Insulating the FLAT ROOF Timber Joist

Insulation guide for general building contractors, self builders and builders merchants.

  • U value guide.
  • Insulation suggestions and advice.
  • Airtight advice.
  • Ventilation advice.
  • Site visits and inspections available.

Ventilated Flat Roof - Timber Joist Construction. Roof Type 1:

High performance glasswool insulation roll or batt applied Partially Filling the timber joist Plus PIR insulated plasterboards fixed below the timber joist.

 

Ventilated Flat Roof - Timber Joist Construction. Roof Type 2:

High Performance glasswool insulation roll or batt applied Partially Filling the timber joist Plus Glasswool insulation roll or batt applied below the timber joist between timber counter battens.

Roof Build-Up for Flat Roof Type 1.

6: Water tight layer.

5:  Plywood or OSB roof decking.

4:  Crossflow ventilated airspace.

3:  High performance glasswool insulation roll or batt applied partially filling the timber roof joist.

2:  Rothoblaas Vaporvlies 120 airtight membrane applied taped and sealed below the timber roof joist.

1:  PIR Insulated plasterboards fixed below the timber roof joist.

Flat Roof Type 1

 

Roof Build-Up for Flat Roof Type 2.

8:  Water tight layer.

7:  Plywood or OSB roof decking.

6:  Crossflow ventilated airspace.

5:  High performance glasswool insulation roll or batt applied partially filling the timber roof joist.

4:  Rothoblaas Vaporvlies 120 airtight membrane applied taped and sealed below the timber roof joist.

3:  High performance glasswool insulation roll or batt applied between the timber battens.

2:  50mm Timber counter battens fixed below the timber roof joist.

1:  Plasterboard fixed below the timber roof joist.

Flat Roof Image 2

What we need to know prior to insulating the ventilated flat roof.

Q. What type of watertight roofing system are you applying over the plywood/OSB roof deck?

Is it torch on felt, fibre glass roof or other?

 

Q. What is the total depth of the roof joist?

Are they 150mm, 175mm, 225mm or other?

The answer to this question will determine the insulation space available between the timber joist.

 

Existing timber joist: Timber joist can be extended to increase the insulation space using timber counter battens.

 

 Q. What is the spacing/centers between the timber joist?

Are they 400mm centres, 600mm centres or other?

The answer to this question will determine the thermal bridging factor.

 

Q. Are you applying an airtight membrane below the timber joist?

Yes, is the best option, airtightness helps to reduce heat loss. An airtight membrane is also considered a vapour check or vapour control layer.

 

Q. Are you applying insulated plasterboards below the timber joist?

Yes, is the best option, the insulated plasterboards will help reduce thermal bridging and improve the overall U value result.

 

NOTE: Crossflow ventilation is required between the insulation layer applied between the roof joist and the underside of the plywood/OSB roof deck.

NOTE: For the purpose of calculating the condensation risk analysis the foil facing on the reverse side of insulated plasterboards or plasterboards should not be considered a vapour control layer as the board joints, penetrations and abutments cannot be adequately sealed.

 

High Performance Glasswool Insulation Thermal Conductivity

For the purpose of calculating the U values in this insulation guide the high performance glasswool insulation materials applied between the timber roof joist has a thermal conductivity of 0.034 W/mK and the high performance glasswool insulation materials applied between the timber battens has a thermal conductivity of 0.035 W/mK

 

PIR Insulated Plasterboards - Thermal Resistance Guide

PIR = Polyisocyanurate foam insulation.

The thermal resistance is calculated to include the insulation board and the 12.5mm plasterboard

37.5mm                1.205 m²K/W

42.5mm                1.430 m²K/W

50.5mm                1.793 m²K/W

52.5mm                1.884 m²K/W

62.5mm                2.339 m²K/W

72.5mm                2.793 m²K/W

82.5mm                3.248 m²K/W

92.5mm                3.702 m²K/W

 

