Insulating under floorboards, Piglet John?

[TKKW]

Thinking of insulating under our floorboards in out 1930s home. The air bricks are not blocked. Under the floorboards are about a foot and a half, suspended above the soil, bricks, foundations etc.

We have been advised not rock wool/wire to hold as over time this bunches up and doesnt wick away moisture if air gets trapped/ cools/warms up.

We have been advised to install kingspan or celotex between the joists which would be held in place by dozens of little bars between the joists.

My DH is mad for the idea but Im not so sure. We have had a lot of renovating mishaps (poor workmen/ changing our minds on things installed etc) so I really dont want another mistake.

Basically, Im worried about mould or rot or condensation forming.

Has anyone used kingspan or celotex?

We've been quoted about £1000 for materials, labour and sanding boards leaving us ready to oil or varnish.

Thanks.

The quote is for 3 rooms and 30- 32 ish metres squared.

Watching with interest.

I would probably seal the gaps in your floorboards instead. Over insulation could well result in condensation and mould. We shoved rockwool in one end of our living room and it didn't work nearly as well as sticking stopgap between the gaps in our floorboards. I do sympathise though, our living room was freezing and so draughty.

But just a single layer of planks between you and the cold air coming in through the airbricks is hardly 'good' on the insulation stakes MtM

We found that what made it cold was the draughts, not the lack of insulation. Stopgap made an instant difference to how warm our living room felt (we didn't bother with the hall and the kitchen is tiled anyway). Our boards feel quite warm underfoot actually, but you can really tell where the draughts are (our patio doors were very badly fitted and it felt like a stream of cold air in the winter). Are they proposing to seal the gaps between your floorboards? If they're anything like our thirties boards, they'll be quite wide in places. I've been warned off making our house too airtight as it will likely develop damp and condensation as they just weren't built to be non-draughty. This could all be hearsay of course!

Also watching with interest. Stopgap has improved things but is not magical. .

Thanks for replies.

Mummy to mog, Ive just looked up stopgap. That looks good. Will talk to DH about that.

Its about 10cm thick and our space under floorboards is about 45cm.

No, not sealing gaps, laying this board stuff under the floor between the joists which hold the boards up by little bars screwed between the joists.

Taking up all the floorboards and fitting new floor boards where a previous owner took out a fireplace and left us with a big concrete gap.

Mummytomog, who told you about making your house too airtight?

I've heard the same about airflow. Modern houses are too insulted and condensation results (apparently?)

We put hardboard on our floor under the carpet - noticeable improvement in the temperature

I think it was the chimney sweep when I was bitching about having to put yet another air vent in the floor (because of the size of the stove). We had an ex-local authority flat with shite double glazing for years and it was riddled with damp on the first floor. Consequently OH is obsessed with opening fucking windows all the time even though it is clearly not needed in this house. In the end I locked the bathroom windows and 'lost' the key.

I didn't understand
"We have been advised not rock wool/wire to hold as over time this bunches up and doesnt wick away moisture if air gets trapped/ cools/warms up.

We have been advised to install kingspan or celotex between the joists which would be held in place by dozens of little bars between the joists."

Mineral wool is somewhat porous, unlike Celotex and Kingspan which are airtight, so I don't believe the story that rockwool would cause trapped air and moisture and the foams wouldn't. Interstital condensation can occur, but it won't under a floor.

Many floors are covered with vinyl or lino so do not permit passage of air or miosture through them anyway.

In fact, the subfloor should be well-ventilated with clear airbricks on all sides and an airflow to take away moist air (moisture will evaporate up from the concrete or earth beneath the house, unless it is relatively modern and the groundslab incorporates a DPM)

the installers may have some other reason for preferring the foam slabs, for example they may find it easier, quicker and cleaner to handle. The material is more expensive. Whichever you use would normally be fitted between the joists. the wool is not rigid so is easier to stuff into irregular gaps and fill them. the foam slabs are better insulators for the same thickness, but losses through floors are relatively low so either would do. Mineral woold would be better for blocking draughts as it can be stuffed tight against the walls and joists. the foam slabs would need expanding foam to block cracks and gaps and stop draughts, for example under the skirting board.

While the floor is up, take the opportunity to clear away all underfloor rubbish, dead rats, sacks of sovereigns etc, and to clean out the airbricks and honeycombs. Do any plumbing or wiring. Insulate any pipes. It is all very easy when the floors are up.

We used a kingspan (I think) insulation board type things. We nailed them up on to the joists rather than cutting them to size to fit between joists. Bought them online for about £250 and fitted ourselves. We're not diyers but it was easy with 2 of you and ours is a large double sided cellar. Yes it makes a big difference but we're now looking at installing 2 pvc double glazed windows to close of the coal shafts/vents to block out draughts completely in winter.

Thanks Piglet and AngieM2.

