Top 3 things to consider when choosing retrofit underfloor insulation

Underfloor insulation is a key thing to consider to help make your home more comfortable and cheaper to run. There are some questions you need to answer about its suitability and long term performance :

  • Is it possible to install?

If you have a timber floor suspended above the ground with 600mm clearance everywhere and an entry point,  then yes. Some installers will work with only 400mm space. For homes on a slope with enough clearance at one end and not enough at the other, you could consider digging out the earth  to get enough room to work or just leave that area uninsulated. If you’re on a concrete slab on the ground then it’s not possible to add insulation. If you have steel floor framing, you’ll need to talk to your insulation provider about alternatives. If you have a lot of rubble or ducting and stored materials under the house making it even harder to move around, it is more difficult and expensive.

  •  Which material to use?

Options include polyester and glass wool batts, foil lined polystyrene boards, reflective films and products like Aircell, a foil lined air filled polymer product. It’s important to understand how these products work in order to make the best choice. The aim is to prevent the movement of heat across the floor structure so that in cold weather you keep the heat inside your home and in summer you prevent the heat coming in. Heat is transferred by radiation (eg the warmth you feel in front of a fire), convection (heat carried by moving air) and conduction (eg heat you feel through the ceramic mug of a hot drink).

Batts, whether they are made of polyester, wool or glass wool work by trapping air within them that can’t move. That means heat has a hard time moving through them. To be effective they must sit in contact with the underside of the floor and stay there for years and years. Our picture shows glass fibre batts being held in place (sort of) by chicken wire. It has sagged a lot meaning air can now move freely between the batt and the floor which also means there is very little insulation value.

Polystyrene boards also contain trapped air and being rigid, you can imagine that once they are attached under the floor they’ll stay there. However, rats and mice love to burrow in polystyrene, making a plastic pollution mess and leading to multiple holes in the boards which again leads to a failure of insulation value.

Foil products work by reflecting the radiant heat coming from the floor back upwards. To be effective they need to have an air gap between the foil and the floor boards and the foil surface must remain shiny to be reflective, so any build up of dust which is pretty much inevitable under a floor, makes them less and less effective over time.

  • Get quotes and recommendations.

We recommend Autex Green Stuff rolls for underfloor insulation. They’re made from recycled plastic, they come in widths to suit most homes, they are made to remain stiff and in place against the floor material for the life of the product and rats and mice hate it. They won’t make holes or burrows in it. It is soft, non irritating and easy for the installers to handle and it is effective! Avoid any product that will sag as you see in the picture and we believe you will be disappointed with foil lined products over time. Check our links page for recommended suppliers.

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