Educate to Insulate

Natural Building Materials

  • Timber roof joist are natural building materials and will continue to expand and contract over the entire life time of the building.
  • Choosing flexible high performance glasswool insulation materials can make it easier to ensure a snug sustainable fit between the insulation layer and the sides of the timber roof joist thus reducing the risk of cold air moving from the ventilated airspace into the warm side (inside) of the roof.
  • A snug fit is easily achieved by cutting the high performance glasswool insulation slightly oversize and compression fitting between the timber joist.
  • High performance glasswool insulation will not slump between the timber roof joist when applied compression fitted.
  • High performance glasswool insulation is vapour permeable and fire safe. Fire classification A1 Non-Combustible and will also provide a level of sound reduction from external noise intrusion.
  • Airtight membranes when applied correctly will reduce heat loss by restricting the movement of air and also provide protection against moisture damage.
  • Insulation that is installed correctly provides protection from the cold in winter, protection from the heat in summer and contributes to reducing fuel bills.

Key words: Protection and applied correctly………

 

Educate to Insulate

People fail products; products do not fail people.

  • A U value calculation is a paper exercise and in many cases the expected U value results are not achieved on site due to poor application standards and incorrect choice of insulation materials.
  • To insulate is to protect.
  • Radon barriers protect.
  • Fire barriers protect.
  • Vapour control membranes protect.
  • Breathable roofing membranes protect.
  • Ventilation protects.

If you don’t inspect the installation of all products or systems mentioned above; don’t expect the best results.

 

Thermal insulation, acoustic insulation, firestopping insulation, ventilation, breathability and airtight systems are inextricability linked.

A good building specification provides protection and will assist the builder in achieving the best results and a healthy home.

 

Educate to Insulate

The U value Results - Flat Roof Type 1:Ventilation (layer 4)

  • The insulation thickness applied between the roof joist is 100mm

6:  Water tight layer.

5:  Plywood or OSB roof decking.

4:  Crossflow ventilated airspace.

3:  100mm High performance glasswool insulation roll or batt applied partially filling the timber roof joist.

2:  Rothoblaas Vaporvlies 120 airtight membrane applied taped and sealed below the timber roof joist.

1:  37.5mm PIR Insulated plasterboards fixed below the timber roof joist.

U-value (rounded) 0.27 W/m²K

Flat Roof Type 1

To improve the U value result, increase the PIR insulated plasterboard thickness to;

Insulated plasterboard thickness                Thermal resistance                        U-value (rounded)        

                  42.5mm                                          1.430 m²K/W                                   0.25 W/m²K

                  50.5mm                                          1.793 m²K/W                                   0.23 W/m²K

                  52.5mm                                          1.884 m²K/W                                   0.22 W/m²K

                  62.5mm                                          2.339 m²K/W                                   0.20 W/m²K

                  72.5mm                                          2.793 m²K/W                                   0.19 W/m²K

                  82.5mm                                          3.248 m²K/W                                   0.17 W/m²K

                  92.5mm                                          3.702 m²K/W                                   0.16 W/m²K

 

  • Minimum roof joist depth. 150mm
  • Minimum crossflow ventilated airspace. 50mm
  • Roof joist centres. 400mm
  • Insulation type and thermal conductivity in layer 3, High performance glasswool insulation roll with a thermal conductivity of not less than. 0.034 W/mK
  • PIR Insulated plasterboard thermal resistance guide. (see above)
  • Roof joist centres set at 600mm will show a marginal improvement on the U value result.

Rothoblaas Vaporvlies 120 (layer 2) is a fixed sd airtight membrane and is suitable for ventilated roof spaces.

The brand of the high performance glasswool insulation roll 34 (layer 3) is not important. If the insulation is the same thickness and has a thermal conductivity of 0.034 W/mK or better you will achieve at least the same result. The brand of the PIR insulated plasterboard is not important (layer 1). If the overall thermal resistance for the PIR insulated plasterboard is the same or better you will achieve at least the same result.

 

The U value Results - Flat Roof Type 1:Ventilation (layer 4)

  • The insulation thickness applied between the roof joist is 150mm

6:  Water tight layer.

5:  Plywood or OSB roof decking.

4:  Crossflow ventilated airspace.

3:  150mm High performance glasswool insulation roll or batt applied partially filling the timber roof joist.

2:  Rothoblaas Vaporvlies 120 airtight membrane applied taped and sealed below the timber roof joist.

1:  37.5mm PIR Insulated plasterboards fixed below the timber roof joist.

