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Insulating Roofs with Pavatex Wood Fibre Insulation – Externally and Internally

Insulating Roofs

External Roof Insulation Sarking Boards

Isolair Sarking Board
Isolair
A thinner water resistant sarking board for externally insulating roofs as well as masonry or timber frame walls which are cladded with a ventilated façade.
(Similar to Hofatex UD)
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Pavatherm Plus Sarking Board
Pavatherm-Plus
A thicker water resistant sarking board which is a combination of Pavatherm and Isolair for externally insulating roofs and masonry or timber frame walls behind a ventilated façade.
(Similar to Hofatex Kombi)
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Pavaflex Wood Fibre
Pavaflex
A flexible and breathable wood fibre batt for snugly insulating between joists, rafters and studs to provide excellent thermal insulation results.
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The Benefits of External Roof Insulation

Externally insulating a roof with sarking boards is very effective because the complete roof structure is covered in insulation and so cold bridges through the rafters are avoided. This type of roof is called a warm roof. This heat retention is very considerable because the rafters typically take up 15 to 20% of the entire roof surface and the heat will escape through these timber elements. Isolair or Pavatherm-Plus wood fibre insulation boards are installed directly on top of the rafters and they must be fixed into place with battens and counter battens. The roof covering, such as tiles are fitted onto the counter battens. It is recommended to place an airtight membrane below the wood fibre for optimum results.

 

Because Isolair and Pavatherm-Plus boards are both water resistant they can be left exposed on roofs for up to three months, except in heavy snow conditions. This means that the internal building structure will remain dry to allow construction work to proceed. Isolair and Pavatherm-Plus tongue and groove boards are generally used on new builds but if the roof in a renovation project is being removed it is highly recommended to insulate above the rafters as well as between the rafters. Isolair comes in various thicknesses from 20 to 52 mm and Pavatherm-Plus is available in 60 to 160 mm thicknesses, so there are plenty of options to meet U-value requirements. It is also strongly recommended to insulate internally between the rafters with the flexible and breathable Pavaflex wood fibre insulation material. There is no need to leave a cavity between the wood fibre external roof insulation and the insulation between the rafters because there will be a ventilated cavity above the wood fibre sarking boards.

 

Pavatex wood fibre boards are a totally natural and renewable product, which allow water vapour to pass through the structure and so ensure a healthier and warmer indoor atmosphere. The water vapour will then evaporate through the ventilated cavity above the insulation. Sound insulation through the roof will be greatly enhanced with the addition of dense wood fibre sarking boards so noise from rain, traffic or airplanes should be significantly reduced. Attic areas of buildings tend to overheat during the summer because there is virtually no thermal mass in a roof covering. With the addition of wood fibre boards, this excess heat from the sun will be stored for between 10 and 12 hours when it will then be released as temperatures drop, whereas other insulation materials can only provide 5 to 8 hours lag time. This keeps buildings a few degrees cooler during the warmer summer months. The reverse happens in winter when the heat from the central heating system will be stored in the wood fibre layer and released at night-time as temperatures drop. This is particularly crucial when the loft space is used as living accommodation.

 

Example of Low U-value Application on a Pitched Roof – External

  • Concrete interlocking roof tiles
  • Battens and counter battens
  • 100 mm Pavatherm-Plus Wood Fibre Sarking Board above the rafters (0.043 W/mK)
  • 180 mm Pavaflex Flexible Wood Fibre Insulation (0.038 W/mK) between 175mm deep timber rafters
  • 22 mm OSB
  • 25 mm service cavity
  • 12.5 mm plasterboard & skim

Masonry Walls

U-value: 0.13 (subject to specification)
Approx Thermal Lag = 12 Hours

Internal Roof Insulation

Pavaflex Wood Fibre
Pavaflex
A flexible and breathable wood fibre batt for snugly insulating between joists, rafters and studs to provide excellent thermal insulation results.
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Pavatherm Wood Fibre Board
Pavatherm
A universal rigid wood fibre board for insulating internally below roof rafters, above floor sub-decks or flush with walls to reduce thermal bridging.
(Similar to Hofatex Therm)
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Pavadentro Insulation Board
Pavadentro
An internal insulation board that can be plastered directly with a breathable plaster. Excellent hygroscopic properties due to unique mineral layer that controls moisture transfer.
(Similar to Hofatex SysTem IA)
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The Benefits of Internal Roof Insulation

Insulating a roof internally is the most common upgrade method and excellent roof insulation U-values can be achieved. For best results it is recommended to fix a water resistant wood fibre board such as Isolair or Pavatherm-Plus between the rafters leaving a minimum of a 45 mm ventilated cavity between the insulation and the roof covering so that any moisture can escape to prevent rot in the timbers.  Any remaining space in the cavity can be filled with the flexible insulation, Pavaflex. Pavaflex is a flexible and breathable insulation product which provides excellent thermal resistance (0.038 W/mK) and is easy and safe to handle. The batts come in standard widths of 375 mm and 575 mm to fit snugly between most rafters.

 

To achieve the best U-value results the underside of the rafters should be also insulated with a layer of Pavatherm wood fibre which is available in thicknesses from 40 mm to 120 mm. The wood fibre boards have many advantages including the considerable reduction of thermal bridging through the rafters and also the addition of a thermal mass layer to the roof to reduce summer-time overheating. Wood fibre has the highest Specific Heat Capacity (2,100 J/kgK) and highest density of all insulation materials and these factors combined mean that wood fibre is the best insulation material at storing excess solar heat which comes in through the lightweight roof and through the windows during summer. The storage capacity of the wood fibre layer will provide a very welcome relief to this over-heating which can make living in loft spaces very uncomfortable in the warm weather.

 

The roof will be better soundproofed and because the Pavatex wood fibre is breathable, condensation will not accumulate on the ceiling. Always ensure that the insulation between the rafters is also breathable or hygroscopic so that all internal moisture and humidity from showers, cooking, wet clothes and breathing can pass through all the insulation layers and evaporate out through the roof vents. It is believed that internal condensation and mould growth can exacerbate conditions like allergies and asthma which are becoming more and more prevalent. Also because the insulation is totally natural it will not emit any harmful chemicals into the atmosphere, as many insulation materials do when there is a change in air pressure between the inside and outside of buildings. This contributes to a much healthier living environment.

 

Pavatherm can be covered with plasterboard or other similar ceiling board. However if an optimal breathable solution is required, the underside of the rafters should be insulated with Pavadentro which can be plastered directly with lime or clay plaster which is much more vapour-open than plasterboard.

 

Example of Low U-value Application on a Pitched Roof – Internal

  • Concrete interlocking roof tiles on rafters
  • 50mm min cavity under tiles
  • 125 mm Pavaflex Flexible Wood Fibre Insulation (0.038 W/mK) between 175mm deep timber rafters
  • 120 mm Pavatherm Wood Fibre under rafters (0.038 W/mK)
  • 22 mm OSB
  • 25 mm service cavity
  • 12.5 mm plasterboard & skim

Masonry Walls

U-value: 0.15
(subject to specification) Approx Thermal Lag = 11 Hours

 

U-value Calculator

To calculate your own U-value figures on various structures try the Free U-Value Calculator.

 

Pavatex Wood Fibre Stockists