Housing Retrofit: Unventilated pitched roof insulation

   
picture of unvented roof

Introduction


• The concept of an ‘unventilated’ roofing is relatively new. Developed over the last two decades, the principle is based upon the introduction of low vapour resistance (LR), ‘breathable’ sarking membranes to the market. A ‘breather’ membrane allows internally produced water vapour to pass through the membrane, whilst preventing the ingress of rain and snow.
• Moisture passing through the breather membrane is dispersed through the ventilated cavity formed by counter battens over the rafters.
• A breather membrane for pitched roofing is defined by BS 5250:2002 as a one that has a water vapour resistance figure of less than 0.25 MNs/g.
• For designers, the advantage of using ‘unventilated’ construction comes from removing the necessity to provide a ventilated cavity below the sarking. Together with not having to provide a ventilation pathway, the technique permits insulation to be installed within the full depth of the rafter.

vented and unvented roofs

breather membrane
Breather membrane ( in colours )


U-value target


The EST target u-value for the rafter-level insulation of a pitched roof is 0.20 W/m2K, though much higher values are achievable.


NB

• The roof insulation methods illustrated below are for general guidance only. The performance of products and methods of fixing vary. When specifying, confirm with the product manufacturer their installation methods and expected performance.
• Beware when combining different insulation materials. Check with respective manufacturers regarding possible condensation risks.

 

Insulation positions

 

The existing (ventilated) roof


existing ventilated roof

In addition to the possibility of defective / decaying materials, existing roofs are unlikely to meet with current regulations – even less with best practice standards of thermal performance and air permeability. Check condition of timbers before proceeding.

 

Between the rafters only


insulation between the rafters only

The most straightforward treatment is to add insulation to the full depth of the rafters.

Pro

Minimal raising of existing roofline

Pro

Maintains internal space and head height

Con

The shallow depth of the rafters might inhibit achievement of performance standards

Con

Thermal bridging through the rafters

Typical U-values: between and below rafters

 

MIneral wool U-value W/m2K
150mm 0.26
PUR / PIR
150mm 0.22

 

 

Between and below rafters with added counter battens


Between and below rafters with added counter battens

By effectively extending the depth of the existing rafters, it is possible to improve the potential thermal performance of the roof through providing an extra thickness of insulation.

Pro

Minimal raising of existing roofline

Pro

Provides for extra depth of insulation

Con

Reduces internal space and can compromise head height within the room

Con

Thermal bridging through the rafters

 

Typical U-values: between and below rafters

 

Between (mm) Below (mm) U-value W/m2K
Mineral wool between and below*
100 100 0.18
160 100 0.15
200 100 0.12
Sheep's wool
100    

Sources: *Knauf;

 


Between and below rafters with added rigid insulation


Between and below rafters with added rigid insulation

By effectively extending the depth of the existing rafters, it is possible to improve the potential thermal performance of the roof through providing an extra thickness of insulation.

Pro

Minimal raising of existing roofline

Pro

Provides for extra depth of insulation

Con

Reduces internal space and can compromise head height within the room

 

Typical U-values: between and below rafters with added rigid insulation

 

Between (mm) Below (mm) U-value W/m2K
Mineral wool between and PUR / PIR below*
100 40 0.23
160 40 0.17
200 40 0.15
Phenolic foam between and below**
75 60 0.18
100 60 0.16
150 60 0.12
200 60 0.10

Sources: *Knauf; **Kingspan

 


Between and above the rafters

 

Breathable insulating sarking above rafters


Breathable insulating sarking above rafters

 

 

Insulation above the rafters


insulation above rafters

Adding insulation above the rafters is the ideal solution where internal space is at a premium. Together with insulation between the rafters, this solution can achieve high levels of thermal performance.

In addition to the more common rigid forms of insulation, products new to the UK market have been introduced that combine breathable sarking with insulation. These wood fibre-based boards have been well-proven on the continent since the 1930s. They provide (subject to specification) the potential for a completely ‘breathable’ form of construction by the use of ‘breathable’ insulation materials between the rafters. In a ‘breathing’ roof, VCLs are not required at ceiling level – and only air-tightness membranes are required if using a service void.

Pro

Provides for extra depth of insulation

Pro

Maintains existing internal space and head heights

Pro

Insulation attached over the rafters reduces thermal bridging

Con

Requires raising of the roof line

Con

Combining an insulation material of vapour resistance over the rafters (eg foam board) with insulation of relatively low resistance (mineral wool) between the rafters is not advised unless an effective VCL is used on the u/s of the rafters

 

Typical U-values: between and and over rafters

 

Between (mm) Over (mm) U-value W/m2K
Sheep's wool between and over*
125 50 0.24
150 75 0.19
150 100 0.17
150 175 0.14
Hemp / cotton between and insulating wood fibre over**
100 100 0.21
150 100 0.17
Mineral wool between and over
100 70 0.23
125 70 0.20
150 70 0.18
Phenolic foam between and over***
60 70 0.16
80 90 0.13
100 130 0.10
Extruded polystyrene (XPS) between and over ****
100 75 0.17
125 75 0.15
150 75 0.14

Sources: *Second Nature; **NBT; ***Kingspan ; ****Knauf;

 

Air tightness


• In most instances, a combined air tightness / vapour control layer (VCL) should be located on the warm side of the insulation.
• It is essential to maintain the air tightness of the construction. If services are run behind the air tightness layer, punctures are to be expected through pipes and wires entering the habitable space. These punctures should be located through rigorous site inspections and sealed accordingly.
• An alternative is to form a ‘service zone’ (shown in these examples) by adding battens to the underside of the VCL before covering with plasterboard.

 

General design and installation notes


• Before commencing work, examine the roof timbers for rot, damp and infestation
• Confirm with the insulation manufacturer the expected u-values and suitability of planned construction particularly where condensation control is required.
• Ensure that fire protection is provided to the underside of the rafters by using 12.5mm plasterboard or similar.
• When using rigid insulation boards, preference should be made for interlocking boards. All joints should be taped and the joint and the junction between the rafter and insulation is sealed with foam (leave a 5mm gap).
• Air cavities where occurring between the vapour control layer and the u/s of the insulation should be sealed top and bottom.
• Ensure that the space between the wall and roof insulations is continuous at eaves.
• Avoid recessed lighting
• Cables less than 50mm away from plasterboard should be enclosed in metal conduits

 

 

Publications

Building Research Establishment (BRE)

Thermal Insulation: Avoiding Risks, C.Stirling, BRE Press, 2001
Installing Thermal Insulation, BRE Press, 2006

 

 

 

 

Further information

Insulation materials compared
TRADA
National Insulation Association
British Urethane Foam Contractors Association


Pitched roof insulation products on GreenSpec

 


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