U-value (rounded) 0.20 W/m²K

Flat Roof Type 1

To improve the U value result, increase the PIR insulated plasterboard thickness to;

Insulated plasterboard thickness                Thermal resistance                        U-value (rounded)        

                  42.5mm                                          1.430 m²K/W                                   0.19 W/m²K

                  50.5mm                                          1.793 m²K/W                                   0.18 W/m²K

                  52.5mm                                          1.884 m²K/W                                   0.18 W/m²K

                  62.5mm                                          2.339 m²K/W                                   0.16 W/m²K

                  72.5mm                                          2.793 m²K/W                                   0.15 W/m²K

                  82.5mm                                          3.248 m²K/W                                   0.14 W/m²K

                  92.5mm                                          3.702 m²K/W                                   0.13 W/m²K

 

  • Minimum roof joist depth. 200mm
  • Minimum crossflow ventilated airspace. 50mm
  • Roof joist centres. 400mm
  • Insulation type and thermal conductivity in layer 3, High performance glasswool insulation roll with a thermal conductivity of not less than. 0.034 W/mK
  • PIR Insulated plasterboard thermal resistance guide. (see above)
  • Roof joist centres set at 600mm will show a marginal improvement on the U value result.

Rothoblaas Vaporvlies 120 (layer 4) is a fixed sd airtight membrane and is suitable for ventilated roof spaces.

The brand of the high performance glasswool insulation roll 34 (layer 3) is not important. If the insulation is the same thickness and has a thermal conductivity of 0.034 W/mK or better you will achieve at least the same result. The brand of the PIR insulated plasterboard is not important (layer 1). If the overall thermal resistance for the PIR insulated plasterboard is the same or better you will achieve at least the same result.

 

The U value Results - Flat Roof Type 1:Ventilation (layer 4)

  • The insulation thickness applied between the roof joist is 180mm

6:  Water tight layer.

5:  Plywood or OSB roof decking.

4:  Crossflow ventilated airspace.

3:  180mm High performance glasswool insulation roll or batt applied partially filling the timber roof joist.

2:  Rothoblaas Vaporvlies 120 airtight membrane applied taped and sealed below the timber roof joist.

1:  37.5mm PIR Insulated plasterboards fixed below the timber roof joist.

U-value (rounded) 0.18 W/m²K

Flat Roof Type 1

To improve the U value result, increase the PIR insulated plasterboard thickness to;

Insulated plasterboard thickness                Thermal resistance                        U-value (rounded)        

                  42.5mm                                          1.430 m²K/W                                   0.17 W/m²K

                  50.5mm                                          1.793 m²K/W                                   0.16 W/m²K

                  52.5mm                                          1.884 m²K/W                                   0.16 W/m²K

                  62.5mm                                          2.339 m²K/W                                   0.15 W/m²K

                  72.5mm                                          2.793 m²K/W                                   0.14 W/m²K

                  82.5mm                                          3.248 m²K/W                                   0.13 W/m²K

                  92.5mm                                          3.702 m²K/W                                   0.12 W/m²K

 

  • Minimum roof joist depth. 230mm
  • Minimum crossflow ventilated airspace. 50mm
  • Roof joist centres. 400mm
  • Insulation type and thermal conductivity in layer 3, High performance glasswool insulation roll with a thermal conductivity of not less than. 0.034 W/mK
  • PIR Insulated plasterboard thermal resistance guide. (see above)
  • Roof joist centres set at 600mm will show a marginal improvement on the U value result.

Rothoblaas Vaporvlies 120 (layer 4) is a fixed sd airtight membrane and is suitable for ventilated roof spaces.

The brand of the high performance glasswool insulation roll 34 (layer 3) is not important. If the insulation is the same thickness and has a thermal conductivity of 0.034 W/mK or better you will achieve at least the same result. The brand of the PIR insulated plasterboard is not important (layer 1). If the overall thermal resistance for the PIR insulated plasterboard is the same or better you will achieve at least the same result.

 

The U value Results - Flat Roof Type 1:Ventilation (layer 4)

  • The insulation thickness applied between the roof joist is 220mm

6:  Water tight layer.

5:  Plywood or OSB roof decking.

4:  Crossflow ventilated airspace.

3:  220mm High performance glasswool insulation roll or batt applied partially filling the timber roof joist.

