Is fibreglass insulation affecting subfloor ventilation airflow?

[RobWW]

Hello,

Below the floorboards, the last owner of our property (a later Victorian terraced house) had recently (2018) installed fibreglass insulation, which they have pinned up in rolls between the joists. I'm worried about some damp we have in the house, and so I pulled up a floorboard and pulled back the insulation. While the joists are dry, the dirt floor below is moist.

We have two air vents, one original one at the front, and a replacement big one at the back (it's a narrow house with a single room spanning the ground floor, 9m long, 3.5m wide). The front vent needed a lot of cleaning to allow air to flow, but looking into it from outside I saw the fibreglass came very close to the vent, and was at the exact height of it. I've decided to pull the fibreglass back a bit from the vent, to allow more airflow.

However, I'm now concerned about whether rolled out fibreglass, pined to the bottom of the floorboards, might itself cause damp issues. There is about a foot and a half between the dirt floor and the fibreglass, but it seems to me that the fibreglass is likely to absorb damp air and so keep the subfloor area more humid, and this might mean the air vents have to work harder just to keep it dry under there.

Does anyone have experience of this? Is it OK to have this fibreglass under the floors like this?

Thanks!

 

[JohnD]

what is the depth, below floor level, of the airbricks?

What is the depth, below floor level, of the bottom of the insulation?

What is the depth, below floor level, of the earth or oversite concrete?

Is there really only one airbrick at the front and one at the back?

Have you got a water meter

 

[RobWW]

what is the depth, below floor level, of the airbricks?

What is the depth, below floor level, of the bottom of the insulation?

What is the depth, below floor level, of the earth or oversite concrete?

Is there really only one airbrick at the front and one at the back?

Have you got a water meter

Airbrick is high up, only about 5 centimetres lower than floorboards.

The bottom of the insulation is about half a foot below the flooboard (i.e., roughly in line with airbrick, hence why I pulled it back.

The earth is then about a foot below the bottom of the insulation (i.e., air would have to draw down from the vent to below the insulation).

Yes, there's only one airbrick at front (size of two standard bricks on top of each other) and one at back (bigger, probably size of about four bricks stacked).

 

[RobWW]

Airbrick is high up, only about 5 centimetres lower than floorboards.

The bottom of the insulation is about half a foot below the flooboard (i.e., roughly in line with airbrick, hence why I pulled it back.

The earth is then about a foot below the bottom of the insulation (i.e., air would have to draw down from the vent to below the insulation).

Yes, there's only one airbrick at front (size of two standard bricks on top of each other) and one at back (bigger, probably size of about four bricks stacked).

And no, we are not metered as the pipe serves both our property and next door's. The water company has told us there is a leak in next door's pipe, but apparently not ours (although of course we may well have it in ours too, or be getting water comign in from the leak next door).

 

[KenGMac]

Is there an air gap between the fiberglass and the flooring??

 

[JP_]

How long since you cleared the air brick? It can take a while for things to dry out. Any space for adding another air brick or two? S couple of extra single brick vents front and back should make a big difference.

 

[Leofric]

I'm now concerned about whether rolled out fibreglass, pined to the bottom of the floorboards, might itself cause damp issues.

The fibreglass itself shouldn't cause damp issues but it sounds like there aren't enough air bricks and the fibreglass quilt will obstruct the airflow anyway.

 

[noseall]

Is there an air gap between the fiberglass and the flooring??

Why would there be?

The correct place for any air gap would be on the cold side.

 

[Leofric]

The correct place for any air gap would be on the cold side.

Yes, ventilated air space below the insulation on a suspended timber floor .This air gap requirement ,usually specified in relation to the 50mm gap above insulation to sloping ceilings, seems to get misinterpreted.

 

[John D v2.0]

Yes, ventilated air space below the insulation on a suspended timber floor .This air gap requirement ,usually specified in relation to the 50mm gap above insulation to sloping ceilings, seems to get misinterpreted.

Yes and further to that the ventilated gap is only needed if the opposing surface is vapour closed eg flat roof covering or old style roof felt. This would of course apply to the oversite concrete too.

 

[KenGMac]

Is there an air gap between the fiberglass and the flooring??

The reason I asked was that on a couple of occasions I have seen "unsupported" fiber glass which had been pinned to the joists sagging badly downwards and away from the underside of the floor boards.

The shall I call it "more conventional" method of installing glass wool in this location was to use plastic netting slung between the joists and thus the glass wool was completely supported in the correct position.