Insulating a suspended floor

[kkliskey]

Hi everyone. We are in the middle of decorating our ground floor living/dining room in a victorian house. What was meant to take a month or two has now taken us 3 months and we are still no where near finished We have got to the stage where we are working on the floor. The carpet was ripped up a while ago with the intention of having exposed floor boards. The majority of the floor boards are sound and those that aren't will be replaced with reclaimed boards which have already been bought and are aclimatising in the same room. We have read about incorporating insulation under the floor boards which we have decied we would like to do. Now this is where we don't really know what we are doing and my many questions start....
1. Is there an easy way of lifting the floor boards with out spliting the wood. This has happened to a couple of the boards that we have lifted and we don't want to have to replace the lot.
2. I have read that you can use loft insulation secured with nylon garden netting. Can this loft insulation be the space blanket type that is encased in foil and plastic sheets?
3. Does the netting run above the joists or does it need to be fixed to the underside of the joists. I've read that both are possible but not sure what standard practice is.
4. We have a number of pipes (gas and water) and electrics running under the floor boards. Most are level with the bottom of the joist but some are higher up. I worry that if the insulation is in contact with these pipes/cables it might cause a fire hazard.

I think that's it for now but I'm sure I can think of more later :? After I ask myself all these questions and try and work out the answers, I wonder if its all worth it. Will the insulation make that much difference?

Looking forward to any advice. Many thanks, Karolina

 

[MdB]

1. I can't :-( but I was only lifting modern T&G which was going to be replaced anyway so I didn't take the greatest care.

2. I can't see any reason why not.

3. Don't know.

4. I can't see gas or water pipes as being a problem. Electrics is a problem. The maximum current a wire can take depends on how well it can get rid of the heat that is generated as current flows through it. This means that wires going through insulation have to be thicker. Wires on top of or beneath insulation is less of a problem as the heat can escape to a certain extent. Either way, you need somebody who is Part P qualified to work out what is acceptable and what is not. (A wire going to a light fitting is probably fine. A wire going to an electric shower probably isn't.)

 

[kkliskey]

Thanks MdB, I didn't realise we would need an electrician to look at it. If we re-routed the cables so that were tacked to the underside of the joists, would this be acceptable and solve the problem?

 

[another_richard]

On number 1 no it is a nightmare. Jemmy bar, old screwdrivers. <mental idea>If the floor is low enough can you remove a few boards and climb under?</mental idea> I have heard tell of a special electric saw that cuts joints over the joists. Might not be necessary if you read my number 3.

On number 2 I was supplied with and used fibreglass

On number 3 I screwed in screws about 3/4 way down the joists and ran fishing wire across then pressed in the insulation.

On number 4 we recently had a spark in and he said it is fine about the wires and insulation (in another job I was doing) BUT I would get a qualified opinion

 

[MdB]

Thanks MdB, I didn't realise we would need an electrician to look at it. If we re-routed the cables so that were tacked to the underside of the joists, would this be acceptable and solve the problem?

Very likely yes but it is difficult for anyone to give an authoritative answer without actually being there to have a look. Tacking to beams is a good way to conduct heat away apparently (wood isn't a good conductor of heat but it is a lot better than insulation).

To give you an idea of the issues involved, 6kW (two bar fires) with cables clipped to a solid wall can be supplied by 2.5mm^2 cable. For the same load inside insulation requires 10mm^2 cable.

 

[Andrew Bodenham]

Insulating your suspended floor is certainly a good idea if you can manage it.

Lifting boards without damage, especially if they are tongue and groove, is very difficult. Nails are usually rusted into the joists, and will be very reluctant to come out. Levering one board up is OK, but getting it's neighbours without splitting off the tongues is more of a challenge. Long hardwood wedges hammered in along between the board and the top of the joist is one approach, once you've got the first 2 or 3 boards up. Another way is to use a heavy hammer and punch to drive the nails right down through the boards and into the joists, so releasing the boards. As you are looking at a ground floor room this may be possible - if you try this upstairs the vibration can damage the ceilings below. In any case you need to identify the position of any cables of pipes which might be in the firing line.