2:  Rothoblaas Vaporvlies 120 airtight membrane applied taped and sealed below the timber roof joist.

1:  37.5mm PIR Insulated plasterboards fixed below the timber roof joist.

U-value (rounded) 0.15 W/m²K

Flat Roof Type 1

To improve the U value result, increase the PIR insulated plasterboard thickness to;

Insulated plasterboard thickness                Thermal resistance                        U-value (rounded)        

                  42.5mm                                          1.430 m²K/W                                   0.15 W/m²K

                  50.5mm                                          1.793 m²K/W                                   0.14 W/m²K

                  52.5mm                                          1.884 m²K/W                                   0.14 W/m²K

                  62.5mm                                          2.339 m²K/W                                   0.13 W/m²K

                  72.5mm                                          2.793 m²K/W                                   0.12 W/m²K

                  82.5mm                                          3.248 m²K/W                                   0.12 W/m²K

                  92.5mm                                          3.702 m²K/W                                   0.11 W/m²K

 

  • Minimum roof joist depth. 270mm
  • Minimum crossflow ventilated airspace. 50mm
  • Roof joist centres. 400mm
  • Insulation type and thermal conductivity in layer 3, High performance glasswool insulation roll with a thermal conductivity of not less than. 0.034 W/mK
  • PIR Insulated plasterboard thermal resistance guide. (see above)
  • Roof joist centres set at 600mm will show a marginal improvement on the U value result.

Rothoblaas Vaporvlies 120 (layer 4) is a fixed sd airtight membrane and is suitable for ventilated roof spaces.

The brand of the high performance glasswool insulation roll 34 (layer 3) is not important. If the insulation is the same thickness and has a thermal conductivity of 0.034 W/mK or better you will achieve at least the same result. The brand of the PIR insulated plasterboard is not important (layer 1). If the overall thermal resistance for the PIR insulated plasterboard is the same or better you will achieve at least the same result.

 

Educate to Insulate

The U value Results - Flat Roof Type 2:Ventilation (layer 6)

  • The insulation thickness applied between the roof joist is 100mm
  • The insulation thickness applied between the timber counter battens is 50mm

8:  Water tight layer.

7:  Plywood or OSB roof decking.

6:  Crossflow ventilated airspace.

5:  100mm High performance glasswool insulation roll or batt applied partially filling the timber roof joist.

4:  Rothoblaas Vaporvlies 120 airtight membrane taped and sealed below the timber roof joist.

3:  50mm High performance glasswool insulation roll or batt applied between timber battens.

2:  50mm Counter battens.

1:  Plasterboard fixed below the timber roof joist.

 U-value (rounded) 0.27 W/m²K

 Flat Roof Image 2

  • Minimum roof joist depth. 150mm
  • Minimum crossflow ventilated airspace. 50mm
  • Roof joist centres. 400mm
  • Insulation type and thermal conductivity in layer 5, High performance glasswool insulation roll with a thermal conductivity of not less than. 0.034 W/mK
  • Roof joist centres set at 600mm will show a marginal improvement on the U value result.

Rothoblaas Vaporvlies 120 (layer 3) is a fixed sd airtight membrane and is suitable for ventilated roof spaces.

The brand of the high performance glasswool insulation roll 34 (layer 4) is not important. If the insulation is the same thickness and has a thermal conductivity of 0.034 W/mK or better you will achieve at least the same result. The brand of the high performance glasswool insulation roll 35 (layer 2) is not important. If the insulation is the same thickness and has a thermal conductivity of 0.035 W/mK or better you will achieve at least the same result.

 

The U value Results - Flat Roof Type 2:Ventilation (layer 6)

  • The insulation thickness applied between the roof joist is 150mm
  • The insulation thickness applied between the timber counter battens is 50mm

8:  Water tight layer.

7:  Plywood or OSB roof decking.

6:  Crossflow ventilated airspace.

5:  150mm High performance glasswool insulation roll or batt applied partially filling the timber roof joist.

4:  Rothoblaas Vaporvlies 120 airtight membrane taped and sealed below the timber roof joist.

3:  50mm High performance glasswool insulation roll or batt applied between timber battens.

2:  50mm Counter battens.

1:  Plasterboard fixed below the timber roof joist.

