Insulating under a suspended floor

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Insulating under the suspended wood floor was our one bit of DIY. We were so concerned to get it right, we just didn’t trust anyone else to be careful enough.

My husband John loves a challenge, so he spent many a happy hour pondering how to do it before landing on a process that was both do-able and effective. Imagine our delight when we found this blog on the Ecological Building Systems website  and discovered he had worked out best practice all on his own!

There were four steps to the process:

  1. Lay an airtightness membrane across the joists to create ‘saddles’ to hold the insulation. The membrane needed to hang down between each joist but never cover the air bricks. John found a piece of wood the correct length and used that when he covered each joist.
  1. Measure the exact size of each inter-joist gap. In an old house – and if you have a suspended floor your house is pre-1950’s – the gaps between each joist will be different. Maybe not by much, but certainly different.
  1. Cut the insulation to size and stuff it into the gap. Make sure it is a tight fit – you have to really shove it into the space, so there are no gaps for air to come through.
  1. Either seal the joins between each insulation batt or cover the whole floor with an airtightness membrane and seal it to the wall at the edge with something like Contega tape.

We very soon got into a routine. I’d cut two or three lots of wood fibre according to instructions and take them through to John who would then call out the measurements for the next three gaps so I could return to the garden with my saw. It’s not a tough material to saw up, unless like me you’re not used to such an activity – then you get blisters from the handle. Mind you, I felt quite proud of mine!

Deciding what materials to use

We were always going to use wood fibre insulation from an environmental perspective, but I also discovered this is the best option anyway for under the floor or in the loft. Flexible insulation will always be best where there could be movement because it will flex and retain the fit. There are a number of options out there and the range is increasing. Our main go to providers are Ecological Building Systems and Unity Lime – both also have very helpful technical experts who will talk you through the pro’s and con’s plus the best methods of installation.

Use of PIR

I can’t write about insulation without a mention of PIR. This commonly used petrochemical material – you’ll recognise it by its distinctive tin foil cover – is not an eco friendly option overall and it’s especially poor under the floor, because it has no give. Between joists that are uneven (and most of them are) it’s almost impossible to create a perfect fit and seal. The inevitable movement of the floorboards can shift it out of place, creating an airtightness nightmare.

I realise it’s a tempting option: it is cheaper than wood fibre, very efficient with a low U value and easy to get hold of at short notice. However its production is a disaster for the environment, it off-gases (releases volatile organic compounds into the air for months after installation) and should the material catch fire, it will release higher levels of toxic gas than with other insulation material.

Benefits of insulating under the floor

If it was only possible to do two things to improve energy usage in the house, I’d insulate the loft and under the floor. A suspended floor over a sub floor void is marvellous for natural ventilation: cold air flows through the air bricks, doing a great job of keeping the foundations of the house dry and along the way massive amounts of air come up through the floorboards, ‘airing’ the whole house. All those times you’ve sat watching the TV with your freezing feet – this is why. Put a cosy duvet of insulation between you and those much needed air bricks –  you’ll be so much more comfortable and you’ll save considerable amounts on your energy bill.

Judith will be showing people around the house for the local Sustainability Festival in June, so if you’re in the area and interested in learning more about retrofit and insulation, take a look. 

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