Depending how much of a void there is below the floor you may be able to work from below without removing the boards (except for an access hatch, of course).

As for materials, you can certainly use rockwool or glassfibre laid between the joists supported on netting or wires, but a quicker, cleaner and (in my opinion) easier method is to use rigid polyurethane foam sheet (Kingspan, Celotex ). This can be cut to width (saw, breadknife) so as to be a wedging fit between the joists. Screws let into the joists every 150mm or so to support it if you've not quite got a tight fit. I've used 50mm thick insulation board, which is thin enough to be clear of any pipes / cables etc running through the joists.

The alternative to insulating below is to fit a nice thick carpet with nice thick underlay. Quiet, too.

 

[kkliskey]

Thanks Andrew,

Our boards are straight edge so at least we don't have the tongue and groove problem. I will have a look at the rigid insulation - this might be easier to install than the space blanket. When you install the insulation should there be a pocket of air between the bottom of the board and the insulation or should the insulation be directly below the floor board. Finally, where can you buy Kingspan or celotex from? Builder merchants or specialist suppliers?

Thanks again,

Karolina

 

[AMc]

I would also go for insulation boards as they are thickness for thickness more efficient.
Ours is being fitted on top of tile batterns attached to the joists the right depth below.
I've heard good reports of this company who supply seconds which is a cost effective way to do the job - no personal involvement on my part mind! This link also has details on fixing though it is aimed at a new build
http://www.secondsandco.co.uk/page_show/read_solidground/index.html

 

[MdB]

I saw yesterday that Wickes do sprung polystyrene for going between joists. It is basically slab of poly with many slots cut into it to allow it to compress a bit to give a tight joint. Sounds like a terrible idea as you would lose a lot of the thermal performance and poly is highly flammable. Thought I would mention it as it was something I had not seen before.

Re lifting boards: Penners (I think) and others have mentioned before the trick of screwing another plank to the top of the board you are trying to lift to increase leverage.

 

[Feltwell]

*IF* you can get underneath the floor and there is enough space to crawl about, I would say that is the easiest way, although it's not a pleasant job. I used Rockwool this way and found it had enough "grip" when pushed up between the joists to hold itself in place, then once complete I tacked chicken wire to the bottom of the joists with a staple gun to hold it in place permanently.

There's no right or wrong way - Celotex or Kingspan rigid boards will do the job just as well but requires cutting to exactly the right width, Rockwool is more forgiving of wonky joists / poor cutting!

Seconds and Co are a good supplier if you go down the rigid board route, otherwise for fibreglass or similar most of the DIY sheds have offers on now - my local B&Q had 2 for 1 at the weekend.

You could be very green and go for sheep's wool insulation, if you can stand the price - environmentally supposedly the best and it's also nice stuff to work with, unlike fibreglass which is evil stuff! B&Q had an offer on "Thermafleece" brand recently, the offer may still be on now.

 

[Penners]

The alternative to insulating below is to fit a nice thick carpet with nice thick underlay. Quiet, too.

Hear, hear!

I'm sorry to sound negative about your plans, but have you considered the amount of cost and fabric damage that your desire for bare floorboards is potentially creating? You could end up ruining a Victorian floor, having to re-route electric cables and inserting extensive amounts of insulation. And unless you get that insulation really effectively butted up against all the joists, you'll probably still have draughts coming up between the boards.

Even if you achieve all of the above, you're then faced with the problem (often aired on this forum) of getting your original boards and your bought-in boards to match in colour, and of sanding them down and finishing them.

Isn't a good quality underlay and fitted carpet starting to sound very attractive, and very cheap, by comparison?

Your house, your money, your choice. But it bears thinking about!



MdB - thanks for the credit, but actually I seem to remember that was Lime's idea.

 

[Lime]

I'm sorry to sound negative about your plans, but have you considered the amount of cost and fabric damage that your desire for bare floorboards is potentially creating? You could end up ruining a Victorian floor, having to re-route electric cables and inserting extensive amounts of insulation. And unless you get that insulation really effectively butted up against all the joists, you'll probably still have draughts coming up between the boards.