U-value (rounded) 0.21 W/m²K

 

  • Minimum roof joist depth. 200mm
  • Minimum crossflow ventilated airspace. 50mm
  • Roof joist centres. 400mm
  • Insulation type and thermal conductivity in layer 5, High performance glasswool insulation roll with a thermal conductivity of not less than. 0.034 W/mK
  • Roof joist centres set at 600mm will show a marginal improvement on the U value result.

Rothoblaas Vaporvlies 120 (layer 3) is a fixed sd airtight membrane and is suitable for ventilated roof spaces.

The brand of the high performance glasswool insulation roll 34 (layer 4) is not important. If the insulation is the same thickness and has a thermal conductivity of 0.034 W/mK or better you will achieve at least the same result. The brand of the high performance glasswool insulation roll 35 (layer 2) is not important. If the insulation is the same thickness and has a thermal conductivity of 0.035 W/mK or better you will achieve at least the same result.

 

The U value Results - Flat Roof Type 2:Ventilation (layer 6)

  • The insulation thickness applied between the roof joist is 180mm
  • The insulation thickness applied between the timber counter battens is 50mm

8:  Water tight layer.

7:  Plywood or OSB roof decking.

6:  Crossflow ventilated airspace.

5:  180mm High performance glasswool insulation roll or batt applied partially filling the timber roof joist.

4:  Rothoblaas Vaporvlies 120 airtight membrane taped and sealed below the timber roof joist.

3:  50mm High performance glasswool insulation roll or batt applied between timber battens.

2:  50mm Counter battens.

1:  Plasterboard fixed below the timber roof joist.

U-value (rounded) 0.18 W/m²K

 

  • Minimum roof joist depth. 230mm
  • Minimum crossflow ventilated airspace. 50mm
  • Roof joist centres. 400mm
  • Insulation type and thermal conductivity in layer 5, High performance glasswool insulation roll with a thermal conductivity of not less than. 0.034 W/mK
  • Roof joist centres set at 600mm will show a marginal improvement on the U value result.

Rothoblaas Vaporvlies 120 (layer 3) is a fixed sd airtight membrane and is suitable for ventilated roof spaces.

The brand of the high performance glasswool insulation roll 34 (layer 4) is not important. If the insulation is the same thickness and has a thermal conductivity of 0.034 W/mK or better you will achieve at least the same result. The brand of the high performance glasswool insulation roll 35 (layer 2) is not important. If the insulation is the same thickness and has a thermal conductivity of 0.035 W/mK or better you will achieve at least the same result.

 

The U value Results - Flat Roof Type 2:Ventilation (layer 6)

  • The insulation thickness applied between the roof joist is 220mm
  • The insulation thickness applied between the timber counter battens is 50mm

8:  Water tight layer.

7:  Plywood or OSB roof decking.

6:  Crossflow ventilated airspace.

5:  180mm High performance glasswool insulation roll or batt applied partially filling the timber roof joist.

4:  Rothoblaas Vaporvlies 120 airtight membrane taped and sealed below the timber roof joist.

3:  50mm High performance glasswool insulation roll or batt applied between timber battens.

2:  50mm Counter battens.

1:  Plasterboard fixed below the timber roof joist.

U-value (rounded) 0.18 W/m²K

 

  • Minimum roof joist depth. 270mm
  • Minimum crossflow ventilated airspace. 50mm
  • Roof joist centres. 400mm
  • Insulation type and thermal conductivity in layer 5, High performance glasswool insulation roll with a thermal conductivity of not less than. 0.034 W/mK
  • Roof joist centres set at 600mm will show a marginal improvement on the U value result.

Rothoblaas Vaporvlies 120 (layer 3) is a fixed sd airtight membrane and is suitable for ventilated roof spaces.

The brand of the high performance glasswool insulation roll 34 (layer 4) is not important. If the insulation is the same thickness and has a thermal conductivity of 0.034 W/mK or better you will achieve at least the same result. The brand of the high performance glasswool insulation roll 35 (layer 2) is not important. If the insulation is the same thickness and has a thermal conductivity of 0.035 W/mK or better you will achieve at least the same result.

 

Insulation and Associated Building Materials; Pack sizes, Roll sizes.

