External Wall Insulation Q+A

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Email Correspondence with Architect

Do you know the difference between Rockwool external insulation and Steico wood fibreboard from a sustainability point of view?

We have a client who thinks Rockwool isn’t very environmentally friendly?

I really appreciate your time in responding to us.

All a bit of a science I am not very knowledgeable with, particularly the over heating aspects of rockwool and its tendency to hold moisture against timber – Rot wool!

In our case we are looking at using it in external insulation with a render finish so presumably not an issue here.

We have been looking at Sto systems and they use polystyrene as the insulant.

This seems to have better insulating qualities but our client doesn’t like the idea of using polystyrene.

Presumably with a better insulating material you are gaining over the lifetime of the building in the energy saved versus the use of the plastic with its embodied energy and environmental bad press.

Stone Mineral Wool Insulation

Definitions:

Generically referred to as stone wool or stone mineral wool
Proprietary Name: Rockwool
Nickname: Rot wool

Basics:

Stone in its raw state is a low impact material that can be part of a long term solution
Turning stone into mineral fibre takes a lot of heat energy, air and water (and depending upon fuel choice a lot of carbon dioxide)
There are other ingredients e.g. plastic fibres added for loft, that add more energy and carbon dioxide
There are other ingredients e.g. fire and rot protection that add chemistry (and their impacts) to the recipe
In the event of fire these ingredients can add to pollution.
Stone wool is economic to purchase and has middle of the road winter thermal conductivity performance
Stone wool is effective as a winter thermal insulation at the right thicknesses
Stone wool is a poor summer insulation that lets solar radiant heat to pass through easily and is then effective at trapping the heat internally, leading to summer overheating in attic rooms and lofts and Northern hemisphere southern rooms in other lightweight construction.
As UK suffer warmer summers stone wool will increase the number of overheating failures from 20% of housing to higher percentages.
Nicknamed ‘Rot wool’: Stone mineral fibres are hydro-phobic and rejects water and water vapour from their fibres, their performance is reduced in the presence of water, moisture or water vapour in the air spaces between the fibres, and can hold water or water vapour against timber structures making them vulnerable to rot.
Competent vapour closed construction with vapour control and breather membrane systems and heating or summer solar heat gains should overcome risks of moisture build up
The embodied energy and carbon both increase at the beginning of the materials life and then in use they reduce in-use energy and depending upon fuel choice, reduce carbon in operating the building.
UK has been blessed with the Gulf stream to provide mild wet weather, and 4 weather fronts competing providing short spells of extremes of weather, UK buildings have never had to perform well.

External Insulated render:

Relatively safe from inclement weather during assembly
Needs a waterproof thermal break plinth, at abutments and behind services
- Enviroform Solutions Ltd. (Aluminium clad aerogel insulation)
- Polystyrene with waterproof membranes
- Cellular glass with waterproof membranes below ground
Avoid creating a cavity between insulation and solid masonry walls
Use a moisture permeable render system on solid wall construction e.g. lime, silica,

Wood Fibre Insulation:

Definitions:

Generically referred to as: Wood fibre board, dense wood fibreboard, flexible wood fibre batts
Proprietary Name: many, Steico, Diffutherm, etc.

Basics:

Wood and wood fibre in its raw state is a low impact material that can be part of a long term solution
Turning wood into wood fibre takes far less energy than stone into stone wool fibre; and water (and depending upon fuel choice less carbon dioxide)
There are other ingredients e.g. plastic fibres for loft that add more energy and carbon dioxide
There are other ingredients e.g. fire and rot protection that add chemistry (and their impacts) to the recipe
In the event of fire these ingredients can add to pollution.
Wood fibre is not as economic to purchase as stone wool and has middle of the road winter thermal performance
Wood fibre is effective as a winter thermal insulation when used at suitable thicknesses.
Wood fibre is an excellent summer insulation that stops solar radiant heat passing through easily and prevents summer overheating in attic rooms and lofts and in northern hemisphere southerly rooms behind lightweight construction.
Brick or stone walls should provide sufficient decrement delay.
Brick outer leaf with ventilated cavity should do the same.
Timber frame without a masonry outer leaf needs decrement delay in the insulation provided.
Dense wood fibre provides decrement delay if of sufficient thickness
Flat open sites with complete uninterrupted solar access need 10-12 hours of decrement delay

