Insulating suspended timber floors

Now that we’re (almost) through another winter, we have had a number of calls from clients who felt the cold through their wooden floors and have asked us about insulating them.

The first thing to say is that underfloor insulation can make a dramatic difference to comfort levels, particularly if you have bare suspended timber floors.

The second thing is that draughts significantly affect the performance of the insulation.  This means that even if you install lots of insulation, if there are gaps which allow the cold air through, it won’t improve your comfort levels as much you might have hoped.  It’s therefore better to close the draughts and ensure that the work is as airtight as possible.  Our preferred method is to install celotex boards, cut to size to fit in between the joists, and to then tape up the junctions between the boards, the timbers and the walls.

This obviously requires all the floor boards and skirting boards to be removed. Whilst there are a number of other methods that do not require this level of disruption, they generally require the existence of a large crawl space underneath the floor. However even if a crawl space exists it can be very difficult to achieve satisfactory levels of airtightness. There are shortcuts, but nothing beats taking up the floors and ensuing that it’s done properly.

This means that the work is likely to be disruptive and expensive, particularly if the flooring is tongue and groove flooring because this needs to be taken up very carefully and replaced in exactly the same way as it was installed. Very often it might be impossible to take all the boards up undamaged. Sometimes these can be replaced with reclaimed boards, or you might think of it as a good opportunity to replace all your boards. The latter option increases the material cost but can reduce the cost of installation as less care is needed when taking the boards up.  This means that it’s unlikely to meet the golden rule for green deal purposes, but if your ground floor is cold and uncomfortable, this work will make the floor just as comfortable as your upper floors !

If you do decide to insulate under your suspended floor, one crucial issue to consider is ventilation! If the void under the floor is not sufficiently ventilated, installing an airtight layer between the floor and the ground will trap moisture and lead to damp and the potential decay of the timbers.