Musty smell from oversite soil. Liner?

[Timw]

We suspect that the musty smell near our front door may be due to moisture in the oversite soil beneath the floorboards. There is indeed a high moisture level in the soil near the wall, and the gaps between the old pine boards are not sealed.

A damp surveyor has suggested that we cover the oversite soil with a butal liner or polythene to inhibit the release of water vapour - and odour - into the cavity.

A couple of questions:

1. Is covering the oversite soil with liner considered good practice?

2. Should the gaps in ground floor boards be sealed or left open?

Tim

 

[Penners]

I wouldn't cover the soil under your suspended floor with any kind of impermeable sheet. The moisture will still be there and could accumulate to make the soil even damper. This could then migrate to the walls and make the masonry damp.

It sounds to me as though you need to do something about the underfloor ventilation. Are there ventilation grilles in the external walls, below interior floor level? There should be. And ideally they should create a crossflow draught through that void.

As for the floorboards, once you have ensured that the void below is well ventilated, you'll definitely want to do something to close up the gaps. How wide are these gaps and are the boards tongued-and-grooved?

 

[stuart45]

Concrete is normally used on the oversites now.

 

[Feltwell]

I agree with Penners, and would say a definite no to concrete!

Insulate well underneath your floorboards, and ensure the void below is well ventilated with a good cross-draught - make sure all the airbricks are completely clear of soil & vegetation, you may need to consider installing more. Do not start putting plastic sheet down. As for the gaps between the boards, if you've insulated well enough I'm not sure they'll matter that much anyway.

 

[MdB]

Lots of ventilation, insulation and no covering get my vote too.

 

[Lime]

I agree with all the above advice except the concrete suggestion.
You could also rake off the top layer of oversite soil which should remove much of the rotted detritus originally dumped (often shavings and wood) and rubbish fallen through the boards that has built up over the years.

 

[Feltwell]

You could also rake off the top layer of oversite soil which should remove much of the rotted detritus originally dumped (often shavings and wood) and rubbish fallen through the boards that has built up over the years.

Good idea.

If you need any more suggeston that plastic sheet or concrete is a bad idea, I would say that I have recently built a new garage with a suspended floor, albeit block and beam rather than timber. There was no requirement from Building Control to concrete the oversite - we actually used the void to take spoil from taking up the old driveway. All Building Control were interested in was in seeing that we put enough airbricks in to keep the underfloor void well ventilated, and that we didn't fill the void too much with spoil.

 

[stuart45]

The regs are diferent for timber floors, compared to block and beam. For timber BC like to see either a concrete oversite or a membrane.
I have worked on a number of old houses in the past where a concrete oversite was installed to cure ponding problems and as far as I know it didn't cause damp problems in the walls. The problem with many older properties is that the oversite is much lower than the external ground level which can sometimes mean that there can be a lot of water laying under the floorboards. Ventilation alone will not always shift it.
The oversite needs to be above the highest point of the external ground level, or laid to fall to drainage.
You could always use limecrete if really concerned. Although I wouldn't normally recommend OPC for older properties, there can be times when it could be used.

 

[Lime]

The regs are diferent for timber floors, compared to block and beam. For timber BC like to see either a concrete oversite or a membrane.
I have worked on a number of old houses in the past where a concrete oversite was installed to cure ponding problems and as far as I know it didn't cause damp problems in the walls. The problem with many older properties is that the oversite is much lower than the external ground level which can sometimes mean that there can be a lot of water laying under the floorboards. Ventilation alone will not always shift it.
The oversite needs to be above the highest point of the external ground level, or laid to fall to drainage.
You could always use limecrete if really concerned. Although I wouldn't normally recommend OPC for older properties, there can be times when it could be used.

Not a very good solution for older properties.
The idea of placing tons of concrete under a floor to deal with damp and ponding is a modern "solution" for a problem that is normally easily sorted by other means.
If there is ponding under the floor the source of water needs to be dealt with, it won't go away if covered in concrete, it will just go somewhere else in the structure.
The floors in many older houses are now almost at ground level due to build up of soil and "improvements" surrounding the house.
(The Romans didn't build houses 12 feet below ground level in London)
To pour in concrete to above the highest point of the external ground level is not possible in most instances.

If I was purchasing an older house with an infilled concrete oversite under the wooden floors I would not buy it.
It means trouble.

 

[Feltwell]

I'd still say concrete is a bad idea, for all the reasons Lime mentions. Also, for TimW, increasing the subfloor ventilation and insulating will be substantially quicker, easier, cleaner and cheaper - and it's all easily reversible.

 

[stuart45]

The regs are diferent for timber floors, compared to block and beam. For timber BC like to see either a concrete oversite or a membrane.
I have worked on a number of old houses in the past where a concrete oversite was installed to cure ponding problems and as far as I know it didn't cause damp problems in the walls. The problem with many older properties is that the oversite is much lower than the external ground level which can sometimes mean that there can be a lot of water laying under the floorboards. Ventilation alone will not always shift it.
The oversite needs to be above the highest point of the external ground level, or laid to fall to drainage.
You could always use limecrete if really concerned. Although I wouldn't normally recommend OPC for older properties, there can be times when it could be used.

