flooring options

[MetManMark]

We need new flooring as a result of our dry rot problem We had initially hoped to go for oak floorboards with celotex underneath. I have now gone off that idea and will remove the celotex to maximise ventillation. Our thoughts are now turning to something flat (potentially 22mm moisture resistant chipboard + good underlay + thick carpets). Any thoughts on what to use for the floor - is there anything with some sort of insulation built in? I don't want to reduce the airflow under the floor but was thinking that there must be something more effective than chipboard?
Any thoughts / other considerations?
Thanks
Mark

 

[middi]

Presumably you have floor joists and wodden floors that have rotted, from what you describe. Also , presumably, you have void under the floor joists that is ventilated externally.
If you confirm that this is the case then there options you can use in order to keep insulated, warm and ventilated.
Thowing away celotex insulation in favour of chipboard and carpets doen not seem to be a good idea as at present. Please specify how the ground floor is constructed.

Middi

 

[MetManMark]

Hi Middi
Please see my other post "Not happy with Building Control advice Advice now needed!"
Essentially there is about a foot under the floorboards before the earth. External walls are brick and stone - lime rendered inside but cement rendered outside (to be replaced by lime when we have the time / pennies).
Hope that is all the info you need to comment further? I liked the idea of celotex in that it keeps the cold out but I am not so keen if it reduces ventilation.
Thanks
M

 

[middi]

Hi.
In nutshell. make absolutely sure that there is adequate ventilation ( free and unobstructed air movement in the the void below the joists)
In most cases, and in older buildings, the external air inlets below the floor joists are inadequate. If they are perforated bricks or cast iron grids the size of a brick these should be at approx. 2 metres apart on opposing sides along the house elevations.

Obviously the more ventilation you create the colder the internal floor is going to get. Short of making a solid and insulated floor ( if you have the option) the next best thing is to insulate correctly between the joists. This can be done by nailing 28x28mm p/treated roof battens along the lower opposing lips of the floor joists and rest celotex (or other) tight between the joists and resting on the battens, then seal the upper lip of the celotex against the floor joists with expander foam. ( in case there are gaps ) Stretch a breatheable membrane across the top of of the joists and finish the floor with whatever you fancy. You can use moisture resistant chipboard or proper floor boards.

Apart from the labour of a few extra air bricks to be installed the above is reletively easy and cheap to do. Also you do not disturb the floor height and should be able to install 100 mm thick insulation boards with ease
( if the floor joists are pine ( or softwood) it is a good idea to drown them in wood preserver once they are exposed)
Middi

 

[JoceAndChris]

Disagreeing with middi is a bit like a gnat arguing with a lion, but I'm going to throw in the pot the idea that insulation is in this case seems like a bad idea. I've read through your other threads, and whilst I take on board Matt's point that the dry rot must stem from a water source that you need to eliminate, at the same time if it were my house I'd want to maximise ventilation and throw another log on the fire.

We've got boards over earth with no insulation, with rugs on top ( no fitted carpets). Yes it's a bit cold Nov-Feb but we never have any problems with damp or condensation or moulds in those rooms. That's what I'd want to do in your case, lovely reclaimed boards, no insulation between, and thick rugs that leave around a foot exposed around the edge. I would definitely be arguing my corner with any misguided Building Inspector ( though I've no intention of ever involving a building inspector here)

That advice is probably unpopular and I'm sorry for that. Your story is just so sad, you must be infuriated.

 

[tycarregydwr]

Disagreeing with middi is a bit like a gnat arguing with a lion, but I'm going to throw in the pot the idea that insulation is in this case seems like a bad idea. I've read through your other threads, and whilst I take on board Matt's point that the dry rot must stem from a water source that you need to eliminate, at the same time if it were my house I'd want to maximise ventilation and throw another log on the fire.

We've got boards over earth with no insulation, with rugs on top ( no fitted carpets). Yes it's a bit cold Nov-Feb but we never have any problems with damp or condensation or moulds in those rooms. That's what I'd want to do in your case, lovely reclaimed boards, no insulation between, and thick rugs that leave around a foot exposed around the edge. I would definitely be arguing my corner with any misguided Building Inspector ( though I've no intention of ever involving a building inspector here)

That advice is probably unpopular and I'm sorry for that. Your story is just so sad, you must be infuriated.

