.58-0.7
perm) provided by a normal vapor
retarder. Finally, it should be
noted that the last winter of exposure
was very cold.
Only the reference roof is
found to be very wet, which is
explained by unsatisfactory ventilation,
as there is no connection
from eave to eave; i.e., the ventilation
gaps in the two elements on
each side of the house have no
connection, and consequently
there is very restricted possibility
for wind to pass through the construction
and remove moist air.
Computer simulation of
moisture transfer
The planning of the full-scale
tests was supported by computer
simulation of the moisture transfer
in a roof construction by
means of the simulation program
MATCH. MATCH is a dynamic,
one-dimensional model for combined
heat and moisture transfer
in composite constructions. The
model makes it possible to simulate
the temperature and moisture
conditions on an hourly basis
in the individual layers of the construction
based on knowledge of
the materials properties of the
individual layers and the boundary
conditions to which the construction
is subjected. The model
takes vapor as well as liquid
transfer into account (but not precipitation).
The climatic data used
are from “The Danish Test Reference
For practical purposes, a
vapor retarder installed in a
building might be expected to
have a Z-value of 30 GPa s m2/kg
(0.58 perm). On the safe side, a
value of eight was chosen for the
calculations. With this assumption,
the simulation showed that,
dependent on the roof underlayment
material used, condensation
might occur occasionally.
Laboratory tests
Laboratory tests were made
for the most important of the performance
properties related to
moisture transfer, i.e.:
• Water vapor permeability
• Tightness against precipitation
• Tent effect
The water vapor permeability
was tested according to ASTM
E96, wet-cup – i.e., with 100% RH
in the cup and 50% outside. Some
25 materials were tested, including
the new type of underlayment
materials as well as the previous
types. Those new types available
on a commercial basis all had Z
values below 3 GPa s m2/kg (5.8
perm), whereas the old type had Z
Figure 5 – Relative humidity and moisture content just beneath
the vapor-permeable underlayment as calculated with MATCH
for a 5-year period. Two underlayments with Z values of 3
(open) and 25 (tight) respectively, are compared. The upper figure
shows the relative humidity in the insulation just beneath
the underlayment. The lower figure shows that equilibrium is
reached for both structures, but on different moisture levels.
Proceedings of the RCI 22nd International Convention Brandt, Hansen, and Bunch-Nielsen – 17
values ranging from 30 –
500 GPa s m2/kg (0.003 –
0.58 perm).
Tightness against
precipitation was tested
at Velux’s (a major European
skylight producer)
wind tunnel. Two different
investigations were
carried out.
The first was primarily
intended to assess the
differences in the
amounts of water penetrating
through various
roof coverings and various
underlayments. The
tests were made on a test
roof of 2.6 x 3.6 m. The
tests on underlayments
were made with the
slopes 25° and 45° and
wind directions of 0 and
22° (0° is wind perpendicular
to the building/
eave). For roof coverings,
slopes of 25°, 35°, and
45° and wind directions 0°, 22°,
and 45° were used.
For this first test round, a test
simulating driving rain was used.
The water is equivalent to 120
mm/h m2 and the wind is a
dynamic wind profile with gusts
up to 20 m/s and 30 m/s for
underlayments and roof coverings
The results were that considerably
more water penetrated
when the underlayment was
mounted perpendicular to instead
of parallel to the rafters. For the
board materials, water penetrated
the joints, irrespective of whether
these were made with overlapping
or with joint profiles. Some of the
roof coverings had virtually no
water penetrating, with a few
drops at the most, whereas considerable
amounts penetrated
others (i.e., water was running in
at some joints). The amount of
water penetrating the roof covering
depends on the slope and the
wind direction, but the dependence
varies from roof covering to
roof covering.
In the second test round, the
underlayment mounted perpendicular
to the rafters was tested,
together with the roof covering,
where the highest amount of
water penetrated in the first test.
The result of this was that no
water penetrated to the inside of
the underlayment when the roof
covering was present. The problem
with water penetration of the
underlayment is therefore considered
mainly to be a problem during
the building period.
For the testing of tent effect,
no suitable existing methods were
found. Instead, a proposal for a
test method was made by the SBi.
The test method is comprised of
an accelerated aging followed by a
test of watertightness of the product
when placed in contact with
the substrate. The aging is assessed
to simulate roughly six
months’ exposure to Danish sunlight.
