Weatherproofing and Airtightness for Mass Timber

Wood is vulnerable to moisture damage. The moisture levels in structural mass timber must be kept under control by applying a temporary or permanent cover. 

The design of weather protection is critical for protecting timber elements from rain, wind and snow during all phases of construction.

Figure 1 - Mass timber weather protection solution using Wetguard^®

Weather protection (blue) ensures moisture-sensitive elements stay dry during construction. 

The airtightness layer (red) provides moisture control and makes the building more energy efficient when it is in use.

Figure 2 - Airtightness layer and weather protection layers

The airtightness of mass timber elements like cross-laminated timber (CLT) and laminated veneer lumber (LVL) depends on the way in which individual planks are joined together.

Figure 3 - Gluing and stacking of timber boards to create a mass timber panel

• Airtightness is low when the adhesive is only applied to the affixed surface of each lamella.
• Airtightness is generally high when adhesive is applied to all surfaces of the wood including edges.
• Processing errors or uncontrolled weather can also affect the airtightness of mass timber elements causing gaps to appear during the drying phase.
                                                                                                                                                The airtightness of mass timber elements is defined by the manufacturer.

1. Seal joints
Mass timber elements can be made airtight by sealing the panel connections from the outside using tapes. This solution means that the open surfaces of the mass timber remain exposed to atmospheric influences.

Figure 4 - Sealing with tapes

2. Use weatherproof membranes

It is possible to achieve a continuous air barrier using a weatherproof membrane with mass timber.

Mechanical fixings are needed to fix the membrane to the mass timber elements.

Figure 5 - Sealing with a weatherproof membrane

3. Use self-adhesive membranes
It is also possible to achieve a continuous air barrier using a self-adhesive membrane on mass timber elements. 

No additional fixings are needed for this solution.

Figure 6 - Using a self-adhesive membrane

4. Use vapour control membranes
If the internal surface of the mass timber is covered (e.g. using gypsum boards), then airtightness can be achieved from the inside.

Figure 7 - Sealing with a vapour control membrane

But the most common way to seal mass timber is from the outside, combining weather protection with airtightness.

Sealing joints is critical for achieving airtightness in mass timber construction. These areas must be sealed to prevent uncontrolled convection and moisture damage.

The most important details are:

• Vertical and horizontal connections of mass timber elements
• Joints with roof elements
• Connections to other materials (e.g. concrete)
• Connections with the ground floor or foundation
• Openings and penetrations

Figure 9 - Key details for airtight mass timber construction

1. Floor/ceiling elements

2. Roof elements

3. Floor to wall joints

4. Cantilever to wall joints

5. Wall to flat roof joints

6. Wall to pitch roof joints

7. Timber to concrete element

8. Wall to foundation

9. Windows

10. Penetrations

Air- and weathertight connection of floor and ceiling elements demands attention to the following details:

• Joints must be airtight to ensure a continuous airtightness layer
• Joint sealing must be flexible to allow for movement during construction
• Joints on horizontal surfaces must be carefully protected from moisture damage
• Pre-installed, full-surface, self-adhesive membranes protect mass timber elements from moisture during transport, storage, and construction 

Figure 10 - Sealing floor/ceiling joints

Connection of mass timber roof elements requires careful attention to the following technical details:

• Membrane-sealed surfaces must be robust and non-slip for construction safety
• Joints must be airtight to ensure a continuous airtightness layer
• Joint sealing must be flexible to allow for movement during construction
• Joints must be weathertight during the construction phase to protect mass timber elements from moisture damage
• Preinstalled, full-surface, self-adhesive membranes protect mass timber elements from moisture during transport, storage, and construction 

Figure 11 - Roof sealing

With weatherproof membranes on the external wall, the floor to wall joints are sealed.

• Weatherproof membranes and mass timber together provide an airtight layer with vapour control function
• An interior design with exposed mass timber is possible
• Weatherproof membranes guarantee the weather protection of mass timber
• Mechanical fixings must be used with this type of membrane
• Joints on horizontal surfaces must be carefully protected from moisture damage
• Joint seals must be flexible to allow for movement during construction

Figure 12 - Floor to wall joints with weatherproof membranes on the wall (closed timber façade, mass timber interior)

With self-adhesive weatherproof membranes on the wall, the floor to wall joints are sealed.

• Weatherproof membranes and mass timber together provide an airtight layer with vapour control function
• An interior design with exposed mass timber is possible
• Membranes provide weather protection for mass timber
• No mechanical fixings are required
• Joints on horizontal surfaces must be carefully protected from moisture damage
• Joint sealing must be flexible to allow for movement during construction

Figure 13 - Floor to wall joints with self-adhesive weatherproof membranes on the wall (closed timber façade, mass timber interior)

With tapes on the wall, the floor to wall joints are sealed.

• Mass timber elements must be approved as airtight
• Airtightness is achieved by sealing mass timber joints from the outside using tape
• An interior design with exposed mass timber is possible
• Mass timber may need additional weather protection

Figure 14 - Floor to wall joints with tapes on the wall (closed timber façade, mass timber interior)

With self-adhesive weatherproof membranes on the wall, the floor to wall joints are sealed.

• Weatherproof membranes and mass timber together provide an airtight layer with vapour control function
• An interior design with exposed mass timber is possible
• Façade membranes must be UV-stable, permanently weathertight and resistant to ageing
• No mechanical fixings are needed for self-adhesive membranes
• Joints on horizontal surfaces must be carefully protected from moisture damage

Figure 15 - Floor to wall joints with self-adhesive weatherproof membranes on the wall (open timber façade, mass timber interior)

With weatherproof membranes on the wall, the floor to wall joints are sealed.