I can just post quickly on here so right now, so:

Piglet, do you know if the boards could cause condensation or mould?
We have cavity wall and we noticed some white mildew on the clothes stored at the very top of the built in wardrobe but not in the lower part where we hang clothes and take in/out daily or weekly. Would the floor insulation do anything like that?

Angie, have you noticed anything like more condesation ?

cellars need a lot of ventilation (AKA draughts) to keep them dry. Block it and you can expect damp and mould.

It is OK to block draughts into the living rooms above.

mould at the top of cupboards suggest that water vapour has risen up (it is lighter than air, hence clouds) in the room and been unable to escape.

There is a faint chance of water penetration from leaking roof or gutter, but more likely humid air. It is very common in bathrooms and bedroms, also in lofts if they have incorrectly been sealed and draughtproofed. Increased ventilation is the cure.

We had mould on walls behind a cupboard, was directly above the kitchen and we assumed it was cooking vapour. It's so tricky to know what's causing damp/mould problems. But checking your gutters is always a good start.

tkkw no mould and it was celotex we used. However, we haven't blocked off the big coal thingies yet (they are like big windows at the front). Not sure now whether to go ahead with putting the pvc windows in....they will be down in the cellar, not visible from the outside of the house, and will tilt open so we can have them open all summer and even open a bit in winter....what do you think piglet?...or would we be best to just put some wood panels against the windows for winter to stop draughts?

cellars need to be draughty to blow the water vapour out. It will be coming through the floor and walls.

If you do any panelling, it will be damp behind, so don't use wood. You mentioned damp corners, so existing ventilation is insufficient. Ventilation gets damp out, heat just makes it warm.

If you want to spend any money, consider digging out the floor. Provided you do not destabilise the wall foundations, you can probably increase headroom as well as laying a better concrete floor with dpm and possibly even insulation. Best to include a gutter round the edges so that if you do line the walls later, any water can be collected and pumped out. Modern approach is that you will never stop damp getting into a cellar, so you have to arrange to take it out. You can get special floor tiles with a drainage gap underneath.

I'm getting confused between two similar threads.

Hi again, Im confused by piglet john. Oh yes, last post not meant for here?

I dont really know how else to explain the rock wool thing. Was just paraphrasing i suppose what I remembered.

So Piglet John, you dont think its a bad/ concerning idea so long as air bricks not blocked?

Thanks for all your replies peoples.

Oh yes, the wardrobe which had the mildew or white dotty areas on folded old clothed at the top of the built in cupboard is not above the kitchen and is not on the adjoining wall with the bathroom either. In fact it is not where near a drain either. Bit confusing.

How would I air, leave the wardrobe doors open and window in room open?

as long as you have plenty of unblocked airbricks at both ends (one every two metres is enough), honeycombed sleeper walls, and a detectable cool breeze blowing under the floor, that will be sufficient ventilation unless you see signs of wet under the floor. I would be happy with mineral wool between the joists. Rigid foam is more often pinned to the underside of the joists as the gaps between them may be uneven and it is messy to cut or trim. I would still pack the edges of the room with wool to block draughts.

The mildew cupboard: ventilate it better, maybe trim the door top and bottom for airflow. Verify the wall is not damp from gutter, roof or plumbing leaks. Verify the loft insulation is correct over that part of the ceiling. Cupboards on external walls can be ventilated to the outside air if condensation is bad, by drilling a couple of holes. This is sometimes necessary in fitted wardrobes. you can also fit foamed insulation slabs tight to the wall but they must have no gaps.

Piglet John - we sometimes have damp on the ground under the floorboards - we're on clay and when its really wet there's no where for the water to go - we don't have a damp problem in the house as a result of this and it does go relatively quickly. We have good airbricks (its a 20's semi). This is why I've been worried about potential damp if I do insulate under the floorboards.

(I should say that when we got here we had a much worse problem, with water hitting the front wall of the house, off the drive, and then bubbling up under the house when it rained hard, but we've put in drains to divert the surface water from the drive into the drainage system so things are a lot better than they were....)

insulating the floor will not cause water under the floor, any more than a carpet or vinyl would. the ventilation to dry it out is almost exclusively through the airbricks.

drains, and sloping the drive or paths away from the house, will carry away rainwater, provided you do not have a high water table which is more work to address, and may need a trench round the house filled with large clean stones.

Any overspilling gutters or downpipes, or cracked drains (which is very common on older houses having salt-glazed clay underground pipes, which crack, on clay soil, which moves, and especially on houses that were shaken during the 1939/45 Unpleasantness) will put a lot of water round the house whuich will often go under the floor.

We do have a high water table... and if you dig a hole to be a 'soakaway' it simply fills up with water... When we first moved here we augered a hole to see if there was a gravel layer that we might be able to dig a soakaway down to - there wasn't (we went about 10m which is quite a signfiicant distance to hand auger!). We've done something like a french drain with a perforated tube at the bottom in front of some of the house and that does seem to have helped a lot - combined with the guttering that takes surface water from the drive off into that area.