Choosing a Wood Fibre Warm Roof System
A well-designed roof insulation system needs to consider more than the insulation material alone.
Thermal insulation, airtightness, vapour control, wind protection and weather protection all need to work together while maintaining appropriate drying potential within the roof construction.
The Roundtower Warm Roof Insulation System brings these functions together using compatible STEICO products.
System Components
The principal components are:
STEICO Flex 036 — flexible wood fibre insulation between rafters.
STEICO Special Dry — rigid continuous wood fibre insulation and sarking board above the rafters.
STEICOmulti Renova — humidity-variable internal airtightness and vapour-control membrane.
STEICOmulti UDB — highly vapour-permeable external windtight and weathering membrane for applications where the roof detail requires it.
STEICOmulti Tape F — high-performance sealing tape for compatible STEICO membranes, boards and junctions.
STEICO Flex 036
STEICO Flex 036 is a flexible wood fibre insulation batt designed for roof, wall and ceiling constructions.
Its technical properties include:
- thermal conductivity: 0.036 W/mK;
- density: approximately 50 kg/m³;
- vapour diffusion resistance factor: μ 2;
- specific heat capacity: approximately 2,100 J/kgK;
- fire classification: Euroclass E.
Within the Roundtower Warm Roof System, Flex 036 is installed between the timber rafters to provide the principal flexible insulation layer.
Installing Flex 036 Between Rafters
Flex 036 is designed to fit closely between structural timber members.
The insulation should be accurately cut to minimise gaps around the rafters and adjoining construction.
Close fitting helps maintain continuity of the insulation layer.
The material should not be unnecessarily compressed, as the intended thermal resistance depends on the installed insulation thickness.
STEICO Special Dry
STEICO Special Dry is a rigid wood fibre sarking and insulation board used above the structural roof.
Its properties include:
- thermal conductivity: 0.040 W/mK;
- vapour diffusion resistance factor: μ 3;
- density: approximately 140 kg/m³;
- specific heat capacity: approximately 2,100 J/kgK;
- compressive strength: at least 100 kPa.
Special Dry combines insulation with external wind and rain protection and provides a continuous wood fibre layer over the roof structure.
Why Insulate Above the Rafters?
When insulation is installed only between timber rafters, the rafters themselves interrupt the insulation layer.
Installing Special Dry continuously above the rafters helps reduce this repeating thermal bridge.
The combination of Flex 036 between the rafters and Special Dry above them therefore creates a more continuous thermal envelope than between-rafter insulation alone.
STEICO Special Dry as a Sarking Board
Special Dry performs more than a purely thermal role.
It is specifically designed for external roof applications and combines:
- thermal insulation;
- wind protection;
- rain protection;
- and a vapour-open external layer.
Its tongue-and-groove format allows the boards to form a continuous external roof layer when installed in accordance with the STEICO system requirements.
The final slate, tile or other roof covering remains the permanent weathering layer.
STEICOmulti Renova
STEICOmulti Renova is a humidity-variable vapour-control membrane for internal applications.
Its Sd-value varies from approximately 0.4m to 35m, allowing its resistance to water-vapour diffusion to respond to changing moisture conditions.
Within the Roundtower Warm Roof System, multi Renova can form the internal airtightness and vapour-control layer beneath the wood fibre insulation.
This helps control air leakage and moisture movement from the occupied space into the roof construction while retaining increased drying potential under appropriate conditions.
Why Use a Humidity-Variable Membrane?
A humidity-variable membrane changes its resistance to water-vapour diffusion depending on surrounding conditions.
Under conditions where greater vapour control is required, the membrane provides increased resistance.
Under suitable drying conditions, its diffusion resistance reduces.
This provides a more adaptable moisture-control strategy than assuming that every roof requires a permanently high-resistance vapour barrier.
STEICOmulti Renova for Roof Retrofit
Multi Renova is also designed for selected sub-and-top roof renovation arrangements.
This can be particularly useful when an existing roof is renovated from above and the internal ceiling is to remain in place.
The membrane can be installed around the existing rafters within an appropriate STEICO roof detail, provided sufficient wood fibre sarking insulation is installed above.
The exact build-up should follow the relevant STEICO design requirements.
Airtightness
Airtightness controls unintended air movement through the roof construction.
This is important both thermally and hygrothermally.