I am a total newbie on this forum and the farthest thing from an expert, but we did exactly this in our previous house (Victorian mid-terrace). The front room had the fireplace bricked up, and fitted carpets, and double glazing, and unsurprisingly, condensation and mould galore. We reopened the fireplace and fitted a woodburner, ripped out the carpets and discovered nice 'shabby chic' pine floorboards that were resting on joists which thank goodness were fine, over a void of about 8-12 inches, over earth. The floor was very gappy - some gap between boards as much as 5mm. After much research and discussions we decided the leave the floor as it was. The very biggest gaps we blocked up but left the smaller ones, and then we put down thick rugs in the center of the room (leaving lots of space around the edges especially by the exterior wall). We worried it would be quite cold, but I'm being completely honest when I say that it was not a hugely noticeable difference in how it felt. Especially with a fire going it was more than toasty. And the condensation and mould problems were reduced almost to zero immediately.

 

[Flyfisher]

Just thinking aloud really, but is it really not possible to combine insulation with ventilation when it comes to suspended floors?

In the case in question it's stated that there is about 12 inches between the ground and the floor. This should allow for perfectly adequate ventilation. In such circumstances, would the addition of insulation between the floor joists really compromise the underfloor ventilation? I find that hard to imagine. And if the ventilation isn't compromised then surely there should be no damp problems arising just from the bare ground below the floor, unless there is also a stream running through the house or, perhaps more likely, a problem with downpipes/soakaways.

So if the insulation is tucked up only between the joists and the underfloor ventilation is not compromised, how could the damp and rot problems be exacerbated by the insulation?

I can see that sealing off gappy floorboards might affect underfloor ventilation, but surely only if the existing air bricks are inadequate for the task in the first place.

 

[MetManMark]

Thanks for all your advice. Having double checked the gap is about 9 inches under the floorboards so the celotex did really limit the ventilation in my book. Will let you know how we get on with our complaint.
Mark

 

[Flyfisher]

Did the celotex extend below the level of the floor joists? How deep are the joists? Six inch joists would still allow for a three inch gap between the ground and the bottom of the joists, which seems plenty for adequate ventilation, assuming adequate and functioning air bricks.

 

[Feltwell]

I agree with FF - I don't think the insulation is the cause of your problems, although it may have made the problem worse. If you have water ingress that is the first thing that needs to be solved. If the underfloor ventilation is inadequate, then I can see it's quite possible that ventilation though the gaps in the floorboards was helping. However the solution to this is not really to reinstate ventilation into the room, it's to ensure that the ventilation beneath the joists to the outside is adequate.

Plenty of us on here have insulated under our suspended wooden floors without causing any problems with rot. Mine are well insulated, but there is a veritable gale blowing under there on a windy day! In my case the problem was not a lack of air bricks, but that those that were there were blocked.

 

[middi]

Quite correct from Feltwell and F/fisher.

If you have " a gale" blowing below the floor joists with "tight" insulation in between these joists, there should be no cause for rot affecting the joists and floor.

Middi

 

[MdB]

My thoughts agree with Middi, Feltwell, Flyfisher too.

 

[JoceAndChris]

That's very interesting then and I stand corrected. Xx

At some point we should perhaps lift the old boards and insulate in that case.

Hope Mark can get his problem sorted.

 

[AMc]

We need new flooring as a result of our dry rot problem We had initially hoped to go for oak floorboards with celotex underneath. I have now gone off that idea and will remove the celotex to maximise ventillation. Our thoughts are now turning to something flat (potentially 22mm moisture resistant chipboard + good underlay + thick carpets). Any thoughts on what to use for the floor - is there anything with some sort of insulation built in? I don't want to reduce the airflow under the floor but was thinking that there must be something more effective than chipboard?
Any thoughts / other considerations?
Thanks
Mark

My 2p'th worth - a solid layer of chipboard across the joists will block all ventilation into the room as effectively as celotex (other insulation boards are available) and it will have no insulative effect.
Moisture resistant chipboard is going to create a (more or less) impermeable barrier between the joists and the room reducing ventilation from the sub floor into the room to nothing - if it's wet down there then your joists will stay wet and continue to rot under the chipboard and the first you'll know about it is when the floor gives way.

The insulation should have been installed so it doesn't extend below the level of the joists or block the airbricks if it wasn't it may have created a bridge to transmit water from the ground to the joists.
If the walls are very wet then I suppose the insulation might have helped water to move along the joists and made the damage worse than "just" the end of the joists themselves.

I think you need to work out where the moisture has come from and then work out how to deal with it.

I understand dry rot likes fresh water and that would be where I would start to look first but there are proper rot experts to give a more informed opinion.

