Insulating floor in a room with an open fire?

[leew2]

Not sure where best to put this, but I am thinking of re-decorating our front room. As part of this work I'd like to insulate the floor by taking up the floorboards, fitting pieces of kingspan between the joists, supported on screws screwed into the joists so that they stick out a little bit to support the bits of kingspan, then either put back the original floorboards or lay a chipboard floor or lay new floorboards.
However this room has an open fire and I have been told that air needs to be able to enter the room from the gaps in the floorboards for the fire to work correctly, is that true. If I was to replace the fireplace with a living flame gas fire would the same problem still exist? Would putting slot vents in the window frame help?

 

[stuart45]

You need air, but it doesn't have to come from under the floorboards.

 

[geraldthehamster]

Not sure where best to put this, but I am thinking of re-decorating our front room. As part of this work I'd like to insulate the floor by taking up the floorboards, fitting pieces of kingspan between the joists, supported on screws screwed into the joists so that they stick out a little bit to support the bits of kingspan, then either put back the original floorboards or lay a chipboard floor or lay new floorboards.
However this room has an open fire and I have been told that air needs to be able to enter the roof from the gaps in the floorboards for the fire to work correctly, is that true. If I was to replace the fireplace with a living flame gas fire would the same problem still exist? Would putting slot vents in the window frame help?

An open fire (or heating stove) needs ventilation so it car draw air, but not, by design, from gaps between floorboards. Tongue and groove floorboards aren't designed to have gaps. Depending on the heat output of your fire you, might need fixed ventilation by means of an air brick to the outside. Approved Document J of the building regs covers requirements for new installtions, and will give you an idea.

I did what you are proposing to do, with my floor. I have a suspended timber floor with a 9 inch void under the joists, which is ventilated by air bricks. I put 50mm Kingspan under the joists, and another 50mm between. Expanding foam is your friend here: I propped up the pieces that were under the joists on offcuts, and foamed round where they touched the joists. This glues and seals them into place. Then placed the top sheets between the joists, and foamed around those.

It's important to make sure that the air bricks still ventilate the space under the insulation, to avoid damp.

I also had a wood burning stove installed. Being over 5 kW output, there was a requirement for fixed ventilation. Rather than bang a hole through the wall, I ducted air from the ventilated under floor void up behind the new hearth, to a vent in the wall behind the stove. I'm not sure how much difference this makes, as I have plenty of adventitous ventilation from other rooms, and at least one draughty door to the outside.

Hope some of this helps.

Cheers
Richard