Help! My builder says I need a membrane.
Which one do I need and why?
There are a number of different membranes which can be used within the building structure, each performs a different role and has different properties to offer. It is important to understand what each one does before you go ahead and purchase. Below, we give you a brief overview of the 4 most commonly used types of building membrane, then go on to answer one of the most common questions we get asked: “What is the difference between a VSD membrane and a Breather Membrane?”
The 4 most commonly used types of building membrane are:
- High vapour resistance Roofing Felt
- Vapour control layer
- Variable Surface Diffusion ‘Intelligent’ membrane
- Breather Membrane
High vapour resistance Roofing Felt:
Why do we use High vapour resistance Roofing Felt?
Traditional roofing felt is a material typically made of a base fibre which is then coated with asphalt or tar. It is waterproof and is highly resistant to moisture/water vapour.
Although breather membranes have become increasingly popular due to their improved performance and durability, roofing felt is still commonly specified in building work if bats are present in the structure or by a conservation officer in a listed property / historic building to preserve the originality of the structure.
Vapour Control Membrane
What is a Vapour Control Membrane?
An air and vapour control membrane is an airtight membrane made of plastic or foil which is used to reduce the movement of warm moist air from the inside of the building and stop it penetrating through to the actual building fabric. It is installed on the warm side of the insulation i.e., the inside of a pitched roof or the interior walls.
How do Vapour Control Membranes actually work?
Vapour control membranes have one job and that is to stop moisture travelling from the inside of the building (i.e., the internal rooms) and through to the building fabric itself. This in turn designed to protect the building fabric from mould growth, decay and low air quality. Vapour control membranes do not allow moisture to travel back in to the building fabric which can be problematic as moisture gets trapped behind the membrane and has no means of escape.
Variable Surface Diffusion (VSD) Membrane
What is a Variable Surface Diffusion (VSD) Membrane?
Thermofloc Variable Surface Diffusion (VSD) is classed as an intelligent membrane, acting as an air tightness and vapour control membrane whilst allowing two-way movement of water vapour as conditions within the building allow.
Where do you install a VSD Membrane?
The VSD membrane is installed on the inside (warm side) of roof construction and the inside (warm side) of walls of timber frame structures and studwork,
What does a VSD membrane do?
VSD Membranes are used in building construction to control the movement of water vapour from the inside of the building and through to the building fabric itself, helping to prevent condensation from forming on the inside of the building fabric which can lead to mould growth , decay and low air quality. Because VSD membranes can effectively resist air pressure, they are used to improve air tightness and minimise air leaking within the building fabric.
How does a VSD membrane work and what is the difference between a vapour control membrane and an intelligent membrane?
As with a vapour control membrane, when humidity levels are high within the building, a VSD membrane will be vapour closed, i.e. they have a high vapour resistance. This prevents the water vapour in the humid air travelling through to the building fabric and forming interstitial condensation which in turn creates mould and decay.
The difference between a vapour control membrane and an intelligent membrane is that once the humidity levels drop then the ‘intelligent’ membrane will open up (vapour open state) and allow any trapped water vapour to escape. So, moisture that does enter the building fabric is able to back diffuse out of the building as the humidity levels decrease.
So now we have looked at the difference between the membranes installed on the warm side let’s take a journey to the cold side. ( the outer side nearest the wall/roof.)
What is a Breather Membrane and why do I need one?
Breather Membranes are semi-permeable membranes designed to allow excess water vapour to pass from the inside of the building fabric out into the atmosphere, whilst preventing moisture, surface water or rainwater from entering from the outside.
They are installed on the cold side (outer) of the insulation – so the outside of a pitched roof, on the underside of suspended floor joists, or the exterior walls of a timber frame building, or sometimes against the inside surface of a solid wall after the insulation.
The use of a Breather Membrane will help to prevent condensation, mould and decay within the fabric of the building which in turn helps to maintain humidity levels and create a healthier living/working environment.
How do Breather Membranes work?
Installed on the cold side of the insulation -The inside face of the Breather Membrane contains pores or holes which allow water vapour and other gas molecules to pass through from inside to the outside of the building.
Because these gas molecules are incredibly small (less than 3 millionths of 1mm) the membrane can appear solid to the naked eye but will still allow large quantities of water vapour to pass through.
The outside of the membrane stops liquid water ingress (rain/snow) from the outside of the building to the inside, serving as an extra layer of protection within the roof or wall when the cladding material itself may not be watertight (e.g. tiles)
Where do I use a Breather membrane?
Breather membranes are primarily used in roofs and walls but can be used on the underside of suspended floor joists from below.
They help keep the roof space dry, prevent condensation and maintain the effectiveness of the insulation.
In walls, breather membranes are installed behind the exterior cladding. They contribute to moisture management, a more energy-efficient building and will assist in maintaining a consistent indoor temperature by reducing heat loss through the walls and preventing drafts.
What if I don’t use a Breather Membrane in my roof or wall?
If you decide not to use a breather membrane you risk water passing through the tiles / stone/cladding on the external side. A build-up of water in a structure is never good news. Excess moisture rots timbers, creates damp, allows mould spores to thrive and renders any insulation ineffective, all of which leads to an unhealthy building and in turn an unhealthy living environment.
Thermafleece Breather Membrane – which way is up?
Thermafleece Breather Membrane has two sides. A grey side which has a higher UV stability than the other white side. When installing, the grey side should face outward (the cold side) and the white side should face inward toward the inside of the building (the warm side). For example, when installing in a floor, the grey side faces the ground and in a roof, the grey side faces the sky.
Joints in the membrane can be sealed with a breathable tape to prevent accidental air leakage, although this is not essential.
Do I need to use a Breather Membrane when insulating a suspended floor?
A Breather Membrane can be used to support insulation (hammocking) between suspended floor joists
The membrane can be nailed or stapled to the underside of the floor joist if access permits.
Coverage and overlapping a breather membrane
When installing onto a wall or pitched roof, 1 roll of a membrane with 75m2 coverage will actually cover about 60m2 allowing for overlaps.
If you are hammocking the membrane for a suspended floor then one 75m2 roll will cover approx. 45m2
There is no need for a membrane if you are insulating your loft, although because lofts are inherently dirty, and debris tends to fall down from the roof, many people use them as a cover over the insulation to help keep it clean.
For further advice and installation diagrams take a look at our help and advice section https://naturalinsulations.co.uk/help-advice/
For product information and factsheets go our product section https://naturalinsulations.co.uk/membranes/
For further information on Eden Renewable Innovations Ltd – The manufacturers of Thermafleece go to the website www.thermafleece.com