Damp loft

[autolycusd]

Not soaking, but cardboard storage boxes, for example, lose their stiffness after a while and become limp (I know).

After an extension a couple of years ago, the loft is well insulated - about half has high mass acoustic fibre glass between the rafters and 70 mm celotex above, and the rest has two layers of conventional fibreglass. In excess of Building regs at the time. There is a bathroom fan that ducts through the ceiling to a false tile, but this seems to be leakproof and the loft door fits snugly in the hatch.

The roof is new. When the roofers were fitting the breathable roofing felt, they managed to rip it in several places before fitting the tiles. I pointed it out to them and i think they just put another layer of felt on top and then tiled the roof. There is no other loft ventilation.

I was wondering if this 'double layer' might be what is causing the problem? Or something else?

 

[kbdiy]

I would think it is more likely to be a lack of through flow ventilation that is causing the dampness. Are there gaps at the eaves or are they blocked with insulation? You say it is insulated at the rafters - is there any provision for airflow above that insulation?

 

[autolycusd]

I would think it is more likely to be a lack of through flow ventilation that is causing the dampness. Are there gaps at the eaves or are they blocked with insulation? You say it is insulated at the rafters - is there any provision for airflow above that insulation?

Hi kbdiy - thanks for the reply.

Firstly, i do apologise for my terminology - i meant to say joists, not rafters, which i guess makes a big difference and answers your second question.

There are no gaps as such at the eaves. There are skillings, so there is celotex insulation between the rafters for a coupe of feet with an inch or so gap between the top of this insulation and the (breathable) roofing felt. I'm not sure what purpose this gap serves as there is no airflow - air vents - in the eaves.

Hope you understand the problem better now and sorry again for the confusion.

 

[autolycusd]

Just a quick update, i contacted the manufacturer as there were no further responses here.

They said: "due to this area being un heated you will always get this type of issue".

Frankly, i was somewhat amazed at this statement and i wondered if anyone here might care to comment?

 

[tony1851]

Yes, that is ridiculous.
Your problem is obviously condensation. You prevent that by providing both heating and ventilation. Reduce one, and you need to increase the other.
Clearly you don't want to heat the roof void, so you need plenty of ventilation - a draughty roof is a healthy roof

 

[autolycusd]

Thanks Tony1851. But i thought the whole point of using breathable felt was that you didn't need to ventilate the loft space? So you'd recommend some soffit vents perhaps? I was wondering about using a tubular greenhouse-type heater that uses very little power, or a dehumidifier with a humidistat?

 

[Alastairreid]

Yes breathable membrane provides ventilation to an extent, but more often than not requires additional ventilation, ie soffit, over fascia, ridge or tile.

 

[tony1851]

+1