Conflicting advice over suspended timber floor - can anyone help.

3 Jun 2021
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United Kingdom
I have a ground floor , suspended timber floor with rockwool insulation between the joists. Do I need a DPM/vapour barrier between the insulation and the floorboards.

My setup is simple - hope this is enough information.

1) floorboards
2) 8" joists
3) rockwool between joists - held up with netting
4) concrete base
5) good ventilation from airbricks etc
6) ground floor
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if you have good airflow with no blocked air bricks, then it should stay dry down there.
Last edited:
No, you don't need a DPM at the position you have indicated. Building regulations for a suspended timber floor:

Suspended timber floor
As a requirement of the Building Regulations the structure should be protected against the growth of weeds and other plant-life. The ground should have a layer of concrete poured across and there should be a ventilated gap of at least 150mm between the underside of the timbers and the concrete, to prevent moisture gathering and affecting the condition of the joists. The timber floor joists should be sized correctly depending on their span (length between supports) and are normally laid across the shortest span from wall to wall with a gap underneath.

An intermediate wall with a small foundation may be needed to reduce the span and keep the thickness of the floor joists to a minimum. A damp proof course (DPC) should be placed between the timber and the wall. Insulation is then placed between the joists (thickness depends on the product used). Air vents should be placed underneath to provide ventilation to the void and the air should be able to travel from one side of the building to the other.
Thanks for this.. i should have used the term "Vapor Barrier" its condensation i am worried about on the warm side..

Do i need a vapour barrier??
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Thanks for this.. i should have used the term "Vapor Barrier" its condensation i am worried about on the warm side..

Do i need a vapour barrier??

A vapour control layer is fitted on the warm side to stop warm, moisture laden air travelling to the cold side and then forming condensation.

On the floor it can go under the floorboards or it can be laid on top of the floorboards - carpet and laminate underlays often include a VCL.

A VCL is correctly taped also acts to help air tightness in the living space.

If your void is well ventilated, you may not have a problem

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