Anyone done this? Does it make a big difference?

I’ve got a 3ft void under the house that is cold and damp during the winter – if I insultated it would it improve the feel for the rest of the house?

We have a timber floor on the ground floor level of our house; in the winter you can feel the cold air through the gaps in the floor boards which makes those rooms very cold. Our landlord did insulate between the joists and there was a significant improvement. However, if you do insulate the floor make sure you maintain free air flow through the vents in the external wall as this air flow will prevent the timbers from rotting.

I haven’t done this. I expect it would make a difference.

The 3ft void should be ventilated and so insulating could create issues.

Perhaps you could cut foam/ploystyrene sheeting to fit between the joists, close up to the underside of the floorboarding, but I would speak to an expert.

Yeah, there are air bricks which allow air to circulate around the space.

I was thinking of getting fibre glass and fill the space between the joists up.

Heat rises.

@ funkymonkey: not sure how that answers the question.

TheFunkyMonkey – Member
Heat rises.

Which is why you can feel the cold air from the void underneath…

Celotex sheets/blocks are the preferred choice these days & have much better thermal properties than good old fibreglass. You do have to maintain the ventilation though or risk the timbers rotting.

a surveyer told of to use netting between the joists and fill with loft insulation.

Heat rises.

and as the heat rises what fills the void?

So, I onsidered doing this, as our living room is north facng and has a cellar beneath it.

However, we resorted instead for fitting carpet and very thick underlay. It seems to have warmed the room up enormously in the winter. No longer do we have to wear coats when watching TV..!

Things that put me off:

1. Ventilation. The void is there for a very good reason. I wasn’t sure we could maintain the ventilation, especialyl as the cellar is already a little damp.
2. Hassle. The cellar ceiling is lath and plastered, so I’d have had to ripped that off to insulate.

This is what it’s like under the floor…

Behind the far joist lining the wall is the air brick so I’d keep those uncovered to allow for the air to circulate.

Just about to get a new floor laid in a couple of rooms downstairs for this reason – we also get slugs popping up at night for a wander – they get up through the radiator pipes…

They’re laying ply on top of the existing floorboards, then new boards on top. Am leaving the void as is for air to circulate – I guess you could put celotex between the joists if it was really cold?

The floor above is hardboarded in one room and both room have underlay and carpet on them but the carpet gets quite cold.

Wondering if investing in double glazing might be a better option to reduce heat loss.

i’me re-furbing a house at the mo & we’ve put 150mm of pu foam between the joists but obviously kept the air bricks clear.

My point is that as heat rises, it causes convection air currents. For this to draw air up from below, the warm air has to escape somewhere above.
Draught proofing the floor is the cheapest and easiest option by far. It’s probably more effective than insulation too. Just use decorators caulk or silicone between all the gaps. Expanding foam for the larger ones.

Yep, just cut insulating foam (Kingspan, etc.) to fit in between the floor joists. If you cut them well they will just push into place with no extra securing required. Should make a difference to how the room feels.

As has already been said, there must be a decent airflow under the floor otherwise you’ll start getting problems with rot.

While you’re down there you had better have a look at the floor joists where they are sitting on the brick supports. In your pic it looks like they’ve been getting wet at some point which ain’t brilliant.
Ideally, rather than directly on the brick, they should be sitting on something that cannot hold water such as slate or have some sheet plastic forming a waterproof membrane between the supporting wall and the wooden joist.

sharkbait, i was just about to add that, there is most definatly a damp
mark going on there, i had to remove two downstairs floors in my house because of a similar issue, both the lounge and dining room joists were rotted out,

when i came to put new joists down i laid them on a membrane using roofing felt strips, then before we naled down the new oak floors, we laid
a plastic sheet across the joists then tounge and grooved the new oak floorboards into place with secret nailing, this solved the problem of drafts from those floors,

There is slate under them – I’ve checked. Its all dry down there above the damp course.

From looking at it it looks like certain joists were replaced about 50 yrs ago (house was built in the 1850s) and had slate put on them.

sharkbait, i was just about to add that, there is most definatly a damp
mark going on there, i had to remove two downstairs floors in my house because of a similar issue, both the lounge and dining room joists were rotted out,

Yes, anywhere the wood is resting on brick (so both ends and any intermediate supports) there should be a membrane installed. Those brick supports look like there were added later to stop the floor bouncing – the right hand one looks a bit wobbly!

We have the same issue.

Rather than insulating between the joists we are (very carefully) lifting the boards, putting down 20mm ThermoSafe 040 board and then relaying the whole floor.

Having to put some open and close vents for when we have the fire lit though.

I used kingspan solid insulation under my front room last year.

It worked a treat, as well as the kingspan (thermawall TW55) between the joists I over boarded as well.