insulating under wooden floors

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Someone has suggested to us that we could insulate the floors of our Victorian house by lifting the floorboards, putting in battens and boards and laying rockwool over the top before relaying the floorboards. Big sections of floorboards are going to be lifted for plumbing and electric work so this wouldn't be a lot of extra work and wouldn't be too expensive.
It seems like a good idea - is there any reason not to?
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Anthony James wrote

Insulating ground floors is less worthwile than insulating roofs and walls. Although the U value of a floor can look good on paper, the actual heat loss depends upon the temperature differential, and in situations with normal radiators or warm air heating, the air temp at floor level can be much lower than at ceiling level due to convection and draughts. It's far more effective to insulate roofs and walls to a good standard before worrying too much about floors.
But overall it is a good idea. At least it will make the floor much less draughty, especially round the skirting boards, and it'll stop that dirt staining you get round the edges of light carpets. While you've got the floorboards up, don't forget to check for damp, rot and woodworm and any other defects with the plates and sleeper walls, and make sure the air bricks are clear.
BTW, it's much easier and cheaper to support the Rockwool on Netlon plastic garden netting fixed to the joists using a staple gun.
Peter
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Peter Taylor snipped-for-privacy@DELETETOMAILMEclara.co.uk typed:

, <snip>

Err well depends on the Victorian house, IMO unless you have Really good underfloor ventilation in old house's you could be letting your-self in for big problems later. There is no such thing as rising damp/wet rot,! Only a lack of adequate air flow.
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Mark wrote

This is true with or without any insulation, which is exactly why I recommended checking the air bricks. .

No such thing as rising damp? Wet rot doesn't exist? Then please could you tell me what you think is causing my floor timbers to be so wet all the time and damaged by weevils. Why is there a musty smell and what are these black strands all over the place?
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On Wed, 11 Feb 2004 10:02:38 -0000, "Peter Taylor"

Wet rot certainly exists as you describe. A common reason is that the DPC is bridged by the ground outside having been built up over the years, coupled with lack of ventilation.
.andy
To email, substitute .nospam with .gl
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Andy Hall wrote

and
strands
But if rising damp doesn't exist then why bother with DPC's?
Peter
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Peter Taylor wrote:

What are you doing living in my house?? :) In the place I've just bought I've had to spray for woodworm and weevils. Not too much damage thankfully. I'll be tackling the wet rot problem next. Polythene sheeting on the ground under the floor has been recommended.
-Duncan
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"Duncan Lees" <duncan-at-snsys-dot-com> wrote in message

you
time and

strands
Solve the rot problem and you'll solve the weevil problem - they are attracted to, eat, and thrive on wood that has been attacked by rot fungi. And voracious little critters they are as well.
Replace the affected joists with pressure treated stuff - I put DP membrane between the bearers and the new joists for good measure.
Also removed about 20 bags of rubble from under the floor and cleared the air bricks. That should help air flow...
-- Richard Sampson
email me at richard at olifant d-ot co do-t uk
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RichardS wrote:

The weevils should be in bug heaven now, along with the woodworm. If they were still active that is. Wrong time of year to be sure. The main thing was to get the treatment guarantee for when I sell the house, as any survey that actually looks under the carpets would spot the worm damage. Unlike mine.

Thankfully the few affected joists are still sound. Just the wood on top of the sleepers and floorboard are shot. The guy who came round to look at the house (Rent-a-kill Home Care, good service) suggested replacing the wood supporting the joists with simple plastic roof tiles. Anything like that as long as it's plastic and solid.

Yeah, I removed quite a bit of rotten wood and dirt that was sitting on the substrate. Looks like the previous "treatment" for the rot was to replace some of the rotten floorboard and drop the old wood under the floor. You've got to love some peoples idea of a job well done...
-Duncan
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If you reduce the area for the moisture to evaporate, you will make the walls even wetter.
You need a channel round the edge and a pipe to a drain, or a pump if below the drain level.
In a well designed sub floor void there will be sufficient area of masonry to evaporate any moisture emanating from the floor. We have a coal seam basseting under our house, and it carried a continuous stream of water. The bricks sit on permanently wet clay like slip, but there is no sign of moisture in the bricks. Heat and ventilation is the answer.
J.
--
John Rouse

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John Rouse wrote:

The walls are fine. I just need to stop the moisture from the ground finding it's way into the timbers under the floor. It doesn't matter if it remains damp underneath the polythene.

It's not that bad. I'm on a gentle hill, so the problem is likely due to rain finding it's way down hill. I think it would be a different matter if I was at the bottom of a hill and the water didn't have anywhere to go.

That's the main thing the house has lacked. The ventilation is okay, but could be better. I'll put in a couple of extra ventilation bricks while I'm doind the floor.
-Duncan
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Peter Taylor snipped-for-privacy@DELETETOMAILMEclara.co.uk typed:

Yes I know you did, but "checking the air bricks" IMO is not enough you need a small Gale blowing under old houses to keep the 100 year old oh so porous bricks and timber dry. And old builder's practice of dumping waste under floorboards is another recipe for later disaster. I have just filled a 4 cu.mt skip with crap found under my latest buy,

If you take up a floorboard and stick you head below floor level, are you gassed by the smell, or can you feel a breeze blowing.! Its unlikely to be both.
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Mark wrote

But didn't you say that rising dampness doesn't exist? See below

I agree. In that sense my advice to check the airbricks didn't go far enough. Fortunately this situation is still quite uncommon.