• Weatherproof membranes and mass timber together provide an airtight layer with vapour control function
• An interior design with exposed mass timber is possible
• Plastering provides weather protection for mass timber
• Joints on horizontal surfaces need careful protection from moisture damage
• Joint sealing must be flexible to allow for movement during construction

Figure 16 - Floor to wall joints with weatherproof membranes on the wall (plastered exterior wall, mass timber interior)

With vapour control membranes on the wall, airtightness is achieved.

• Vapour control membranes provide an airtight layer with vapour control function
• Joints must be airtight to ensure a continuous airtightness layer
• Joint sealing must be flexible to allow for movement during construction
• Mass timber covered with interior board; the airtight layer can be installed behind the interior board

Figure 17 - Floor to wall joints with vapour control membrane on the wall (closed timber façade, covered interior wall)

By sealing connections with tapes, airtightness can be achieved.

• Vapour control membranes provide an airtight layer with vapour control function
• Min. 60 mm thick insulation over the vapour control layer is required
• The end grain of wall elements is pre-covered with robust self-adhesive membranes
• Joints must be airtight to ensure a continuous airtightness layer
• Joint sealing must be flexible to allow for movement during construction

Figure 18 - Floor to wall joints with tapes (prefabricated wall elements, pre-installed vapour control membranes)

With self-adhesive, weatherproof membranes under the cantilever, the cantilever to wall joints are sealed.

• No mechanical fixings are needed
• Weatherproof membranes and mass timber together provide an airtight layer with vapour control function
• Membranes provide weather protection for mass timber
• Joints on horizontal surfaces must be carefully protected from moisture damage

Figure 19 - Cantilever to wall joints with self-adhesive weatherproof membranes under the cantilever 

Weatherproof membranes and mass timber together provide an airtight layer with vapour control function.

• An interior design with exposed mass timber is possible
• Weatherproof membranes protect mass timber
• The specified weather protection for flat roofs must be compatible with the connected waterproof material.
• Contact your SIGA specialist for specific vapour control requirements in your region
• Joints on horizontal surfaces must be carefully protected from moisture damage

Figure 20 - Wall to flat roof joints with weatherproof membranes

Vapour control membranes provide an airtight layer with vapour control function.

• Weatherproof membranes protect mass timber
• The specified weather protection for flat roofs must be compatible with the connected waterproof material.
• Contact your SIGA specialist for specific vapour control requirements in your region
• Joints on horizontal surfaces must be carefully protected from moisture damage

Figure 21 - Wall to flat roof joints with vapour control membranes

Weatherproof membranes and mass timber together provide an airtight layer with vapour control function.

• Airtight layers and weathertight layers must form a continuous seal from the roof to the wall
• An interior design with exposed mass timber is possible 

Figure 22 - Wall to pitched roof joint with weatherproof membranes

Vapour control membranes provide an airtight layer with vapour control function.

• The weathertight layer must form a continuous seal from the roof to the wall

Figure 23 - Wall to pitched roof joints with vapour control membranes

Weatherproof membranes and mass timber together provide an airtight layer with vapour control function. 

• Joints must be airtight to ensure a continuous airtightness layer
• Joint sealing must be flexible to allow for movement during construction
• Joint sealing must be resistant to mechanical impact and weather
• Strengthen difficult substrates with primer to ensure secure bonding of adhesive tapes

Figure 24 - Timber to concrete joint

Weatherproof membranes and mass timber together provide an airtight layer with vapour control function.

• Airtight/weathertight materials to be applied min. 150mm above standing water
• Joint sealing must be resistant to mechanical impact and weather
• Joints must be flexible to allow for movement during construction
• Horizontal surfaces must be carefully sealed and weatherproofed
• Joint sealing material must be compatible with overlapped waterproof material

Figure 25 - Wall to foundation joint

Weatherproof membranes and mass timber together provide an airtight layer with vapour control function.

• Joints must be airtight to ensure a continuous airtightness layer
• Joint sealing must be resistant to mechanical impact and weather
• An interior design with exposed mass timber is possible

Figure 26 - Hanging window fixed with brackets, mass timber wall with plastered external insulation

Weatherproof membranes and mass timber together provide an airtight layer with vapour control function.

• Joints must be airtight to ensure a continuous airtightness layer
• The opening subsill must be carefully protected from moisture damage
• An interior design with exposed mass timber is possible 

Figure 27 - Window fixed within mass timber wall with external insulation

Vapour control membranes provide an airtight layer with vapour control function.

• Joints must be airtight to ensure a continuous airtightness layer
• The opening subsill must be carefully protected from moisture damage
• Weathertight membranes and tapes can protect the mass timber wall during construction

Figure 28 - Window fixed within mass timber wall with internal insulation

Weatherproof membranes and mass timber together provide an airtight layer with vapour control function.

• Penetrations must be airtight to ensure a continuous airtightness layer
• No fixing is needed
• Mass timber and insulation must be protected from the weather during construction

Figure 29 - Wall penetrations with external insulation, sealed with self-adhesive grommet

Vapour control membranes provide an airtight layer with vapour control function.

• Penetrations must be airtight to ensure a continuous airtightness layer 
• No fixing is needed
• Mass timber and insulation must be protected from the weather during construction

Figure 30 - Pitched roof penetrations with internal insulation covered, sealed with self-adhesive grommet

A hygrothermal simulation must be carried out in the case of build-ups
with a diffusion tight outer layer (e.g. compact wooden roofs) to ensure the
construction is safe from moisture.
 

SIGA can perform a computer simulation to assist with this evaluation. It’s free of charge as long as at least one SIGA membrane is specified in the building.

Fill in the component calculation request form and send it to us.