Roof insulation

I'm trying to do a rapid makeover of a house that was up for sale and now my daughter may rent it from another family member. It's 1920s semi, solid walls, high ceilings and was re roofed 20 years ago from bare slate to felt plus concrete tiles. The eaves have vents but there's no ridge vent and the felt is entire from eaves to ridge.

The loft space is too small to be useful (house is only about 4m width) but is boarded and a 50mm thickness of fibre glass wool was laid on top of the boards. In the last winter I noticed some condensation on the fibre glass, so I rolled it back . The ceilings are strange in that they are hardboard fixed to the joists with battens and glued to the battens are 9" square fibre board tiles. So there is quite a lot of scope for warm, moist air to leave the room and condense in the roof.

I'm guessing my first step is to put a couple of vent tiles near the ridge and then lay 100mm of wool in the loft or should I make the whole roof warm by adding celotex between and to the rafters?

AJH

Reply to
AJH
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No to this last one. What you would have then is the condensation forming on the cold side of that insulation where it can't be ventilated away.

Firstly, you should look at removing the moisture at source: use (or install and use) extract fans in wet rooms; make sure ventilation in other rooms is used.

Secondly, you may want to look at a plasterboard ceiling. A vapour barrier would be a good idea, but isn't essential. Any loft hatches should be draught stripped.

Thirdly, make sure there is enough ventilation to the eaves, and that it's not blocked. If necessary, install extra vent tiles, but not too much at high level; it can create a stack effect, pulling moist air from the inside of the house rather than from outside.

Some condensation is acceptable in a roof space. Provided it is intermittent and dries up, it shouldn't be detrimental to the timber or the insulation.

Reply to
Hugo Nebula

The eaves have vents, so if I seal the loft space from the rafters with celotex they will vent from the eaves to the ridge, as long as I allow some space to collect at the eaves and ridge to which the rafter spaces are open, or am I having a fundamental misunderstanding?

I think there will always be warm moist air in the living space whose dew point will be met once it cools in the loft. Replacing the ceilings would be the only way of sealing them, not a bad idea in itself as there is 30cm of height I could lose.

I was wondering about an insulated "drylined" ceiling but not yet.

The eaves have two metal vents in each roof (it's a stepped building, two roofs at different levels, separate loft spaces) I was looking at putting two matching vent tiles near the ridge and slitting the felt.

Mmm, doing nothing is appealing but how do I know how much condensation to put up with? I do remember when it was a slate roof that snow would blow into the loft between the slates.

Perhaps I'll initially relay the fiberglass but put some chicken wire around the eaves vents to keep them free. As it is 9" solid brick with double glazing the roof losses are about the only thing I can address, short of dry lining, and I'm not up to that.

AJH

Reply to
AJH

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