This is a picture from the space below my (1930s detached) house. A couple of months ago I insulated between the floor joists but when I went down there today was shocked to see condensation on the bottom of the insulation boards. There is no drainage outside the walls and the space floods in the winter. The ground below is still quite wet. It is always damp down there but I’ve never seen any condensation before I put the insulation in. There is ventilation, (4 x 8”x3”) but perhaps not enough. I am digging away soil from the external poured concrete foundation walls, but apart from getting this done and drainage installed asap, what can I do? Dehumidifiers? What about a polythene damp course, would that help, or just prevent the ground from ever drying out?

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Is it over all the insulation or in a particular part of the void?

Had you taken moisture readings for all the joist ends before you started?

A bit too late to change insulants but this may be of some use:

"The act of insulating a suspended timber floor cools down the floor structure and may increase the risk of condensation occurring on the floor timbers.

These risks can be reduced somewhat if there is excellent cross-ventilation below the suspended floor which can ‘blow away’ any water vapour-laden air before it can condense out.

Nevertheless I recommend that a ‘risk management approach’ should be taken in every case before proceeding with insulation of a suspended timber floor.

First I would suggest that you measure the moisture content of the joist ends as they enter the ‘pockets’ in the wall. Crudely, less than 18% may be good, 20% may be acceptable, but much higher may be indicative of a risk factor.

Get an idea of the general level of moisture below the floor. Is it quite wet? Does water come in when it rains? Can you prevent it coming in? If large quantities come in (standing water) can you pump it out? Perhaps as a one-off action, or perhaps you might need a sump with a float-switch-operated pump.

Then look and see whether there is adequate cross-ventilation. For a rectangular floor-plan the ideal arrangement is to have ventilation at each corner. In a small terraced house, for example, a 225mm x 150mm air-brick at each corner may be sufficient. However even if adequate vents exist at front and rear typical cellars impede this ventilation by means of a stone staircase. In certain cases it may be possible (though not low-impact) to knock sufficiently large holes in the central spine wall (with the correct structural advice).

The choice of insulation material can be difficult. Some sources suggest that a breathable fibrous insulant will help to manage moisture. Others take the view that fibrous insulants are absorbent and may worsen the situation by transferring moisture to the structural timbers. Opinions vary!

What is clear is that, if the perimeter walls are at all damp it may be advisable to fix a perimeter layer of impermeable insulation such as extruded polystyrene (XPS) to prevent other insulants coming into contact with any moisture from the walls.

Note that polystyrene should not come into contact with the PVC sheathing of electrical cables as it may make the casing brittle. Equally electrical cables should not be ‘tucked up warm’ in insulation. If they are, consult an electrician as the electrical regulations may require the circuit-breaker to be de-rated to reduce the risk of fire".


Under-floor insulation guidance:

https://www.gov.uk/government/publications/insulating-suspended-timber-floors-best-practice

https://www.greenbuildingstore.co.uk/information-hub/insulating-suspended-timber-floors/

https://www.ecologicalbuildingsystems.com/post/best-practice-approach-insulating-suspended-timber-floors

http://www.sofiepelsmakers.com/suspended-timber-ground-floors.html

I'd suggest your air-bricks may be a tad under-sized. I'd normally do for a minimum of 4 225 x 150s at diagonally-opposite corners.

Wot Nick said about getting an idea about the amount of moisture / water present during the year +1

Given that the condensation is all over except in the location of the air bricks shows that ventilation controls the problem. Perhaps increase the ventilation (temporally or permanently) by the installation of a low wattage fan at an air brick or as an extra.

I have seen this before with EPS stuck unde concrete planks, the water was so heavy that it pulled some of the less well stuck on insulation off

1) don’t panic!

