Martin Twamley from Steico explains how to use breathable internal insulation, while avoiding the risks of condensation and mould.
Interview with Martin Twamley
Martin Twamley is Technical Director of Steico UK. Steico have been producing timber-based materials, predominantly structural timber and insulation materials, since the 1960s and selling them across Europe. Using only pine and spruce they make a number of different products, and will always provide guidance as to what the correct product would be for the required application.
“Once people start using timber and understanding it, it's a sort of material that you fall in love with, and you then change your building process.”
The products have low embodied energy
Structural products like I-joists were first used in the USA and the motivation was that it was the most sensible use of the raw material. However, as a natural material and one that goes through little processing it also has a low embodied energy, and will even lock up carbon.
Laminating the veneers strengthens the product
The I-joists are created from Laminated Veneer Lumber. They are made by peeling a tree, creating 3mm veneers, and then bonding them back together. This means that individual weaknesses are removed and spread across the whole product. The strength-to-weight ratio on engineered timber is then much greater than it is for solid section timber as you're getting a much stronger product for less raw material.
There is very little manufacturing waste
Martin explains that the good thing about producing both structural and insulation products is that there is very little waste. For the structural timber they use the de-barking and peeling process. For the insulation materials they use chipping and milling. Therefore, what they can't use for the structural timber they can use for the insulation. What they can't use for the insulation they can use for producing their own pallets. And if they can't use it for pallets they have a biomass power station where they can use it for electricity generation.
UK housing stock has a need for internal insulation
The UK housing stock is some of the oldest in Europe and is in desperate need of retrofitting. With a reluctance to insulate externally and change the appearance of this historic solid wall housing, internal insulation may be a better solution.
The traditional way of doing this is with systems that try to block the moisture, rather than being able to control it. When you try to block moisture there can be an issue with getting it trapped inside the building instead. This potentially adds risk to the fabric of the building, because of mould and condensation problems.
The Steico wood fibre internal insulation system allows water vapour in, controls it, holds it, and then lets it out again.
Standard insulation installations are vapour closed
These typically are the foil-backed polyurethane type materials that are either on a frame system or dot and dabbed onto brickwork. In theory they don't let any moisture in, which is a good thing for the fabric. This can however lead to high moisture levels internally, so you might need to consider an MVHR for example. Mould will happen when warm air gets behind the plaster board and hits the cold wall surface. It might not even be until years later that the mould shows itself on the internal surface.
“When people think about air quality, they generally think about external air, but we spend 90% of our time indoors. It's the internal air that we're breathing. So that's what's impacting our health.”
These are the kinds of systems that tradespeople and DIYers are most likely to use, because they're readily available. Also, homeowners are primarily just thinking about thermal performance in the winter. But actually, when you consider a holistic approach that takes into account all the possible issues, the material choice is incredibly important.
Internal insulation doesn't have to significantly impact on the room size
Martin talks about applying a thin layer, typically an optimum amount of 40-60mm. Even in relatively small rooms that's an amount that won't really be noticed.
What works for one building, won't work for every building
What works for one elevation of a building might not even work for a different elevation of the same building, because of different weathering exposure. This is why it is so important to have an assessment done, particularly if you're looking for a really low U-value. Martin explains that generally speaking a 40mm or 60mm board on standard brick will work in the majority of situations.
Take the wall system back to bare brickwork if possible
The wall system should be as breathable as possible. That means that you don't want anything on the internal side that hinders water vapour movement, such as impermeable layers, cement-based plasters and renders, and paint layers.
The external face is also important to consider (such as renders and re-pointing), because this is where the majority of the water is coming from.
With this natural, breathable approach, there is a full bonding layer (typically a lime based product) between the insulation and the wall. This is in contrast to a traditional approach that would have a cavity. The fact that there is no cavity means that there is no collection of liquid water, which is what leads to mould and condensation issues. Instead, any moisture is either passed onto the brickwork and out the front, or it can go back to the inside.
Pay particular attention to the critical areas
Window reveals are particularly important. If you have a warm wall surface, then the window reveal will be the coldest area. If you don't insulate into that reveal then that is where all your condensation will form. Martin suggests that with a 40mm or 60mm on the wall, a 20mm board on the reveal will be fine.
Another critical area that could be overlooked if somebody is insulating ground and first floor, is the void where the floor joists connect into the wall. These can be cold spots where condensation will be much more of an issue than it was before. To prevent this, you can tape around the junction of the joists to the existing wall. This stops the route for convected airflow into the wall.
Where you have internal walls connecting with external walls can also be problem areas. If you're insulating the external wall, then the corner of that internal junction can be quite cold. Steico make a wedge-type product of thinner insulation that can go into the corner to stop the condensation risk.
There are useful resources available
Martin makes reference to The Guide for Sustainable Renovation, by The Pebble Trust, as being a valuable resource for anyone taking on a retrofit project. It gives advice with a holistic approach that considers all of the moisture risks involved.
The carbon storage element of materials is often overlooked
A frustration for Martin is that in the UK he thinks we tend to be quite product agnostic. It is very much that if you achieve a certain performance level then it doesn't matter what you use to to get there. He compares this with Bavaria, Germany, that has a new system brought in by local government. They are offering a 500 Euro grant for every cubic metre of biogenic carbon locked up within the fabric of the building. In the UK we have U-value targets, but nothing that looks at the amount of carbon stored within the fabric, or the type of material that is used.
Find out more
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See Steico wood fibre insulation being used on Buckinghamshire Passivhaus (a PH15 project). Join The Hub to access our in-depth video case study.