A flat aluminum “tray” is
used to collect any water penetrating
the underlayment during
the water exposure. In the lower
end of the tray, a batt of mineral
wool is placed. In the upper end, a
piece of plywood is placed. Both
materials are slightly higher than
the aluminum tray. The underlayment
is mounted on a wooden
frame which fits around the aluminum
tray. When mounted, the
frame is placed around the tray,
causing the underlayment to rest
around the mineral wool and the
plywood, respectively. The underlayment
is held in place by the
weight of the frame and the underlayment,
which is intended to
simulate the conditions in a real
roof. Water is sprayed from a nozzle
over the entire surface of the
specimen for six hours. The water
pressure in front of the nozzle is
very low, simulating a fine to
medium rain. The test method
has been elaborated into a Nordtest
test method, NT BUILD 488,
“Roof Tile Underlays: Watertightness
– Tent Effect” (this may be
downloaded at www.nordicinnovation.
Figure 6 – Testing of tent effect. The underlayment is placed over a substrate,
partly of plywood and partly of insulation. Any water passing
through the underlayment during exposure to water spray from the nozzle
is accumulated in the aluminum tray and weighed.
Brandt, Hansen, and Bunch-Nielsen – 18 Proceedings of the RCI 22nd International Convention
Some of the materials tested
using this method showed a considerable
penetration of water
during the test, which is unsatisfactory
as water will flow on the
underlayment in most roofs.
In-situ investigations and
experience from practice
A small number of in situ
investigations was made in the
winter of 1995-96. It was found
that some materials suffered from
water leakage – probably due to
tent effect, resulting in a number
of products being withdrawn from
the market. The testing of tent
effect has subsequently been
common for new products ever
since. A number of products that
failed the test have either not been
marketed or have only been marketed
after improvements of the
Some of the underlayments
have been attacked by mold
growth, and in a few cases, ice
has formed on the inside of a roof
underlayment during long-lasting
cold periods. This shows that
some of the underlayments sometimes
have moisture accumulation
on the inside. Visual inspections
in a number of identical
houses suggest that the problem
is especially pronounced where
the vapor retarder is not perfect;
e.g., where it has been perforated
or where there are leaks around
openings. Visual inspections in
the test house showed that ice
was formed on one of the two underlayments
that could be inspected
visually. This stresses the
importance of an airtight construction.
It is especially important
to avoid convection, which
might transport considerably
more water vapor into the roof
construction than can be removed
by diffusion through the underlayment.
Even though the underlayments
are very open to diffusion,
it is now considered necessary to
have at least some ventilation of
open roof spaces to prevent moist
air from accumulating in the top
of the roof.
Quite a few problems are
associated with the detailing and
workmanship of roof underlayments.
Some suppliers
only sold the products but
did not give any advice as to
its installation. Others described
just a few of the
most used and easiest
details. The detailing was
consequently often designed
by the contractor on the
building site with associated
mediocre quality as a
consequence. Over the
years, a number of publications/
leaflets have been
published showing the principles
of the most used
details. Lately, a rather
large number of details have
been issued by the Danish
Timber Information Council
(TOP). These may be downloaded
TOP is currently finishing a
booklet about underlayments
with wooden materials,
e.g. boards, plywood, and
fiberboards (with or without a
supplementary, watertight membrane)
that also will have a number
of details.
Poor workmanship often results
in problems, mainly because
the work is not done as prescribed,
or because repairs are
done incorrectly or not at all. A
common and easy-to-detect example
is that if the underlayment
has not been mounted sufficiently
tight, it might flap, thereby creating
considerable noise, and eventually,
puncture of the underlayment
where it touches the tile
clips. Otherwise, the main problem
is that details are not made
watertight because the correct
procedure is time consuming.
Some of these problems might not
only be difficult to see in the finished
roof (especially if covered by
a ceiling afterwards), but also very
expensive to repair. This calls for
more responsibility by the craftsman
and/or more rigorous supervision.
Figure 7 – Example of detail around ventilation duct – very poor
Proceedings of the RCI 22nd International Convention Brandt, Hansen, and Bunch-Nielsen – 19
For some years, a private
organization collected
information on available
documentation from all the
suppliers of roof underlayments.
A couple of years
ago, this task was handed
over to a new organization
called DUKO (Danish Roof
Underlayment Classification
Scheme). The organization is
owned by a number of the
interested parties, including
contractor representatives
and building owners.