 

[Flyfisher]

Moisture resistant chipboard is going to create a (more or less) impermeable barrier between the joists and the room reducing ventilation from the sub floor into the room to nothing - if it's wet down there then your joists will stay wet and continue to rot under the chipboard and the first you'll know about it is when the floor gives way.

I agree that a layer of chipboard will essentially eliminate ventilation through the floor but surely that's not what a suspended floor should be relying upon for effective ventilation? After all, a fitted carpet would also do much the same in terms of reducing through-floor ventilation.

Surely the most important aspect of a suspended floor is adequate underfloor ventilation, typically via airbricks? This should dispose of any airborne moisture below the floor and keep the timbers adequately dry. Underfloor damp will only build up if ventilation is inadequate - which could mean too few or blocked airbricks or an excessive source of water beyond 'normal' ground moisture.

I think you need to work out where the moisture has come from and then work out how to deal with it.

Agreed.

 

[plasticpigeon]

Are the boards really that bad that they need replacing? I'm sure that without the conditions for dry rot to exist it will not persist. It would be a shame to throw old boards away unless they are rotten to the point of being unsound. I have a feeling that although dry rot is very bad for woodwork, the best way to stop it is to get rid of conditions that it can live in, and that the general panic associated with it is a bit OTT. I hope this is not unsound advice, Matt Green is definitely the expert on this kind of thing.

 

[MetManMark]

All
Thanks very much for taking the time to post - much appreciated. Just a couple of points to clarify.... I am not suggesting that the celotex reduced the airflow into the room I am suggesting that adding 70mm celotex to a 9 inch (yes I know I have mixed my units!) would have reduced the ventilation and exacerbated the situation. The celotex was installed immediately under the boards and rested on the joists. I am committed to finding the source of the ingress - but having done so I am not very keen on then spending another $$$ for celotex that I have been advised may have contributed to the current problems. I am therefore wondering what options there are for me to ensure decent insulation with no reduction of the underfloor space.
Thanks
M

 

[Feltwell]

Mark, might help if you post up some pictures of the problem and the current insulation and how it is installed. In your last post you say the insulation is resting on the joists - it should be between the joists - possible that we're all getting the wrong idea of your problem.

 

[MdB]

Between the joists and the floorboards is one way of doing it though it would need some form of load spreader - like an additional floor. A photo / diagram would be useful though as I'm confused too.

 

[AMc]

I agree that a layer of chipboard will essentially eliminate ventilation through the floor but surely that's not what a suspended floor should be relying upon for effective ventilation? After all, a fitted carpet would also do much the same in terms of reducing through-floor ventilation.

Surely the most important aspect of a suspended floor is adequate underfloor ventilation, typically via airbricks? This should dispose of any airborne moisture below the floor and keep the timbers adequately dry. Underfloor damp will only build up if ventilation is inadequate - which could mean too few or blocked airbricks or an excessive source of water beyond 'normal' ground moisture.

I should say I completely agree with Flyfisher in my understanding of how a ventilated, suspended floor should work.

Some seem to think that wind whistling up from underneath and between the floorboards and out through chimneys and gaps around the windows is the best way to ventilate the subfloor.
Not an opinion I hold.
I have fitted insulation between the joists under the boards and then installed wet underfloor heating above it and now I have nice warm feet. I took care to improve the ventilation under the floor at the same time.

For the sake of clarity I wanted to illustrate the proposed floor solution would be the same or perhaps worse than the current failed solution in terms of sealing the subfloor and reducing ventilation. In either case this isn't IMHO the best way to keep the joists from rotting or to reduce your heating load (which is what the regs are concerned with).

Foam board insulation has to be installed with care to eliminate gaps which cause ventilation heat losses and to avoid bridging for moisture. This level of attention to detail can be lacking in some general builders as I discovered to my ongoing cost. They seem to think that there is some magic at work and a board of insulation loosely dropped into a gap will magically keep the house warm

Celotex make you register to see their technical documents but Kingspan have free PDFs that show the correct way to install their products.
Worth a look, the PDF is linked from this page - pages 2 and 3 relate to suspended floors albeit in cavity walls.
http://www.kingspaninsulation.co.uk/Products/Kooltherm/Kooltherm-K3-Floorboard/Overview.aspx

As far as the celotex reducing the airflow under the floor. If you have the same sized ventilation openings and less subfloor void volume, I would expect greater air flow through the remaining space not more as there should be less turbulent air flow and a smaller volume of air to move? I'm afraid my fluid dynamics isn't up to much but I would expect better flow over a nearly smooth underside to one with 9" joists at 12" intervals?