Air leakage can carry heat and moisture into the roof through gaps and penetrations, so the airtight layer should remain continuous at:
- membrane overlaps;
- eaves;
- ridge details;
- roof-to-wall junctions;
- roof windows;
- chimneys;
- cables;
- pipes;
- and service penetrations.
Multi Renova should therefore be treated as part of a complete airtightness system rather than simply being laid loosely behind the internal finish.
Airtightness vs Vapour Control
Airtightness and vapour control are related but different.
Airtightness controls bulk movement of air through gaps and junctions.
Vapour control influences water-vapour diffusion through the construction layers.
Multi Renova can perform both functions when installed continuously and correctly sealed.
STEICOmulti Tape F
STEICOmulti Tape F is a high-performance sealing tape for internal and protected external applications within the STEICO system.
It can be used for:
- membrane joints;
- membrane connections;
- compatible board connections;
- corners;
- and selected junction details.
The split release liner can assist when forming corner connections.
The substrate should be sound, dry and free from dust or contamination before bonding.
Tape F on Wood Fibre Boards
Where Tape F is bonded directly to a STEICO wood fibre board, the bonding area should be prepared with the appropriate STEICOmulti Primer where required by the STEICO detail.
Correct substrate preparation is important because wood fibre board surfaces are porous.
Tape and primer should therefore be considered together where direct board sealing forms part of the roof detail.
STEICOmulti UDB
STEICOmulti UDB is a highly vapour-permeable external membrane designed for roof and renovation applications.
It has an Sd-value of approximately 0.02m and provides resistance to wind and driving rain while retaining very low resistance to outward water-vapour diffusion.
The membrane can be used as:
- a roofing underlay;
- an external windtight membrane;
- a diffusion-open weathering membrane;
- or part of selected roof-renovation airtightness arrangements.
Is STEICOmulti UDB Always Required?
No.
STEICO Special Dry itself is designed to provide rain and wind protection within suitable roof constructions.
Multi UDB should therefore not automatically be specified over every Special Dry roof.
It can be incorporated where the roof pitch, covering, exposure, renovation detail or chosen STEICO construction requires an additional membrane.
This should be determined as part of the complete roof specification.
UDB as a Supplement to Sarking Boards
STEICO specifically permits multi UDB to be used as a supplement to sarking boards.
This makes it useful where an additional external weathering layer is required while maintaining high outward vapour permeability.
Its low Sd-value allows the external side of the roof to retain strong drying potential.
Temporary Weather Protection
Both the sarking-board and membrane layers can provide useful temporary protection during construction when installed according to their respective requirements.
STEICO currently lists Special Dry for approximately four weeks of outdoor exposure.
Multi UDB is classified W1 for resistance to water penetration and is also suitable as a temporary roof covering for up to four weeks within its technical requirements.
Neither should be confused with the final permanent roof covering.
Airtightness vs Windtightness
The two terms perform different functions within the roof.
Internal airtightness limits uncontrolled movement of warm internal air into the roof construction.
External windtightness protects the insulation from outside air movement and wind washing.
Multi Renova normally provides the principal internal air-control layer, while Special Dry and, where used, multi UDB contribute to the external windtight and weather-protective layers.
Vapour-Open Roof Construction
A vapour-open roof is designed to retain useful drying potential through its construction layers.
Flex 036, Special Dry and multi UDB all have relatively low resistance to water-vapour diffusion.
However, vapour openness does not mean that moisture from the occupied space should be allowed to enter the roof without control.
The combination of internal humidity-variable vapour control and outwardly vapour-open external layers provides a more deliberate moisture strategy.
Summer Comfort
Wood fibre has a relatively high specific heat capacity.
Both Flex 036 and Special Dry have specific heat capacities of approximately 2,100 J/kgK.
This allows the insulation layers to absorb and temporarily store heat.
In roofs receiving substantial solar radiation, this heat-storage capacity can contribute to delaying heat transfer towards the occupied rooms below.
Actual summer comfort still depends on the complete building, glazing, shading, ventilation and insulation thickness.
Acoustic Performance
The fibrous nature of Flex 036 and the greater density of Special Dry can also contribute to acoustic performance within a roof assembly.
However, sound reduction should be assessed at the level of the complete roof construction rather than assigning one acoustic figure to the insulation products alone.
New-Build Roofs
The system is suitable for new pitched-roof construction where the insulation, airtightness and roof junctions can be designed together from the outset.
This makes it possible to establish continuity between the roof airtightness layer and the adjoining walls before construction begins.
Roof Retrofit
The system can also be particularly useful during roof renovation.
