Hi, I realise I may be covering old ground here but forgive me please - I'm going to try and get this sorted before winter, it's been far too long (4 winters of cold draughts and too much gas burnt).

The back half of the house now has phenolic resin (kooltherm) insulation between the joists and wet UFH. No probs there, although the humidity is low I think this will be advantageous as I gradually "seal up" the house. It is a 1910 solid brick wall construction. The back half of the house is over a 400mm ventilated void.

So - to the front half of the house over a brick cellar. I have decided that I do not need/want UFH in this area but I will get a wood burner at some point in the future (maybe this year who knows). Until then it's just a big gas powered central heating radiator.

This consumes far too much fuel in winter because the well ventilated cellar is underneath and the cold air comes straight up between the boards and through the carpet. I would not reduce the cellar ventilation as it is already quite damp (not dripping) in there. I am thinking that in the future I could maybe pipe excess hot dry air (from above the wood burner) down there. And/or draw the air intake from the woodburner through the cellar.

But for now what I want to achieve is to make the floor insulated and airtight, BUT is it possible to do this and allow water vapour to pass through?

I think it is generally the case that water vapour leaves the living area, but in my case I think the cellar is much damper and will continue to be unless the walls are somehow waterproofed against seepage from the ground. The room above is quite dry because of the UFH in the adjacent openplan kitchen, and I think the addition of the woodburner will dry it out even more (in winter obviously).

Either way I think it would be good to let the water vapour dissapate and even out around the house.

So I would appreciate your thoughts on how best to achieve that - I will be fixing from underneath and thinking of maybe sheep's wool (no i do not want to be dragged into a discussion of sheep welfare), mineral wool or similar recycled product. And maybe a breather/vapour permeable membrane under that. Or just net/wire?.
Then probably plasterboard or just leave it for now and do something more eco later.

And finally - any pointers for good deals on insulation would be appreciated. About 20 sq metres. I have used "seconds and co" for my boards before but I think breathable would be advantageous this time, and I don't think the boards are.

Thanks for listening!
Roger

Hi Roger

A form a wool between the joists, underlined with a breather membrane is exactly what I did to my cellar.

My joists are 175mm and I had some 170mm Glasswool I used to top up the loft insulation, so I used that between the joists. I went for a breather underneath for a couple of reasons, vapour permeability, some level of airtightness and also too keep the fibres away. Installing the wool above my eyes was a nightmare so using a net and allowing more fibres to fall was something I wanted to avoid!!

Also picked a breather membrane with a white underside called Web UV10 so that it gave a lighter apprearance down there. All the walls are now white too and it looks much better.

The room above the cellar still takes a long time to warm up (because of the thick solid walls) but a wood burner would tackle that for you wouldn't it. What I have noticed though is that room (which has no radiator) keeps a much steadier temperature through the night and I no longer have to race through my cereal in the morning to make sure my milk doesn't freeze!

I think the DIY stores are by far the cheapest for wool type insulation. I think they get government rebates on traditional loft type insulation and so they undercut the merchants.

Posted By: WatchItHi Roger
A form a wool between the joists, underlined with a breather membrane is exactly what I did to my cellar.

My joists are 175mm and I had some 170mm Glasswool I used to top up the loft insulation, so I used that between the joists. I went for a breather underneath for a couple of reasons, vapour permeability, some level of airtightness and also too keep the fibres away. Installing the wool above my eyes was a nightmare so using a net and allowing more fibres to fall was something I wanted to avoid!!

Also picked a breather membrane with a white underside called Web UV10 so that it gave a lighter apprearance down there. All the walls are now white too and it looks much better.

Thanks that's interesting and glad you got a bit of comfort on winter mornings. I am reluctant to leave bed unless it's toasty! Not very productive. But more sleep is probably eco-friendly? Any excuse...

Did you find any difference to your cellar in terms of dampness? What was/is the ventilation like down there?

Posted By: betterroofYou can fill any cavities with some rokwool or equivalent. then a vcl over everything, sealed to the walls, then your plasterboard

Any Vapour Control Layer should go on the warm side of the insulation, i.e. above it. You can use a breather membrane below the insulation as suggested by WatchIt. This is essential with mineral/sheeps wool to avoid wind degrading the insulation's performance.

It makes sense to avoid doing anything to the structure which will make the cellar damp worse. However, the only reasons to make a floor breathable are to protect the timbers &/or provide a path for water vapour to leave the room. If the room is regularly heated & the cellar is well ventilated, then for most of the year the water vapour pressure will be from the room to the cellar. So a breathable floor can only increase the amount of damp in the cellar. That said, if the cellar is well ventilated it's unlikely to make any difference.

David

OK so am I right in thinking that a breather membrane is airtight, ie it will stop cold air infiltration?

OK so am I right in thinking that a breather membrane is airtight, ie it will stop cold air infiltration?

Yes, MOST of them are airtight, but the joints are not. However, David is right in that air movement next to a loose insulation like wool is detrimental to its performance, so a breather would help that (even if the joints aren't airtight).

Did you find any difference to your cellar in terms of dampness? What was/is the ventilation like down there?

My cellar is fairly dry. The walls were plastered and this was VERY damp, it was holding loads of water. I knocked it off, re-pointed and used some treacle like paint instead. Didn't exepect it to do as well as it has but it the walls are always dry now. The floor was already pretty good so I didn't touch this. We have a window in there which is left open 365 days, this really helps with the ventilation.

Posted By: squowseis the water vapur presuure a result of relative or absolute humidity? I know th RH is much less upstairs.

It's a result of absolute humidity. That's why in winter water vapour passes from the warm house (with low relative humidity) to the cold outdoors (which can be close to 100% relative humidity).

David

Can you try and get some sort of vapour control on the upper side? Otherwise you'd risk condensation in the wool due water vapour (in the higher absolute humidity air in the room above) condensing in the wool as it diffuses downwards?

This could be as simple as laying some polythene under the carpet.

You really want to try and avoid any air movement through the wool insulation, as both damp and air movement through it will (as has been previously said) seriously knacker its performance - this problem will mainly be sideways in your case, so I'd try to be careful about air and vapour sealing where the insulation batt ends meet the wall (e.g. brick, polythene, wool).

If your joists are going into these damp basement walls, I'd be worried about causing possible rot problems in the future with the treatment which you are proposing, as you will be limiting evaporation from the first exposed bit of the timber (i.e. the first bit that you can see where it's not buried in the wall), so you will likely be making the bit that actually sits in the wall damper. It sounds like you should try and water proof the walls if at all possible (either internally or externally). If you can't get the joist ends dry+warm, then maybe fix a timber plate below the existing joists to carry them (with a bit of polythene between the new plate and wall to make sure it doesn't suffer the same fate), and then cut the joist ends back out of the wall?

If you can't do that, then you could just leave the last 200mm of the joists uninsulated (so that your insulation stops 200mm from the walls, on the two walls where the joists are cut in).

Is there any ventilation in the basement i.e. is it possible to get a cross-flow of air outside->basement->outside?

Could you reduce thermal bridging to

e.g. page 5 of this?

http://www.aecb.net/PDFs/Carbonlite_cottage2.pdf