Insulating under a house

[StepOne]

Hi all,

Another insulation question!

We moved house last June into a property built on a slope where the bedrooms are downstairs, below ground level at one side. We are experiencing our first winter there, and it definitely feels chillier than our last place, especially downstairs, and doubly-especially on the floors. It's most noticeable in the one bedroom we've decorated, where we put down laminate and which feels really cold to walk on even with socks on. The carpets in the other rooms will get replaced probably this year, probably with new carpets based on how cold the laminate is. (we did go for the slightly thicker, slightly pricier laminate underlay, but not sure how much difference that has made).

Below the bedrooms is a large basement, tall enough to stand up in, so we have good access to under the floors, and I was wondering if there would be anything to gain from insulating underneath.

Does anyone have a view on whether it would be worthwhile and what would be the best way to do it?

Thanks,
StepOne

[Slarti]

When I first moved in with the now Mrs S she lived in a ground floor flat in a divided up house and below the living room was an open to the elements cellar that the building owner used as a material store. When winter arrived we were freezing!

So remembering things my parents had done when I was a kid, we took up the landlord's fitted carpet and lined the floor with newspaper as thickly as we could, then put the carpet back. Then another layer of some heavy duty brown paper that I bought and finally a thick old carpet that I had in storage.

The difference was immediately noticeable as, although it was colder the next day, we were warmer.

I had already used cling-film to draft-proof the windows and sort of make double glazing.

So, I'd say yes, under floor insulation is a good thing.

Slarti

[dspp]

I have fitted underfloor insulation in the 1.5m high crawl space under my ground floor and it makes a huge difference. In the room above the floor is bare pine (the original, so about 120 years old and with plenty of gaps for wind to whistle through). Now the room is markedly warmer than my various neighbours who similarly have bare pine, but with no insulation and lots of draughts. And my heating bills are lower than theirs.

In my case I used glass wool insulation placed between the joists, with chicken wire to hold it up, and tyvek wrap to draught proof the whole thing. I left about 30cm clear at the edge of the building so that the side wall of the cellar (that is below ground level) gets properly ventilated. You could either do something similar, or use slab insulation (celotex etc). Remember the draught proofing is as important as the insulation. But take care not to leave any wood in cold/moist/stagnant air pockets or you will get rot.

regards, dspp

[Hariseldon58]

I would think insulation would be a very good idea but also think about airflow in the basement space.

My previous home was a townhouse on four levels with a garage on the lowest level. Whilst there was insulation between levels 1 and 2, the garage door was draughty and uninsulated. After a cold winter we improved draught insulation on the garage door and insulated the garage door, the combination certainly meant the garage was warmer and level 2 was also warmer.

[csearle]

Yes. Go for it. Insulation can be added between the floor joists. You will not be disappointed, especially if there are no air-paths between the cellar and the ground floor. C.

[PrincessB]

In my case I used glass wool insulation placed between the joists, with chicken wire to hold it up, and tyvek wrap to draught proof the whole thing.

Hi dspp,

I understand that rockwool and chicken wire is the most cost effective and DIYable option. I did the same in an old 50's house some years ago and while it's not a fun job, it makes a massive difference.

In the old days (BI - Before Internet) I just lightly packed the underfloor. No chicken wire or draught barrier.

Currently I'm in a Victorian cottage and I had to have the floors removed in order to drop pop in Celotex from above as there's no crawl space.

I left about 30cm clear at the edge of the building so that the side wall of the cellar (that is below ground level) gets properly ventilated. You could either do something similar, or use slab insulation (celotex etc). Remember the draught proofing is as important as the insulation. But take care not to leave any wood in cold/moist/stagnant air pockets or you will get rot.

We upgraded the underfloor ventilation using serveral periscopic vents. When we moved in there were four floor level vents, two of which were blocked so there was no real airflow. With massively increased airflow we ran the insulation blocks right up to the walls.

Well worth doing.

B.

[Wizard]

Insulating under the floor will make a big difference IMHO. I do not know how the calculations work, but on new builds underfloor insulation is required to meet the minimum thermal efficiency standards. A while back there was a shortage of rigid insulation board (fire at celotex factory IIRC) which meant the board we had specified was not available, switching to a less efficient board meant a significant upgrade would have been required every else there was insulation. My take from that is that the underfloor insulation is gives very good bang for the buck in terms of overall thermal efficiency.

I think you have three choices:
1. As stated above cavity batts supported between the joists, you can use chicken wire to support them, but depending on the centres of yout joists you may get enough support form nails of screws driven in to the joists, or stainer wires run across the joists. The one think I would avoid doing is 'overstuffing' the space between the joists as it will reduce the efficiency of the insulation.
2. Rigid insulation such as Celotex. Easy to cut and fit but more expensive to buy.
3. You can by 8x4 sheets of polystyrene from DIY sheds and use as per Celotex, harder to cut and clearly less efficient than Celotex, but generally a lot cheaper.

If you have deep joists I would be tempted to look at the relative costs of the options and if it were cheaper go with a thick layer of polystyrene. You will never get it up to the standard of a new build as the joists will be a thermal bridge, but any of the above three would make a huge difference IMHO.

Hope that helps.

[Slarti]

3. You can by 8x4 sheets of polystyrene from DIY sheds and use as per Celotex, harder to cut and clearly less efficient than Celotex, but generally a lot cheaper.

Polystyrene, fire risk?

Slarti

[sg31]

Insulating under the floor will make a big difference IMHO. I do not know how the calculations work, but on new builds underfloor insulation is required to meet the minimum thermal efficiency standards. A while back there was a shortage of rigid insulation board (fire at celotex factory IIRC) which meant the board we had specified was not available, switching to a less efficient board meant a significant upgrade would have been required every else there was insulation. My take from that is that the underfloor insulation is gives very good bang for the buck in terms of overall thermal efficiency.

I think you have three choices:
1. As stated above cavity batts supported between the joists, you can use chicken wire to support them, but depending on the centres of yout joists you may get enough support form nails of screws driven in to the joists, or stainer wires run across the joists. The one think I would avoid doing is 'overstuffing' the space between the joists as it will reduce the efficiency of the insulation.
2. Rigid insulation such as Celotex. Easy to cut and fit but more expensive to buy.
3. You can by 8x4 sheets of polystyrene from DIY sheds and use as per Celotex, harder to cut and clearly less efficient than Celotex, but generally a lot cheaper.

If you have deep joists I would be tempted to look at the relative costs of the options and if it were cheaper go with a thick layer of polystyrene. You will never get it up to the standard of a new build as the joists will be a thermal bridge, but any of the above three would make a huge difference IMHO.

Hope that helps.

I would also consider superquilt

https://www.lbsbmonline.co.uk/superquil ... -10m-rolls

I've not used it underfloor but it has a lot of advantages over Celotex, primarily because it is much less labour intensive to fit. It can be stapled to the joists. Personally I'd cross batten and overboard but it's not necessary. Because it is installed below the joists there's no thermal bridge effect from them.

It's very effective.

[StepOne]

I But take care not to leave any wood in cold/moist/stagnant air pockets or you will get rot.

Thanks for all the responses so far. Picking up on dspp's point about rot... if I use any kind of board type insulation, there is bound to be a gap between the insulation and the floorboards above. Is this an issue?

Thanks,
StepOne

[ClaudiusTheIdiot]

If warm air can get to a colder area there might be condensation, especially if it isn't ventilated. Hence the use of a vapour barrier on the warm side of the insulation. There's an example in the link below, and more that might help on the same site.

http://www.greenbuildingforum.co.uk/new ... e=1#Item_0