Underfloor insulation

[Gib Bogle]

I have a plumbed and wired shack at the bottom of my section, like a small bed-sit. The land is not well drained, and the shack is at the lowest point, and consequently the ground under it is damp for long periods. This makes the inside rather cold and damp, and the chipboard floor is also suffering ill effects underneath. I am proposing to install polystyrene slabs between the joists, then staple a sheet of thick polythene plastic to the joists. A friend has cautioned me that this could trap condensation. Maybe he's right, but I will not believe it until I understand how the plastic sheet could make the situation worse. Any advice?

[Jim Chisholm]

Any damp proof membrane needs to be on the COLD side of insulation.

If not there may be a point within the insulated layer where the temperature drop means any water vapour will condense (at 'Dew Point)

Where the water condenses insulation breaks down and any wood will get wet. So you need the 500/1000 gauge polythene under the insulation and ideally ventilate the area above.

Jim Chisholm

[Gib Bogle]

Thanks. I think you are confirming my impression, since the cold side is on the outside. That is, we have this sequence: floor - insulation - polythene - air - ground. Ventilation will be hard to achieve. I could stop the polythene sheet a few centimeters before the end of the run of the joists, at each end.

[chris]

Suggest you consider the following as it is only a shack... get shot of the floor - lay sand or whatever to get it level if needed - put in

4" of flooring grade PU foam high compressive strength insulation - lay WBP ply on top no ventilation needed nice and warm

Chris

[The Natural Philosopher]

Oh dear. That means every single house lined with foil backed plasterboard surrounded by insulation is wrong.

You had better tell the powers that be, since this has been the accepted way to do it for years.

But there isn't any moisture there, due to the barrier.

No, you dont.

[The Natural Philosopher]

Dont thank hm for totally wrong information.

[The Natural Philosopher]

+1

[Bob Minchin]

The OP wrote of a high water table in the area. For a belt and braces approach I would use a layer of sand, then a heavy duty polythene DPM followed by as much flooring grade insulation. If there were joists before, these might be important to the structure of the building and so incorporate these into the insulation then fit the floor decking with an optional vapour barrier under if desired. Wet insulation is bad news as if it becomes water logged, it is then a much poorer insulant.

Bob

[Gib Bogle]

OK, so I got bad advice confirming my own wrong ideas. But the picture is still not completely clear.

First, as should have been obvious since I talked about installing underfloor insulation, and an air gap under the floor, this shack is on piles. I can crawl underneath. Any major renovation, reflooring, sand foundation etc is totally out of the question - it just isn't worth that much. There is a free flow of air under the building.

The crux of the issue is the relative importance of the sources of moisture. Inside the shack any occupant will create water vapour by breathing, cooking and washing. Clearly this water needs to get out. This is a matter of keeping some windows open. The other source of water is from the wet ground. The question is this: to what extent is this raising the level of moisture inside the shack? I was thinking of the net flow of water through the floor being into rather than out of the shack, hence the plastic idea. Maybe I'm wrong about this. In any case before I thought of attaching the polythene to the joists, I had the idea of putting the plastic on the ground, and maybe this makes more sense.

The insulation is polystyrene foam slabs. I think this is impervious to water, so there is no problem of water being soaked up and causing timbers to rot.

By the way, this is in Auckland, where we have a few frosts each year. I'm not sure that condensation is such a big issue here. The high temperature today, in early winter, is forecast to be 15 deg. C.

[chris]

Now that you have particularised the "obvious" - that it is in NZ and on piles and that it has to be timber - I can help. My father lived in Morrinsville and I have visited so am not unfamiliar. It is likely that the timber, if preserved, will have had 8 kg per cub metre rather than the 4 kgs used in Europe and as such not be likely to rot at all. If they are not CCA treated then they are at risk. First, your idea of putting a DPM on the ground is correct. Hold it down with sand or gravel or bricks or whatever and use a decent thickness. This will reduce vapour pressure from the ground to acceptable levels. Secondly ensure through ventilation if that is not naturally there by having gaps at opposite ends of, ideally, all sides. You now need to consider insulation. If you can put PU or polystyrene foam wedged between the joists to the level of the bottom of the joists and ensure that it is cut to fit and any gaps filled with expanding foam (use a foam gun not the cans) then all you have to worry about is the cold bridge caused by the joists which in reality is not likely to be serious problem. (It is more likely that gaps and draughts will be worse). However, if this is a shack that you do want warm and are prepared to pay for it, then put on at least one inch of PU foam or similar on the underside of the joists with the joins not on the timber but under the installed insulation between the joists. Tack it on with as wide headed plastic 2" washers as you can for support or even with battens (roofing 'cca'-treated) and non ferrous fixings. You will need to mark where the joist are first on the sides of the house and put on a chalk line or the like. The likelihood of decay occurring due to interstitial condensation is remote in the Aukland climate but if you want to avoid or minimise the risk simply apply a polyurethane varnish to the floor boards once they have dried out fully in the summer or you have de humidified. Chris