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Filling in a suspended timber floor

Philippa B-W

Hi there


I have an ex council house built in 1964. It has filled cavity walls (polystrene balls) and the front of the house has a solid concrete floor and the rear is suspended timber floor (presumably because of the site gradient) with a chimney and airbricks providing air flow (one central at the rear, two to the side by the existing chimney).

We are looking to improve the airtightness of our house whilst we remodel internally (we are moving the kitchen to the rear) and want to put in underfloor heating. I would like to do this in the most eco friendly way possible. We are removing the chimney and are looking to eventually externally insulate (lime parge coat, wood fibre board and lime render) so I suspect we will need to block up the airbricks? In blocking the airbricks I suspect a complete floor fill will be required?

We have been told to remove the floor joists completely and fill with hardcore, membrane (taped at the perimeter), yummy chunky celotex and then UFH with a concrete screed. 

I wondered if anyone has done anything similar or can suggest a why of doing this in an more eco friendly and natural way?

I will save the cavity insulation questions for another forum! 


Topic starter Posted : 24/03/2021 12:52 pm
Chris Brookman
Member Admin

Hi Philippa

As you're removing the chimney there should be no need for the air bricks to supply air to an internal heat source, so they can be removed. However, where you have a suspended floor you must provide cross ventilation to keep the underfloor void dry enough not to cause problems in the timber. Also, you should not have bare earth under a suspended floor. Even dry earth can release as much moisture as standing water and so bare earth should be sand-blinded and have a membrane placed over it.

In terms of making the house airtight, this is an important step to reducing heat loss but do remember you will need to add in controllable ventilation to bring fresh air in. This can be in the form of a passive ventilation system or a mechanical ventilation system, which can be as a centralised system or individual units in each room.

There is no need to fill in the floor if you can achieve the right levels of ventilation but if this is not possible then a solid floor could be installed instead. Rather than using a standard modern build-up, including PIR or other plastic insulation, you could use the following:-

This involves no concrete or other wet products and can be completely removed and recycled if required and also incorporates the underfloor heating that you wanted too.

Hope that helps.

Posted : 29/03/2021 6:32 pm

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