Under-floor insulation

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Freeranger
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Under-floor insulation

Post by Freeranger »

Hi - can anyone advise please? We have a wooden floor with quite a large ventilation area underneath, and I'm thinking of insulating with polystyrene sheets. If I stuff them between the joists, this leaves a cold bridge. If I hang them underneath (held by e.g. garden net) so that they butt up against each other and form a continuous raft below the joists, then will this cause a problem with ventilation for the wood? I think I'd have to put a vapour barrier under the floor boards when they're replaced, and just wonder of this would leave the joists in danger of rotting due to lack of air flow. Does anyone know how best to do this, please?
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Re: Under-floor insulation

Post by Totally Scrambled »

It sounds like you live in an oldish house with air bricks below floor level. If so then the floor was designed to breathe so that any moisture could evaporate and leave the joists and floorboards dry so I would think carefully about adding something that would block the flow.
Does your house have cavity walls or is it old enough to have solid brick walls?
Dom
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Freeranger
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Re: Under-floor insulation

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Hi Dom and thanks for responding

You're right - it's a house that dates back at least as far as 1841 (datestone that we think refers just to the porch). It's thick whinstone random rubble walls, and it has air bricks that go from above ground level through a tunnel type affair to a void below the floor which is about 75-100cm deep by eye, and quite damp. The joist ends are built into the walls, but what we can see is fine in all but one spot where there was a leaky downpipe outside. It is of a design that relies on air flow to keep walls dry in a damp environment.

It seems I'm right to be worried about air circulation, but do you think it would be OK to put insulation down between the joists? If not, do you have any other suggestions on how to insulate in a house like this?
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Re: Under-floor insulation

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We have an old house too although we have solid brick walls rather than stone.
We had to do a bit of work to the sitting room when we moved in as previous owners had no idea. The drive outside had been built up so that it was six inches higher than the floor, they had fitted double glazing without trickle vents, had fitted carpets and had wallpapered the walls.
All this meant that the walls and underfloor area couldn't breathe and so the plaster got damp and was only loosely held to the brick and needed redoing. The floorboards down one edge were rotten and two joists had gone so a new floor was needed. The drive needed taking down a foot so that the damp didn't travel through the wall at floor level.
With solid brick walls moisture travels slowly through them and putting modern paints or wallpaper over them means that eventually this moisture pushes off the paint or paper and weakens the plaster, so we use distemper which allows the moisture to move through it and evaporate naturally.
We just have a large rug on the floor where our feet are so that we don't get a draght up our trouser legs and it allows minimal airflow through the gaps between boards which keeps the underfloor dry.
I would think that insulating just between the joists would block any airflow and cause damp to build up but that is just my opinion based on living in an old house rather than an expert opinion. This was why in days of yore people had foot stools so that their feet were kept off the cold two or three inches immediately above the floor.
We don't notice that the sitting room is any colder than any other room in our house and find that generally the house remains cool in Summer and warm in winter with no noticable increase in heating costs over other houses we've lived in.
Solid external walls were designed to breathe so the more airways you block the damper they get and our experience is to use distemper on the inside of external walls, fit double glazing with trickle vents and don't completely cover the floorboards with carpet.
Hope that's some help.
Dom
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Re: Under-floor insulation

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I've just had a thought. I don't know if there is some way that insulation could be put under the floor with some sort of vent system fitted to allow some airflow or wheteher do this would negate any insulation.
It wouldn't hurt to see if there is such a system available and how much it might cost. I don't know if a builder would know or if it is a more specialist type of thing. The National Trust have a lot of old buildings and might have come across this problem and have found a solution, so it might be worth contacting them.
For us it would be a bit pricey as we'd have to rip up the floor to fit insulation and the fit new boards.
Dom
PS I found This from the Energy Saving Trust, which recomends a permiable insulation to fit under floors like yours which would allow the void to breathe. Reading between the lines it would require the floorboards to be taken up, if you cannont get underneath the floor, to fit it.
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Freeranger
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Re: Under-floor insulation

Post by Freeranger »

Thanks, Dom - I found that very helpful. I think I'm going to use mineral wool with the joists rather than foam or polystyrene now, because I think that might help with the breatheability. I couldn't find where semi-permeable membrane was mentioned, but in general I know it's wise to use some sort of vapour-control to stop the condensation problem where warm air meets cold. I'd thought polythene sheet, but again from your comments, maybe something semi-permeable would be better. I'll investigate further. Didn't know that about the footstools, but that's a good idea too.

I remember you posting pictures when you had your chimney coping stones done. Did you have any damp proofing done at the same time? Someone has told us we should take the chimney back to roof level, install a layer of polythene membrane then another under the coping stones. Both our chimneys are wet, and I wonder whether this is necessary. There's concrete cladding on one gable end and I think that might be part of its problem.