Where the roof covering is removed, additional insulation can be introduced between existing rafters and a continuous wood fibre layer installed externally.
Multi Renova also provides options for roof renovation where the internal ceiling is being retained.
The existing roof construction should be assessed before the final arrangement is selected.
Traditional & Historic Buildings
Wood fibre warm-roof insulation can be appropriate for many traditional buildings where a vapour-open roof build-up is desirable.
However, over-rafter insulation increases the depth of the roof.
This can affect:
- eaves;
- gutters;
- fascia boards;
- verges;
- roof windows;
- chimneys;
- ridges;
- and adjoining roof details.
These changes should be considered carefully on architecturally sensitive or protected buildings.
Insulation Thickness
There is no single correct insulation depth for every roof.
The required combination of Flex 036 and Special Dry should respond to:
- existing rafter depth;
- target U-value;
- roof geometry;
- over-rafter build-up;
- junction detailing;
- available construction depth;
- and project requirements.
Roundtower can calculate different combinations before the final specification is selected.
U-Value Calculations
Roundtower can provide U-value calculations for proposed warm-roof systems.
Calculations should include the effect of the timber rafters rather than treating the entire between-rafter zone as insulation.
Different combinations of Flex 036 and Special Dry can then be compared to determine an appropriate thermal build-up.
Typical Roundtower Warm Roof Build-Up
A typical system can include:
internal ceiling finish
→ service cavity where required
→ STEICOmulti Renova airtightness and humidity-variable vapour-control membrane
→ STEICO Flex 036 between rafters
→ STEICO Special Dry continuous over-rafter insulation/sarking
→ STEICOmulti UDB where required by the roof specification
→ counter battens and battens
→ slate, tile or other appropriate roof covering
STEICOmulti Tape F is used at appropriate membrane, board and junction connections throughout the system.
Frequently Asked Questions
What is the Roundtower Warm Roof Insulation System?
It is a coordinated wood fibre roof-insulation system combining STEICO Flex 036, Special Dry and compatible STEICO airtightness, vapour-control and sealing products.
What is the lambda value of STEICO Flex 036?
STEICO Flex 036 has a declared thermal conductivity of 0.036 W/mK.
What is the lambda value of STEICO Special Dry?
STEICO Special Dry has a declared thermal conductivity of 0.040 W/mK.
What does STEICOmulti Renova do?
Multi Renova forms the internal humidity-variable vapour-control and airtightness layer within the roof.
What is the Sd-value of multi Renova?
STEICO states a humidity-variable Sd range of approximately 0.4m to 35m.
What does STEICOmulti UDB do?
Multi UDB is a highly vapour-permeable external membrane providing wind and weather protection in suitable roof details.
What is the Sd-value of multi UDB?
Multi UDB has an Sd-value of approximately 0.02m.
Do I always need multi UDB over Special Dry?
No.
Special Dry can itself provide external rain and wind protection within suitable roof constructions.
Multi UDB is incorporated where the particular roof detail requires an additional diffusion-open membrane.
What is STEICOmulti Tape F used for?
Tape F is used to seal suitable membrane joints, board connections, corners and junctions within the STEICO sealing system.
Does the roof still need airtightness if the insulation is vapour open?
Yes.
Vapour permeability and airtightness are different properties.
A vapour-open roof still requires controlled air leakage and an appropriate moisture strategy.
Does a vapour-open roof prevent condensation?
No system can guarantee this purely because its materials are vapour permeable.
Indoor humidity, airtightness, insulation thickness, external layers, climate and construction detailing all influence moisture behaviour.
Can this system be used for roof renovation?
Yes.
The combination of between-rafter and over-rafter insulation can work particularly well during roof refurbishment, and multi Renova also supports selected renovation arrangements.
Can Roundtower calculate the roof U-value?
Yes.
Roundtower can compare different Flex 036 and Special Dry thicknesses and calculate the estimated U-value of the complete roof.
Can Roundtower help specify the complete system?
Yes.
Roundtower can assist with insulation thickness, membrane selection, airtightness and windtightness detailing, U-value calculations and the overall warm-roof build-up.
Need Help Designing a Warm Roof?
If you are considering a wood fibre warm roof for a new build, retrofit or traditional building, contact Roundtower Lime with drawings or details of the existing roof.
Our technical team can help develop the complete insulation build-up and identify the appropriate STEICO insulation, membranes and sealing products for the project.