The main purpose is to
make a voluntary classification
of products with documentation
for the most
important properties, including
strength, watertightness,
potential for water vapor diffusion,
tent effect, and durability.
Based on physical properties, the
underlayments are classified into
a number of exposure classes.
The suppliers have to provide the
documentation in the form of test
reports, including proof of buildability;
i.e., they must show how
to make details such as eaves,
chimneys, skylights, etc. Proof of
buildability is achieved by the
companies by mounting their roof
underlayments on a mock-up
including all imaginable details.
The basic idea with the classification
system is that the underlayment
is chosen dependent on the
watertightness of the roof covering
when subjected to driving
rain, the slope of the roof, the
complexity of the roof design, the
serviceability of the underlayment
after installation, and the exposure
to wind. For example, some
roof coverings are exposed to UV
light as well as to rain and consequently
require a very durable
Products appearing on the
classification list from DUKO can
be considered to have sufficient
documentation for their properties,
including buildability. Further
information may be downloaded
Roof tile underlayments have
been used for 50+ years in
Denmark. As a whole, they have
functioned properly. Roof tile
underlayments are a prerequisite
for the use of attics over occupied
spaces – i.e., with (at least) part of
the ceiling and the roof parallel
(because earlier tightening of the
tiles with mortar, bitumen, etc. is
not durable and can not be maintained).
The ventilated underlayments
have functioned properly for a
long period of time (after some
early problems with durability of
the products), provided common
knowledge about the construction/
ventilation is used.
As regards unventilated underlayments,
costly errors have
shown the need to fulfill certain
requirements if a well performing
roof underlayment is to be
achieved. These include most of
the following:
• The underlayment shall
remain tight when subjected
to precipitation,
including driving rain. The
underlayment shall pass a
Nordtest Build 118 test (or
similar) without experiencing
water penetration.
• The underlayment must
have no tent effect. Only a
very small amount of
water is allowed to pass
the artificially aged underlayment
(in Denmark, the
requirement is less than
15g) when tested according
to Nordtest Build 488.
• The water vapor permeability
of the underlayment
shall be low. In
Denmark, the requirement
is that the Z-value shall be
less than 3 GPa s m2/kg.
• The inside of the roof construction
shall be sufficiently
tight to avoid diffusion
and convection. The
tightness shall be seen in
relation to the permeability
of the underlayment.
The Z-value of the inside of
the construction should be
Figure 8 – Mock-up to be used for the suppliers’ proof of buildability.
Brandt, Hansen, and Bunch-Nielsen – 20 Proceedings of the RCI 22nd International Convention
greater than 30 GPa s
m2/kg. Normally, this is
achieved by using a vapor
retarder that, when well
mounted with overlapping,
taped joints and no perforations,
will fulfill the
requirements for both permeability
and air tightness.
• Documentation that all
imaginable details can be
made securely shall exist.
Further, the instructions from
the supplier should be taken into
account, especially related to how
long the underlayments can be
used as a temporary roof directly
exposed to UV light and precipitation.
In this connection it should
be noted that some of the roof
tiles on the Danish market will
allow a penetration of as much as
5% of the surrounding UV light.
Finally, the workmanship is crucial
to the overall performance of
the roof construction. Only when
properly mounted may an underlayment
be expected to fulfill its
duty for many years.
Provided that the above are
fulfilled, it is assessed that unventilated
roof underlayments are
able to function in a temperate climate
such as Denmark’s.
Building Detail Sheets, TOP
(Danish Timber Information
Council) (in Danish)
• RoofUnderlayments. Concepts
and properties.
• Roof underlayments.
Typical descriptions.
• Un-ventilated roof underlayments.
• Ventilated roof underlayments.
Byg Erfa data sheets, Byg-
Erfa (Building Experience
Feed-back Council) (in
• Flapping roof underlayments.
• Underlayments. Construction,
and design.
• Underlayments. Mounting
and details.
NT Build Method 488, ”Roof
Tile Underlays: Watertightness
– Tent Effect.”
Undersøgelser af uventilerede
undertage (“Investigations
on Unventilated Roof
Underlayments”), SBI
report 292, 1998. (in Danish)
“Working With the Performance
Approach in Building,”
CIB Report, publication
64 (CIB, Rotterdam,