Insulating Suspended Timber Floors

[Ross85]

Hi,

I'm in the process of renovating my first house, a mid-terrace built around 1890. My original plan was to fit wet, underfloor heating but the damp surveyor didn't seem keen on it. He also doesn't seem keen on me insulating the floor, even if fairly breathable materials are used and the recess is maintained below the joists via the use of netting or similar to prevent it entering the ventilation space below the floor.

Does anyone have some experience with this matter and could advise? I'd like to have exposed wooden floors, but I don't necessarily want the cold floors associated with that. Any help is greatly appreciated, I'm sure I'll have many more questions to ask over the years, this forum seems very informative!

 

[Zebra]

I haven't done it but I know it's been discussed on here -insulating a timber suspended floor with rockwool suspended in netting, I mean. I'm afraid I can't recall the name of the thread. Was it Middi advocating that? Can anyone remember?

 

[MikeG]

I'd really strongly suggest you don't fit underfloor heating with a suspended timber floor. I have physically done it myself twice (slow learner!), and seen it on projects I've been involved with (I'm an architect), and it has never been successful. Underfloor heating is great when there are no voids, and the pipes are in complete contact with the floor, such as when set in screed. Under timber, there is only a small point of contact with the floor, and the pipe is at least partially surrounded by air, which is an insulant. This means the heat doesn't all get transferred by conduction, which is the way it is supposed to work. Most, if not all the jobs I described above had the UFH ripped out after a year or two, despite numerous efforts to bring the performance up to an acceptable level. In one of them, we even pugged between the joists with kiln-dried sand, which was an absolute workmanship nightmare, and it was the only one of the jobs which ended up with a semi-reasonable performance from the system. They still ripped it out.

 

[88v8]

If you allow air circulation between the boards and the insulation, the insulation is largely pointless.
If you don't, in the presence of damp the floor may rot.

Underfloor heating beneath wood suspended or not imho is barmy. Wood is a good insulator. But unless totally totally dry, it will shrink. So your original floor would end up eating fuel, not warm, and gappy. If you want period heating, cast-iron rads.

We hung rockwool under one room of our previous house, but that a new extension and not damp.

Bare wood floors in a 1890 house are not very 'period' unless one is doing a poorhouse emulation.

Ivor

 

[Ross85]

Thanks for the replies.

The articles I've read recommend a damp proof membrane below the floorboards, which would ensure that air can circulate below the floor but the draft could not penetrate into the house. I appreciate that this will stop ventilation into the the house, so perhaps the addition of some ventilation above the floor would be required? Perhaps another option would be to remove the suspended floors and fill the area with a relevant material? Then wet, underfloor heating could be added and an engineered wooden floor on top of that? The kitchen, which is the ground floor of an original double extension at the back has already been retro-fitted with a concrete floor, which I plan on removing for various reasons, the most obvious being that it's of poor construction. Presumably the extension floor would have also been suspended timber originally? I wonder if the addition of the concrete floor in the kitchen is preventing airflow in the hallway area, see the floorplan attached to make sense of what I'm suggesting.

 

[MikeG]

.......We hung rockwool under one room of our previous house, but that a new extension and not damp........

I would always advocate insulating under a suspended ground floor. Always. Damp isn't an issue with respect to mineral wool insulation. The key factor is getting air circulating below the insulation, which may require remedial work to airbricks, or even inserting some for the first time. A damp suspended floor is the result of poor ventilation, and nothing to do with insulation. I'd be more cautious with the PIR board insulants, such as Celotex, because they form a fairly effective vapour barrier. Not a problem if there is never a spillage in the house, but if there is a kitchen, utility or bathroom involved then I would avoid these, and stick with mineral wool.

 

[MatthewC]

My original plan was to fit wet, underfloor heating but the damp surveyor didn't seem keen on it.

That's a slightly worrying professional title. Is he actually a dpc salesman, or is he really someone who knows about damp in old buildings? I do hope it's the latter!

The articles I've read recommend a damp proof membrane below the floorboards, which would ensure that air can circulate below the floor but the draft could not penetrate into the house. I appreciate that this will stop ventilation into the the house, so perhaps the addition of some ventilation above the floor would be required?

Don't be afraid of ventilation making the place cold - if it's damp then you need more ventilation, not less, and if you have a real fire then you will need the ventilation. My lounge only had one vent under the floor, at the front at ground level; I had a tradesman wanting to fit another (more unsightly) vent in the same wall above floor level, but I made him fit one in the floor on the other side of the room, thus ventilating the floor space as well.

Perhaps another option would be to remove the suspended floors and fill the area with a relevant material? Then wet, underfloor heating could be added and an engineered wooden floor on top of that? The kitchen, which is the ground floor of an original double extension at the back has already been retro-fitted with a concrete floor, which I plan on removing for various reasons, the most obvious being that it's of poor construction. Presumably the extension floor would have also been suspended timber originally? I wonder if the addition of the concrete floor in the kitchen is preventing airflow in the hallway area, see the floorplan attached to make sense of what I'm suggesting.

If the extension was always a kitchen or scullery, I would expect that it was originally tiled, possibly with bare earth underneath it. Where you have a suspended floor you need at least two good vents. The Victorians were good at lots of things but didn't understand ventilation - they seemed to think that one small gap would ventilate a huge room adequately, but the result in my local church is a lot of damp, rotting wood under the 1862 pews.

NB No floor plan seen!

 

[plasticpigeon]

.......We hung rockwool under one room of our previous house, but that a new extension and not damp........

I would always advocate insulating under a suspended ground floor. Always. Damp isn't an issue with respect to mineral wool insulation. The key factor is getting air circulating below the insulation, which may require remedial work to airbricks, or even inserting some for the first time. A damp suspended floor is the result of poor ventilation, and nothing to do with insulation. I'd be more cautious with the PIR board insulants, such as Celotex, because they form a fairly effective vapour barrier. Not a problem if there is never a spillage in the house, but if there is a kitchen, utility or bathroom involved then I would avoid these, and stick with mineral wool.

I don't disagree that it is good to insulate inder a suspended floor, but in my house, a terrace from circa 1900, i really think it isn't possible. The floor joists are only 3" square and these rest on solid sleeper walls at around 1 yd intervals. The sleeper walls are topped with 3" x 3" wall plates (is this the right term) that the joists are nailed to. This means that there is only really a 3" gap ie the depth of the joists that air can flow through. Also the crawl space is only around 1ft, so it is impossible to get down there. I would love to insulate, but I really can't, not even if I lift all of the boards, which would probably mean junking the floor. As for bare floors, I believe my house would have had bare floor boards, but with a big rug in the middle. I think this because there is dark stain around the edge of the room, and also in various books I have, it is suggested that by late Victorian/Edwardian times, people were getting concerned about hygiene so this arrangement was in vogue to enable the floor to be swept and the rug to be beaten outside which carpets didn't allow for.

 

[MikeG]

If you can't insulate and still allow air circulation below, then don't do it. The general principal of voids in a building is that they should either be fully filled, or they should be ventilated. In the case you describe, I wouldn't want insulation in contact with the soil below the floor, so the fully-filled option is out, and it sounds as though air circulation would be compromised by any insulation you fitted. Sounds like you're left with little choice.