GBE Slogan:

Build light, Insulate right, Solar tight

Wood fibre is hygroscopic and absorbs water vapour, into the fibres letting the air spaces insulate effectively; their performance is maintained in the presence of high levels of water vapour, and will not hold water or water vapour against timber structures making them vulnerable.
The embodied energy increase at the beginning of the materials life, but due to photosynthesis of growing timber, carbon is reduced to negative levels; resulting in a low carbon material and then in use they reduce in-use energy and depending upon fuel choice, also reduce in-use carbon in operating the building.
There are many other plant fibre based thermal insulation materials that have similar characteristics to wood fibre.
Mainland Europe has had to contend with more intense summers and intense winters, historically Mainland Europe has suffered many more wars and has concentrated populations in defendable walled cities, where city heat island effect has greater impact and insulation materials have been developed to suit these conditions.
Dense wood fibre insulation has been developed to suit these conditions.
UK is beginning to experience warmer climate and these materials become increasingly necessary to control our indoor air conditions.

External Insulated render:

Vulnerable from inclement weather during assembly, especially south west, west and north west coasts
- Build down from eaves to plinth to minimise water entry
- Needs a waterproof thermal break plinth, at abutments and behind services
- Enviroform Solutions Ltd. (Aluminium clad aerogel insulation)
- Polystyrene with waterproof membranes
- Cellular glass with waterproof membranes below ground
Avoid creating a cavity between insulation and solid masonry walls
Use a moisture permeable render system on solid wall construction e.g. lime, silica.

Expanded Polystyrene

Definitions:

Generically referred to as: Polystyrene, Expanded Polystyrene, EPS (Not XPS)
Proprietary Name: many, Polyfoam, etc.

Basics:

Expanded Polystyrene is a high impact material that should not be part of a long term solution
However due to the limited range of materials available for use below ground and subject to water, its likely to persist.
Oil accounts for a 36% of global anthropogenic or manmade CO2 production
Plastics are a bi-product of oil production
Plastics account for 4.5% of global manmade CO2 production that should not be part of a long term solution
Turning black oil into white polystyrene takes lots of energy and CO2 (and depending upon fuel choice more carbon dioxide)
There are other optional ingredients e.g. graphite that add chemistry (and their impacts) to the recipe
In the event of fire polystyrene and these additional ingredients can add to pollution.
Expanded Polystyrene is an economic material and has middle of the road winter thermal performance
Expanded Polystyrene is effective as a winter thermal insulation when used at suitable thicknesses.
Expanded Polystyrene is not a summer insulation it does not stop solar radient heat passing through easily and enables and exacerbates summer overheating in attic rooms and lofts.

GBE Slogan:

Build light, Insulate right, Solar tight

Expanded Polystyrene is hydro-phobic and rejects water vapour and water, insulate effectively; their performance is maintained in the presence of high levels of water, and will hold water against any vulnerable materials.
The embodied energy and carbon both increase at the beginning of the materials life and then in use they reduce in-use energy and depending upon fuel choice, reduce carbon in operating the building.
UK is beginning to experience warmer climate and these materials become increasingly necessary to control our indoor air conditions.

External Insulated render:

Relatively safe from inclement weather during assembly
- Needs a waterproof thermal break plinth, at abutments and behind services
- Enviroform Solutions Ltd. (Aluminium clad aerogel insulation)
- Polystyrene with waterproof membranes
- Cellular glass with waterproof membranes below ground
Avoid creating a cavity between insulation and solid masonry walls
Use a moisture permeable render system on solid wall construction e.g. lime, silica.