Insulating suspended timber floor

[Blair27]

Hi there

Looking for some advice with the colder months coming...

We own an 1860s house with a suspended timber floor, with the floorboards exposed as our main underfoot. In the winter the house gets really cold and just doesn't keep any heat - never gets above 18C/19C with heating on near 24/7. The windows are single glazed and I'm aware that's an issue but the issue I think I can resolve relatively cheaply this year is to insulate under our floorboards to reduce drafts.

The ground floor joists are about 4" / 100mm and I can access the space underneath, there is probably about 1.25m between the ground and the underside of the floorboards. My plan is to buy 100mm loft roll (glass mineral wool rolls specifically), insert this between the joists and hold it up with chicken wire, stapled to the joists

What I'd like to know if anyone has similar experience of doing this? Was it worth it for you from a heat/cost perspective and how did you do it? I'm worried about the choice of insulation leading to damp problems but from everything I read online glass mineral is permeable / breathable and so with a lot of airflow on the underside this shouldn't be a problem? I've also read that 4" joists aren't long enough and I risk creating a bridge that will attract moisture, is that a risk?

Thanks in advance

 

[a twig]

Easiest option is a carpet with a decent underlay?

 

[Blair27]

Easiest option is a carpet with a decent underlay?

We want to keep the floorboards exposed - though one room will have a carpet

 

[my_name_is_slim_daisy]

Also in a similar situation with suspended floors, and didn’t get round to doing anything about it last winter, and probably not this winter either. I may just throw over some dust covers, tape the edges and put some rugs on top. I’m hoping third winters the charm.

I do wonder though, during summer months would this insulation layer trap heat? The heatwave was difficult enough with the bare floors

 

[IHateDamp]

I've thought about doing this.
Are you confident that your suspended floor is well ventilated and insulation won't block through flow ?
I think I heard glass wool may be more likely to hold damp from condensation than say rockwool as it's more hydrophilic ? Perhaps you could check it out.
I would prefer to use natural materials, but there's the cost.
If you've got gaps in your boards then filling them eg with slivers of wood, could go quite a long way

 

[paulc]

Easiest option is a carpet with a decent underlay?

I used 5mm wood fibre boards (sold as laminate underlay) topped off with a foam underlay and then carpet. Made a huge difference in that it stopped all those nasty cold draughts coming up between a few of the floorboards. Then had the old 1970s DG windows replaced which was another quantum leap forward. The ancient central heating system* went from being barely able to keep the frost at bay to keeping the place reasonably warm.

If you are wedded to the idea of exposed floorboards, it would be worth plugging any/all gaps between the boards. Take the skirting off and seal any gaps between the floor and walls. But getting skirting off may cause significant damage to both the walls and skirting..
If you have easy access to the void underneath and there are not any sleeper walls to get in the way, 4" (100mm) of fibreglass or mineral wool will help. Just be careful not to block air vents or disrupt airflow over any sleeper walls. But don't be too surprised if the work doesn't make much difference - Floors typically account for around 15% of the overall heat loss.

 

[Blair27]

I've thought about doing this.
Are you confident that your suspended floor is well ventilated and insulation won't block through flow ?
I think I heard glass wool may be more likely to hold damp from condensation than say rockwool as it's more hydrophilic ? Perhaps you could check it out.
I would prefer to use natural materials, but there's the cost.
If you've got gaps in your boards then filling them eg with slivers of wood, could go quite a long way

I have yes, like I said there is well over a metre between the ground/soil and the floor so I do think thats got to be plenty, there's also a lot of vents throughout and having been down there it seems they are all working.

My concerns are essentially two fold

1) Is glass mineral wool acceptable? I think my answer to that is yes as it is vapour permeable so any moisture at the underside of the floorboards would be able to escape through the insulation and then into the void to he washed away by the clear air.
2) Are 4" joists long enough or will they create a thermal bridge. Essentially with the house being warmer will cold air in the void reach the bottom of the now warmer joist and turn to moisture, risking rot. Or because there is plenty of airflow underneath will that condensation never really happen?

I do take your point and wonder if draft proofing the room first of all is a better alternative...

 

[fernicarry]

I did this a couple of winters ago and, yes, it did make a noticeable difference to comfort levels. If you search my threads you'll find various write-ups of the approach I used. The main advantage is the draft proofing for which I stapled a breathable membrane under the floorboards first. I've done that for most of the ground floor whilst a much smaller area has the insulation fitted.

I have much less crawl space than you so its a pretty arduous job but with over a meter you should be laughing. Well, coughing and scratching if you insist on using glass wool... That wouldn't be my first choice because its horrible stuff to handle and it does tend to hold onto liquid water even if it allows vapour through. My preferred material is flexible hemp batts which I get from a supplier in your city.

I have between 50 and 90mm of insulation which only part fills the depth of the joists but like you say if the ventilation in the solum is good then its unlikely to cause a problem. If your joist ends are fitted into the walls then it would be a good idea to fill any gaps around them with lime mortar to exclude air that might leak out from the habitable space. The ground floor joists here end just short of the main wall on sleeper walls and yours may be the same.