Roof blanket or Vapour Permeable Membrane?

So roof blanket or vapour permeable membrane – which one is better?

This is a conversation we come up against almost daily in our line of work, as homeowners and builders are often stuck between what’s easy and “normal” and what they suspect might be better.  They want to understand the difference between the standard roof blanket and a Vapour Permeable Membrane (VPM), also sometimes known as a Weather Resistive Barrier (WRB) depending on the product.  My answer is that there are pros and cons to both and I will list them here in an easy-to-read table.  The choice between the two is, of course, up to the individual homeowner and/or builder but it’s always better to make an informed choice instead of “doing what we’ve always done”.  Please read the below carefully and get in touch if you have questions!

Builder’s Blanket Vapour Permeable Membrane
Pros Cons Pros Cons
Additional R value which boosts star rating The foil backing is non-vapour permeable When installed correctly, allows a sealed roof cavity which means you have control over the airflow (which occurs above the membrane rather than below) which means higher energy efficiency for your home No associated R-value – and not modelled in energy assessments (yet)
Commonly used product in the industry – installation knowledge high The roof space needs to be ventilated which depletes the thermal performance of your roof insulation due to wind washing Allows vapour to escape preventing high-humidity roof spaces and condensation No acoustic insulation
Acoustic insulation properties You can’t control where the air goes in a ventilated roof space When a high-quality product is installed correctly, a membrane of this type will protect your structure from rain leaks and keep the structure dry and healthy long term. An unfamiliar construction for many builders – so may require some education
Quicker to install The roof blanket is squashed to practically nothing at each purlin where the roof sheets are fixed and the bulk insulation is rendered ineffective. Thermal bridging, and therefore condensation, is concentrated at these areas when it occurs and can wet your purlins and lead to damage over time
Condensation isn’t completely prevented from occurring underneath your roof sheets and this moisture can drip onto the bulk insulation and cause damage.
Higher risk of the “dribble effect” on lower-pitched roofs where rain hangs around on the end of a roof sheet and “dribbles” into the cavity, especially (see image and link below)
Roof blanket can deflect the roof iron


Blanket insulation can cause roof leaks under flat metal roofs

If you’d like to order a waterproof, class 4 vapour permeable, non-flammable, non-perforated and highly durable vapour permeable membrane, such as Pro Clima’s Mento Plus, please get in touch!

I hope this helps!  Happy healthy building!

Jessica Allen


0409 404 924   |


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