FAQs
λ value
Lambda value. Thermal conductivity (W/m.K) demonstrates
the thermal conductivity of a material. The lower the Lambda value the better.
May also be expressed as k.
λ90/90
Lambda 90/90 is a statistical process applied to many
European Standards that ensures that the declared
thermal conductivity can be obtained for 90% of
production within a 90% confidence level. The aim is
to improve the robustness of design values so that
conductivities used in calculations are more realistic
and are met most of the time; as a conservative rule of
thumb a 90/90 value is 0.005 W/m.K higher (worse) than
the typical or mean value.
A1
Non-combustible materials:
No contribution to fire at any stage of the fire
Air Closed
A type of breathable membrane which comprises a vapour
permeable micro porous film which allows vapour transfer by diffusion.
Whilst allowing water vapour to pass through the micro pores in the film
are too small to allow the passage of air.
Air tightness
The resistance of the building envelope to inward or
outward air leakage. Excessive air leakage results in increased energy
consumption and a draughty, cold building.
Air Open
There are two categories of Low Resistance (LR) underlays:
Air Tight, which comprise a vapour permeable film which allows vapour
transfer by diffusion, and Air Open, which have higher vapour permeability
with the additional condensation controlling air permeability. Being Air Open
allows water vapour to pass more easily than by diffusion alone.
AVCL
Air and vapour control layers (AVCLs) are air and vapour tight.
Used on the warm side of a structure, often behind the plasterboard,
they help to reduce convective heat loss and provide effective condensation control.
BS EN5250:2011
Code of practice for control of condensation in buildings
BS EN16012:2012
Thermal insulation for buildings - Reflective insulation
products - Determination of the declared thermal performance:
This is the newly published British and European Standard for determination
of the thermal performance of reflective insulation products.
BS EN13501-1
Current fire classification procedure for all construction products, including products incorporated within building elements with the exception of power, control and communication cables. The classification ranges from A1 (non-combustible) to F (easily flammable)
Class 1
A superseded classification that requires no surface spread of flame, generally to allow escape through a corridor or stairway. A "Class 1" product when tested to the current standard BS EN 13501-1 could still ignite within 15 seconds and have a "high contribution to fire".
Cold roof
A roofspace where insulation is fitted at ceiling level.
emissivity
Emissivity is a measure of the reflectivity of a surface
to infra red radiation, expressed as e or ε. The more reflective a
surface is, the lower its emissivity.
HR (High Resistance) underlay
Defined by BS 5250:2011 as having a water vapour
resistance greater than 0.25 MN.s/g. BS 5250:2011 states:
In roofs with an HR underlay.., whatever form of external covering or
ceiling is provided, there is a risk of interstitial condensation forming
on the underside of the HR underlay; to avoid that risk, an AVCL should be
provided on the warm side of the insulation, and ventilated voids should
be formed between the underside of the underlay and the insulation. Each
void should be at least 25 mm deep and be vented at both high and low level.
The requirement to ventilate below a high resistance (HR) insulating underlay
dramatically reduces any thermal benefit it gives to the roof structure.
interstitial condensation
Occurs when warm, moist air (generally, from inside a
building) penetrates into a wall, roof or floor or permeable forms of
insulation, e.g. mineral wool. If it reaches dew point, condensation
will appear within the structure.
LABC Registered Details
LABC Registered Details allows manufacturers to register
specific solutions so they have national acceptance by all Local Authority
Building Control across England and Wales.
LR
Defined by BS 5250:2011 as having a water vapour resistance
not exceeding 0.25 MN.s/g. BS 5250:2011 states: for warm pitched roofs
constructed with LR underlay there is no requirement to ventilate the
underside of the underlay and there may also not be the requirement to
provide an AVCL on the warm side: PhotonAir low resistance (LR)
insulating underlay upgrades thermal performance without the risk of condensation.
New thermal element
Addition of a new structure to an existing building
e.g. a house extension.
NHBC
2011 NHBC Standards for pitched roofs now includes the
requirement for the provision of ridge or high level ventilation equivalent
to a continuous opening of 5mm when using vapour permeable underlays.
This followed an increasing number of calls concerning roof condensation
during cold weather to the NHBC. This requirement does not apply to vapour
permeable underlays that are third party assessed as being both vapour
and air permeable.
Part L
Part L (Approved Document L) of schedule 1 of the Building
Regulations covers the requirements with respect to Conservation of fuel and power.
PF
Phenolic foam insulation.
PIR
Polyisocyanurate insulation.
PUR
Polyurethane insulation.
Retained thermal element
Adding a new layer or completely replacing an existing
thermal element e.g. re-roofing.
R value
Thermal resistance (m2.K/W), is the rate at
which heat is transferred through a structure.
The higher the R value the better. R = Thickness/Lambda
SAP
The Standard Assessment Procedure (SAP) is the UK
Governments recommended method system for measuring the energy
rating of residential dwellings.
Thermal element
A Thermal Element is a roof, wall or floor which
separates a thermally conditioned (heated or cooled) space from: the
outside, an unheated part of the same building, a structure exempt
from the building regulations (such as porch or conservatory) or
part of the same building heated or cooled to a different temperature.
U-value
Thermal transmittance (W/m2.K) i.e. how much
heat is lost through a structure. It is the inverse of the sum of the
R-values of the components and the lower the U-value the better.
vapour barrier
VCLVapour Control Layer. Used on the warm side of a
structure, often behind the plasterboard, to restrict the passage of
water vapour.
Water vapour resistance
The vapour resistance of a material is a measure of the
materials resistance to let water vapour pass through.
The vapour resistance takes into account the materials thickness,
so can only be quoted for a particular thickness of material.
It is measured in MNs/g (Mega-Newton seconds per gram).
Wind uplift
The principle purpose of the pitched roof underlay
is to substantially reduce the effect of wind loading on the slate or tile
roof covering. Roof underlays have a tendency to balloon with the risk of
dislodging the slates or tiles. This issue has been addressed in the
updated BS5534 Code of Practice for Slating and Tiling.
A wind speed map of the UK has been produced and manufacturers
are required to provide information on packaging to show the zonal limit suitability.
Warm roof
A roof space where insulation is fitted at rafter level.