Not a very good solution for older properties.
The idea of placing tons of concrete under a floor to deal with damp and ponding is a modern "solution" for a problem that is normally easily sorted by other means.
If there is ponding under the floor the source of water needs to be dealt with, it won't go away if covered in concrete, it will just go somewhere else in the structure.
The floors in many older houses are now almost at ground level due to build up of soil and "improvements" surrounding the house.
(The Romans didn't build houses 12 feet below ground level in London)
To pour in concrete to above the highest point of the external ground level is not possible in most instances.

If I was purchasing an older house with an infilled concrete oversite under the wooden floors I would not buy it.
It means trouble.

You don't need to put in tons of concrete, the levels are built up with hardcore or gravel. 3 or 4 inches on top is enough.
The levels need to finish 6-9 inches below the bottom of the joists. If there is a high water table the water won't go away, but the area between the concrete and the floor is easier to control with ventilation.
Many older properties have a DPC, so in these cases moisture shouldn't be a problem going up the walls anyway.
This system has been common since the 30's and I don't know of any houses where problems have been caused by it.
A certain amount of moisture can escape through the oversite as the concrete is not usually that strong.
If the level can't be higher than the ground levels externally, the concrete has to be laid to fall to a drainage sytem.
However without being on site it's not really possible to give a definite solution to a problem like this.

 

[Penners]

You don't need to put in tons of concrete, the levels are built up with hardcore or gravel. 3 or 4 inches on top is enough.

The sheer quantity of concrete is not the issue. It's the impermeability of it that matters.

Quite apart from the arguments against concrete already made above, the ventilation solution will almost certainly be a hell of a lot cheaper. And in the unlikely event that it doesn't have the desired effect, you haven't done anything irrevocable to the building.

 

[stuart45]

Ventilation is important, but the property in question may already be quite well ventilated. I have seen houses that had clear airbricks in place, but still had a few inches of water under the floor. Stagnant water isn't the best thing to have directly under the floor. Weak concrete will still allow some moisture through, otherwise you wouldn't need a vapour barrier under it on floors. However as I said limecrete could be used.
Houses vary in their problems and desighns, so you can't really say whether a method will work or not without being on site. Many of the Victorian and Edwardian properties I worked on had 3ft or more below the floors and I've never heard of any problems connected to the installation of an oversite over more than 30 years. If there was I doubt if the modern regs would ask for this method.
A concrete oversite might not be suitable for this paticular property, but very few details are known for it.

 

[AMc]

We suspect that the musty smell near our front door may be due to moisture in the oversite soil beneath the floorboards.

If the area is directly by the front door does the door fit well? Is there any opportunity for rain to drive under it or under the frame and into the void?
I had quite alarming "damp" in a section we had completely rebuilt which was the result of a rain pipe coming loose and the water being dumped against the wall. The solution was to reconnect and fix the pipe and wait for the wall to dry.

 

[Timw]

Many thanks to all for your comments.

I take from this that:

1. Covering the oversite soil with a membrane is probably a bad idea.
2. I need to work on increasing sub-floor ventilation by lowering the soil level a little and possibly adding more airbricks.
3. I should seal the gaps between the boards (Penners: the boards are old pine, not tongue and grooved. The gaps vary up to about 1/4 inch)

 

[Feltwell]

I'd add insulation to that list Tim - it would be an obvious time to do it if you've got boards up.

 

[Timw]

Insulating the uderside of floor boards? I wasn't aware that you could do that. What do you suggest?

Tim

 

[AMc]

You have a number of options - from slinging chicken wire between the joists with "wool" insulation on it to fixing battens to the joists and solid boards foamed in to create an air tight seal. If you look on the websites for kingspan, celotex, ecotherm etc. they have construction diagrams showing the way to use solid board - which is much more thermally efficient but is a bit more expensive.

We had battens then ecotherm board insulation foamed at the edges - but we were installing underfloor heating so insulation was very important to keep the heat from being sucked out under the floor by the cross draught.

Either approach aims to stop air and heat moving down into the void. This will also help prevent smell coming up, so it's important you deal with the moisture issue properly. There could be extensive rot if the joists are kept moist and unventilated, sealed where you can't smell the problem until it becomes serious.

 

[Lime]

The problem with using solid boards is to get the floorboards touching the insulation so that there is no air gap between the two.
If there is an air gap the insulation is almost useless as cold air can circulate directly under the boards.

 

[Feltwell]

Floor joists, rather handily, are often positioned at the same spacings as ceiling joists - which means ordinary, cheap loft insulation rolls can be used and come in about the right widths. in my case I can get under the floor - I used rockwool pushed up between the joists, with chicken wire stapled below to stop it falling down - not that it really needed it, the rockwool gripped surprisingly tightly on it's own.

From above, if the boards are lifted you can drape plastic garden type mesh loosely over the joists, staple it in place and fill that with insulation

Inch for inch, in terms of thickness, it's nowhere near as good an insulator as a solid foam board like Kingspan or Celotex. But the point here is that you've got plenty of space to play with - the full depth of the joists - so just stuff more rockwool in, it's much cheaper than board and there are no problems with getting a tight seal against the joists.

Just my opinion - others disagree and prefer board.