Contact Your Local Builders Providers for Pricing

 

Knauf Insulation Earthwool OmniFit Stud Roll 34

100mm Knauf Earthwool OmniFit Stud Roll 34

Insulation thermal conductivity 0.034 (W/mK)

Insulation thermal resistance 2.942 W/m²K

Roll size: 1.2mtr x 5.2mtr (6.24m2 per roll)

Price per roll €

 

150mm Knauf Earthwool OmniFit Stud Roll 34

Insulation thermal conductivity 0.034 (W/mK)

Insulation thermal resistance 4.412 W/m²K

Roll size: 1.2mtr x 3.5mtr (4.2m2 per roll)

Price per roll €

 

180mm Knauf Earthwool OmniFit Stud Roll 34

Insulation thermal conductivity 0.034 (W/mK)

Insulation thermal resistance 5.294 W/m²K

Roll size: 1.2mtr x 3mtr (3.6m2 per roll)

Price per roll €

 

220mm Knauf Earthwool OmniFit Stud Roll 34

Insulation thermal conductivity 0.034 (W/mK)

Insulation thermal resistance 6.471 W/m²K

Roll size: 1.2mtr x 2.5mtr (3m2 per roll)

Price per roll €

 

Knauf Earthwool OmniFit Slab

50mm Knauf Earthwool OmniFit Slab 35

Insulation thermal conductivity 0.035 (W/mK)

Insulation thermal resistance 1.428 W/m²K

Sheet size: 1200mm x 600mm (8.64m2 per pack)

Price per pack €

 

50mm Knauf Earthwool OmniFit Slab 35

Insulation thermal conductivity 0.035 (W/mK)

Insulation thermal resistance 1.428 W/m²K

Sheet size: 1200 x 400mm (5.76m2 per pack)

Price per pack €

 

Rothoblaas Airtight Membranes, Tapes, Sealants, Primer & Airtight Foam

Vaporvlies 120 (fixed sd airtight membrane)

Roll size: 1.5mtr x 50mtr (75m2 per roll)

Price per roll €

 

Clima Control 80 (variable sd airtight membrane)

Roll size: 1.5mtr x 50mtr (75m2 per roll)

Price per roll €

 

Flexi Band Airtight Tape

Overlaps, joints, penetrations and abutments. (25 linear mtrs per roll)

60mm Price per roll €

 

Flexi Band Liner (internal and external use)

Split tape (split 50mm+50mm) (25 linear mtrs per roll)

100mm Price per roll €

 

Seal Band (internal use)

Split tape (split 48mm+12mm) for window/door frame to block or plaster reveal. (40 linear mtrs per roll)

60mm Price per roll €

 

Plaster Band In - Plasterable airtight Tape (internal use)

Split tape for window/door frame to block reveal and membrane to block. (25 linear mtrs per roll)

 

  75mm (split 63mm+12mm)     Price per roll €

 

100mm (split 88mm+12mm)     Price per roll €

 

150mm (split 138mm+12mm)   Price per roll €

 

200mm (split 188mm+12mm)   Price per roll €

 

Superb Glue - Airtight Sealant (310ml tube)

Price per tube €

 

Superb Glue - Airtight Sealant (600ml sausage)

Price per sausage €

 

Hermetic Foam - Airtight Closed Cell Acoustic Foam (b2 fire rated) (750ml tin)

Price per tin €

 

Universal Primer Spray (750ml tin)

Price per tin €

 

PIR Insulated Plasterboards

PIR Insulation bonded to 12.5mm plasterboards

PIR = Polyisocyanurate foam insulation.

 

37.5mm x 1200mm x 2400mm

Price per sheet €

 

42.5mm x 1200mm x 2400mm

Price per sheet €

              

50.5mm x 1200mm x 2400mm

Price per sheet €

 

52.5mm x 1200mm x 2400mm

Price per sheet €

 

62.5mm x 1200mm x 2400mm

Price per sheet €

 

72.5mm x 1200mm x 2400mm

Price per sheet €

 

82.5mm x 1200mm x 2400mm

Price per sheet €

 

92.5mm x 1200mm x 2400mm

Price per sheet €

 

For more information call Dermot Kearns.  Mobile: 00353 870526909

Insulation and Associated Building Products Advisor.