Well, yes, quite.
Unless the floor is in such a bad state I wouldn't do it.
Besides, once the boards are up the joists may need re-seating (as the floorboards are often the only things holding them vertical and separate) .... the underfloor need clearing and cleaning .... the vents cleaning or replacing .... the dwarf walls rebuilding ... the DPC cleaning and/or repairing ... the fireplace footing rebuilding ... The services replacing and rehanging (after all you don't want to be taking up the floor again in a hurry ...etc.

Most floors are improved by good carpets and underfelt to stop draughts.
If you ever have reason to remove the skirting-board then sealing with a mastic material between the wall and the floorboard edges can make a huge difference too.

Added.
However if you are able to access the underside without removing most of the boards then installing rigid polyurethane foam sheet (as suggested by Andrew Bodenham) is a non-destructive way of reaching your goal.

 

[Nemesis]

Bare floors can be very noisy and make a room echo. Also, it's hard not it get it scratched with chair legs etc.

 

[Flyfisher]

Also, it's hard not it get it scratched with chair legs etc.

The patina of age, surely?

 

[Nemesis]

It's fine if that's what you want - not all do, so worth thinking about.

 

[kkliskey]

Isn't a good quality underlay and fitted carpet starting to sound very attractive, and very cheap, by comparison?

Yes it is starting to sound like a good idea. At the same time, I hate carpets in living rooms - they get dirty easily and I would much prefer to sweep a floor everyday then hoover it. In the same train of thought though, would it be an idea to lay a solid wooden/engineered wooden floor over the existing floor boards - I know this would increase the level of the floor but seeing as our skirting is currently not in place this wouldn't be a major issue. Would this still require insulation of the subfloor?

 

[Moo]

I would much prefer to sweep a floor everyday then hoover it.

Do you really mean every day?

As a fully paid-up member of WWUS* I'm really impressed.



* World Wide Union of Sluts**

** How to recognise a fellow member : He/she will tell you, as a newsworthy item, that he/she has just cleaned the hall floor/done the ironing/hoovered the stairs...

 

[Andrew Bodenham]

An overlay wood floor could be a very good compromise. You get a wood floor you want, but one which is draught free, and you have an opportunity to incorporate a thin layer of insulation. I've used solid oak overlay flooring (10mm thick) laid over 3mm neoprene underlay, secret nailed through the tongues, and the effect is very pleasing. With overlay flooring the board width is usually quite narrow, 75 - 100mm, so the look is more like an Edwardian strip floor than the wide boards of earlier periods. For some tastes engineered flooring is too regular and (dare I say) can look like laminate. But if you can live with the visual effect and a thickness gain of half an inch (which is about the same as a good carpet + underlay) then an overlay would be a lot easier than lifting and relaying your existing boards. And the original floor is still there for future owners to discover.

 

[Penners]

Would this still require insulation of the subfloor?

As Andrew has said, laying a new floor on top of the existing - assuming it's a well-engineered interlocking system of some kind - would eliminate the draught problem. And with a foam underlay would add a bit of insulation. But not nearly as much as a good carpet on top of a good underlay.

You pays yer money and takes yer choice.

 

[piper]

All this is interesting and relevant to something we're doing which is slightly different. I'm thinking of insulation under the floorbaords of an upstairs bedroom - trying to create a well-insulated "box" over the room below which may not be heated at the times we want the upstairs room to be warm. My idea has been to lift a few floorboards only, where they are loose anyway, and try to push and/or pull the yellow fibreglass stuff along under the other boards. We have quite a lot of the stuff left over from the extension build, and also quite a lot of offcuts of kingspan, which might be easier to use. Not sure yet how practical the idea is, until I try it. We may well decide to have a carpet there, though painted boards will be cheaper, and we tend to share your ideas of sweeping vs hoovering.
In your case, kkliskey, if you use the type of insulation rolls enclosed in plastic, would it be possible to tie string to one end of a roll and pull it along underneath, having only lifted two boards? Probably a daft idea.
Above the bedroom ceiling there is thermafleece plus old yellow stuff where the thermafleece money ran out.