Loft insulation problems

vcl

We have recently insulated our loft with an additional 200mm using rolls of glassfibre. This was certainly effective enough to make the loft much colder than it was previously. As we wanted to use the loft for storage, we thought it would be a good idea to line the inside of the roof with rolls of the foilbacked blisterpack material from Wickes. This we have done, making sure that the gaps between the joists were clear enough to ensure a good steady flow of fresh air from the vents. I now find that the surface of the foil is running with condensation, and I can hear drips of moisture falling from the roofing felt onto the foil. HELP! I thought I had done the job properly, but it seems I must have made some fairly fundamental mistake somewhere. Advice please.

andrew-b

Firstly where you've insulated with the glass fibre insulation you should have left a gap around the eaves to allow air to ventilate the roof. Did you do that?

I wouldn't have put the foil up in the first place. There should be no need for it as the purpose of insulation is to keep the rooms below warm. You don't want to unnecessarily heat the loft space or trap any heat that does escape into the loft either. The loft space should feel cold and airy (at least today it should!) and should be fine for storage without the need for the foil insulation you've added. So i'd take all the foil stuff down again if i were you. I suspect it's stopping the roof from "breathing" through the felt.

Andy

Porky996T

The purpose of the eaves vents is to allow a cross flow of air through the loft to prevent any condensation forming (0.1m/s air flow minimum) and causing rot problems with the roof timbers.

The warmer, moister air is still passing into the loft, but by fitting the foil onto the rafters you have restricted the airflow to below the felt and tiles only. As the moisture has nowhere to go, it is condensing on the colder foil.

I would agree with Andrew-b (previous post) and not have put the foil up in the first place as this is stopping the removal of moisture vapour in the loft space.

vcl

When I installed the foil, I allowed more ventilation than there was previously. This was because the foil was folded back and stapled to the joists:this left a gap of four inches plus the depth of the eves themselves.

I know I must sound like an idiot, but there were no warnings on the packing, other than to leave ventilation gaps as per normal. vcl

Porky996T

You wouldn't get warnings on packets as the material has a variety of use and applications.

I'm getting confused, did you staple the foil to the rafters or the joists? (joists hold the ceiling up in the bedrooms, rafters run at an angle and support the roof structure)

vcl

I stapled the foil to the rafters, then stapled the ends to the top of thejoists to make sure that it did not block the airflow.

27col

Provided that there is insulation in the floor of the loft there is very little point in putting it on the underside of the joists as well. The existing sarking felt under the tiles will effectively keep out the bulk of the dirt. My sister's old house does not have any felt and the amount of dirt building up in the loft has to be seen to be believed. My house has sarking felt and there is almost no build up of dust and dirt on the stored items in the loft, even after 30years.

allan673

remove the foil insulation, its really designed for houses with no loft insulation. the 200mm extra insulation you have laid is plenty believe me, and the foil insulation is making no difference at all.