Ah, now you've changed your tune. At first you said rising dampness and wet rot DON'T exist. Now you are saying that they DO exist but with good ventilation they SHOULDN'T exist, and that "a small Gale blowing under old houses to keep the 100 year old oh so porous bricks and timber dry" renders DPC's unnecessary. And you ducked my question on what causes floor timbers to become damp in the first place. You're not IMM in disguise are you Mark?
Of course rising damp exists, and saying it doesn't only misleads. Floor plates with no damp-proof couse beneath will frequently become damp and subject to rot damage. Yes, good ventilation prevents rot attack, but relying solely on this is a bit too theoretical and ignores practical considerations. As you said, ventilation can become blocked by rubble. It can also be hampered by clogged-up air bricks, building extensions etc. and there are always areas where air circulation is naturally poor. I would feel much happier knowing there is a damp-proof course under the floor timbers preventing them becoming damp in the first place, with or without "a small Gale" under the floor. Dry timbers don't rot.
And, to get back on topic, with a gale blowing around under the floor, there's all the more pressure to provide draught-proofing and insulation. That doesn't sit well with your ventilation only/no-DPC theory, does it?
Peter
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Peter Taylor snipped-for-privacy@DELETETOMAILMEclara.co.uk typed

Obviously rising damp/wet rot exists the real question is WHY above floor level.! Well im about to try an experiment, ive had the usual DPC merchants have a look at this house (Ho yes you need a silicon dpc that will be 3k please) and also a surveyor who I think know what he's talking about. The house is early Victorian with no DPC and built on Very wet sub soil, so im digging out to a depth of 1mt under the floor ( having removed all the rubble from previous owners/builders) that's only another 1ft. and doubling the number of air bricks. The floor timber is all in need of replacement so that will be pressure treated, the plaster on the walls is all coming off to be replaced with a membrane that can be plastered over and then decorated immediately. (the house is for let) I'll let you know in another 20 years/6 months if this approach works. ;-)

Isn't every one on Usenet ?
-- Mark's Signature-Free Zone: No random wordy quotations No attempts at infantile humour And no tedious home pages.
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typed

so
doubling
;-)
He sees me in his sleep.
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I have yet to see the brick which, unless one end is actually submerged in water, does not have sufficient surface area to evaporate any moisture contained within it. Indeed if you think about it, if bricks were that porous the mortar would never set.
J.
--
John Rouse

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John Rouse wrote

plates
rot
That's possibly true but it's the rate of evaporation that's important, and this depends upon the relative humidity. A saturated brick of any size or porosity will evaporate nothing in 100% RH. If you are trying to say that rising dampness can't exist because of evaporation, please explain how a timber plate on the top of a simple unrendered sleeper wall without a damp proof course can become damp and rotten.

That does happen frequently with soft facing bricks, especially in warm dry weather or when they're straight from the kiln. It's quite normal to have to dunk bricks in water before they're laid in order to kill the suction and prevent the cement from becoming dehydrated.
Peter
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Like your house is under water?

Lack of ventilation.
J.
--
John Rouse

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Peter Taylor snipped-for-privacy@DELETETOMAILMEclara.co.uk typed:

Ok so if rising damp exists how far does it rise ? I'd also be interested in your ideas as to how you would stop it rising.
--
Mark




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Mark wrote

There is no definitive answer - it depends upon various factors, such as the rate of evaporation, the porosity of the brickworrk, the amount of moisture. It's generally accepted that in most "normal" situations (external wall, plaster inside/brick outside and no dpc), rising dampness will not extend above about 1 to 1.2m above the source of the moisture, and often as little as 300mm. However I once saw a party wall between two basements where the dampness had risen the whole storey height and was affecting rooms at ground floor level. There was a good reason for this - both owners had had their basements tanked with waterproof rendering at different times, but each had objected to the other inserting a dpc in the wall, which would have prevented the dampness rising. Instead, the rendering on both faces of the wall prevented any evaporation and the dampness just kept on rising until it reached a point where it could evaporate. You sometimes see this when people have put aluminium foil under the wallpaper and the dampness appears at the top of this
Also remember that ground water contains dissolved nitrates, chlorides and sulphates which it can only pick up in the soil. It is a definitive test for dampness coming from the ground whether these salts are present when the moisture in a damp wall is analysed. Penetrating water, say from a defective rainwater pipe, will not contain them. The salts are hygroscipic (they attract vapour from the air) and often they can make the wall appear and feel damp long after the true source of the dampness has been cured. The salts are also partially conductive, and can give a false reading on an electronic moisture meter, even though the wall is actually bone dry. It's a sad fact that many surveyors and so-called damp specialists are unaware of this, and many properties have been treated for dampness unnecessarily and often ineffectively.

I could write a book! I would be very happy to help with your particular case if you like, but you will need to give me more information on the house and the problems you've encountered, ideally with some photographs. I would suggest you take a look at http://www.mill-rise.freeserve.co.uk which has masses of sound advice on dampness and timber problems, written by an independent consultant, Graham Coleman.
HTH Peter
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