The roof is easier than the floor because it's old sarking boards and slate (Scottish roofs have slates directly onto thick planks), with joist ends bedding into lime, so that's naturally ventilated and inclined to keep itself dry. This type of wall acts like a dry stone wall, with moisture being free to get into the centre, since the stones tilt inwards rather than sitting square like dressed blocks or bricks, but then draining downwards rather than passing through the wall - theoretically at least. They tended to lay big flag type stone crossways near the bottom as a rundimentary damp course to channel moisture out and stop it rising as well. They also tended to lime plaster on the hard, though much of ours has been strapped with laths first. I think some of our problems in other rooms are to do with concrete floors laid directly on earth and butting up to the wall, so there's no gap to stop moisture travelling up the plasterwork. I understand the construction methods and logic (I think) but where I have trouble is understanding how to apply modern regulations and materials to this type of old building.

I've been very disappointed with the building control people who seem to have no understanding of or sympathy for old houses and methods. There's a note in the Scottish building regs (which have been drawn up in consultation with some major housebuilding companies) acknowledging the disagreement of specialists in traditional buildings but (and I paraphrase here) it's worth doing the wrong thing for the greater good of reducing carbon emissions. I think their logic is that if the inside of the house is moisture proof then air circulation around the middle will dry the structure, and it's based on the idea that warm air inside is more humid than cold air outside. In Scotland - and up in the hills near the water table and right by a burn, as we are - I suspect this is not true most of the time because it's fairly permanently cool and very humid (wet), so although there is air circulation, it rarely truly dries out. Sorry, now ranting and getting away from the point. }hairout{ Just hard to know what to do for the best.
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Re: Under-floor insulation

Post by Totally Scrambled »

I think you can get a breathable moisture barrier, the sort of stuff they put on the outside of a timber frame building before the wood cladding is put on.

Freeranger wrote:
I remember you posting pictures when you had your chimney coping stones done. Did you have any damp proofing done at the same time? Someone has told us we should take the chimney back to roof level, install a layer of polythene membrane then another under the coping stones. Both our chimneys are wet, and I wonder whether this is necessary. There's concrete cladding on one gable end and I think that might be part of its problem.


No, there is no sort of damp proofing in the chimney. The top of the stack is a type of cement, rounded so that water would run from the centre to the edges of the stack. The stacks themselves step down so that there are a couple of overhangs which helps the water drip away while not soaking into the brickwork too much.
Our chimnies are external so they stick outwards from the sides of the house which means they dry out better, especially if you light a fire, than if they were internal.

In fact our house has no damp course as it was built before they invented them. The ground was leveled and a thick layer of clinker laid down and packed. They then built brick spreaders which started out 8 bricks wide with the courses above reducing in width until they were wall thick at which point they took the walls upward.
Round here houses built a bit later had a layer of slate added a couple of courses up which served as an early type of damp course.
Dom
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silverback
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Re: Under-floor insulation

Post by silverback »

Hi FR, our house is roughly the same age as yours approx 1850, originally a Victorian two up two down, built with soft red brick with lime mortar, 9" solid walls with a slate damp course!. The front and back rooms had wooden joists and floorboards, which were part rotten, and damp rising up the front and side walls of the house!. I had the walls chemically damp proofed, stripped and dug out all the front and rear floors, put down a double heavy duty polythene membrane, laid a concrete floor with 3" polystyrene insulating sheets, then screeded over the top, this was about 25yrs ago, no problem since! )t' .
As you already know, most old houses, especially with soft red brick were designed to get wet and dry out, with varying weather conditions!. But modern things such as central heating and double glazing caused problems with restricting ventilation and air flow thus causing condensation, whereas before these houses had drafts and open fires that kept a good circulation of air and a mainly dry house!.
I would have no idea of the costs involved now, to put in solid floors with insulation, or if it's possible to do it yourselves!, because once it's done it's done!!..

>gl<
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Freeranger
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Re: Under-floor insulation

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I suppose that's the opposite approach, SB, and very practical - stop worrying about the effect of the condensation on the timber by just removing the timber. I had thought of that originally because we wanted to lay flags, but the cost would be mostly in the filling of such a big void with hardcore and then difficulty in compacting it enough. Money became more if an issue then, so this hanging method was a way of doing it on the cheap. Maybe its something we can do when we have a bit more cash. Did you lay your insulation above the concrete or below? We were told that we should lay boards between battens on top of the slab and then marine ply to support the weight of a stone floor, but if all the load is vertical and even, I can't see why you couldn't lay it like a raft on the surface of a solid floor with the stone over a layer of ply or even moisture-resist chipboard.
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Re: Under-floor insulation

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Dom, thanks for reading my rant - more despair than anything - and for telling me about your chimneys and house construction. I find it really interesting how things were done historically and in different areas to take account of materials and conditions. We had a superb builder in our last house who specialised in this kind of thing and was fascinating on different building techniques through history and between Scotland and England. They aren't all cowboys!! We can't afford him at the moment, though, so thanks for your advice.
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Re: Under-floor insulation

Post by silverback »

Hi FR, I laid the insulation under the concrete, just a thought!, have you considered putting down concrete joists!, whereas you can lay thermal block in between them, then cover with marine ply or the likes!..take a look below if it helps. )t'

http://www.homebuilding.co.uk/advice/ke ... ture-guide
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Freeranger
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Re: Under-floor insulation

Post by Freeranger »

That's an interesting idea, SB - I'll look into that. Thanks very much.
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