2) monitor it for a while

3) ask questions, is the wood damp,

I have some solutions

Worth a read:
https://buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces

Lstiburek is a famous building engineer from across the pond, to be fair this article is based on the climate in Maryland which is very hot and humid in the summer. He argues that the soil in the underfloor void stays close to the year-round average temperature (about 10-12⁰C in different parts of the UK, more in the US). The floors were originally warmer than that, so stayed dry, but if the floor is insulated, the lower edge of the insulation drops close to soil temperature (by heat radiation to/from the soil). So at 10-12⁰ there will be condensation whenever the air humidity dewpoint is more than that 10-12⁰, which it might be in humid summer weather.

His proposed solution is 1) more ventilation 2) breathable floor to allow humidity to escape the build-up 3) insulation extended below the floor timbers so the timber remains above dewpoint even if the insulation doesn't.

Reducing the humidity in the void also sounds a good idea, ours is never damp.

Posted By: ChristopherThis is a picture from the space below my (1930s detached) house. A couple of months ago I insulated between the floor joists but when I went down there today was shocked to see condensation on the bottom of the insulation boards. There is no drainage outside the walls and the space floods in the winter. The ground below is still quite wet. It is always damp down there but I’ve never seen any condensation before I put the insulation in. There is ventilation, (4 x 8”x3”) but perhaps not enough. I am digging away soil from the external poured concrete foundation walls, but apart from getting this done and drainage installed asap, what can I do? Dehumidifiers? What about a polythene damp course, would that help, or just prevent the ground from ever drying out?

I'm with Tony; add more insulation, especially under the joists, so they are warmer and condensation forms on the bottom of the new insulation instead. So you'll still have condensation but it will be in a place that it doesn't do any harm.

Why does the space flood in winter? And will your new drainage stop it (where will it drain to)? I don't think a damp proof membrane on the ground will help.

If you can stop it flooding, so the only problem is condensation from the air, then it might be worth considering “fully fill the void with graphite EPS beads” as Dominic suggested. That's the ultimate in adding more insulation

Joe Lstiburek's article is useful, but it's important to remember that he's describing very different conditions to the UK, so some of his advice doesn't translate well. Use his science but draw your own conclusions.

@djh I hope that digging earth away from the walls and putting in drainage will keep things dry(er) below the floor, will drain to a pond in front of the house. But if the condensation is coming from the air, might still be an issue.

@tony Got a moisture meter and checked the joists, they are certainly damp, mostly high 20s some over 30% the highest readings were nearest the vents.

The layer of insulation below the joists sounds like it is worth doing, but presume the wood needs to dry out a bit first?

If the insulation has condensation, then the insulation is working as it is the coldest surface.

Where is your water table? If you dig a hole outside, does it fill with water?

I'd be concerned that the moisture is coming up from below. What is the ground - soil/rubble?

Did you insulate from below? The fact you can get to the joist ends suggest there is good access.

*If* the moisture is coming from below, I'd suggest that you need to stop it. A new build would have a sheet of plastic and a concrete blind over the top.

Posted By: tonyIt is not damp from below that is the problem, it is moisture in the air and that can’t be stopped.

But I am wondering how that moisture is getting into the air. If the ground is damp, that is the source.

Adding more insulation isn't going to stop the condensation (the surface will still be the coldest point). Unless the joists are completely sealed, it could make it worse. You suggest EPS so it will breathe but equally, the dew point could be within the insulation. At worst, directly between the 2 layers of insulation.

I'd be trying to remove the source of the moisture first.

It could also be that the steady state has not been reached (if recently done) so you need greater intervention initially to reduce the moisture content. If other conditions are OK, it will not return, but the 'system' cannot remove the initial excess.

Posted By: tonyIf the house is warmer than the ground moisture will be moving from the house into the ground, driven by partial vapour pressures being higher in warmer places.

If so, then I'd like to find the source. Looked well sealed in the picture.

It could also be damp air from before insulation that needs to be forced out to reach a new steady state.

Still cannot see how more insulation solves it as the new surface will still be the coldest surface.