Ventilating loft space

[Ruffles]

Hi

I'd like to increase the insulation in our loft. However I suspect we need to increase the ventilation first to avoid condensation. I have found evidence of condensation on the roofing felt.

We have two loft spaces - one for the original cottage - the other for a (larger) extension.

I believe the standard way to do this is to provide vents in the eaves and air bricks in the gables. However our cottage has bedroom ceiling level between 1 and 3 feet above the gable level. If I lag the space between the inclined ceiling and the roof this will reduce ventilation.

Is there a standard way of ventilating a roof space such as ours?

 

[MdB]

Others have mentioned this: http://www.lapvent.co.uk/ which I was thinking of using. No idea how good they are.

 

[Ruffles]

Others have mentioned this: http://www.lapvent.co.uk/ which I was thinking of using. No idea how good they are.

Looks ideal. I wonder if it fits in the gap between the rafters in the old roof. Guess I'll have to get up there to measure. Any DIYers out there with experience of fitting them?

Thanks.

 

[MdB]

Let is know if you find out prices.

 

[Flyfisher]

If I lag the space between the inclined ceiling and the roof this will reduce ventilation.

I suppose it will 'reduce' the ventilation in the sense of restricting any howling gales blowing around inside the roof space, but I don't see how it would reduce ventilation in the bad, moisture-trapping sense, providing the lagging material itself is permeable.

(I'd like an expert opinion though!).

 

[Penners]

If I lag the space between the inclined ceiling and the roof this will reduce ventilation.

I suppose it will 'reduce' the ventilation in the sense of restricting any howling gales blowing around inside the roof space, but I don't see how it would reduce ventilation in the bad, moisture-trapping sense, providing the lagging material itself is permeable.

(I'd like an expert opinion though!).

Well, this ain't no expert opinion, but I'm not sure I agree with you.

Lagging or insulation material is specifically designed to trap squillions of pockets of still air. So if a gap is entirely filled with lagging material, I'm not sure that it could still be regarded as permeable, or that it could effectively allow the passage of ventilating air.

But, like you, I'd be interested in an expert view.

 

[Mark]

we have some lengths of plastic pipe stuck between the sheets of felt - like a diy version of the lapvent thing

 

[Flyfisher]

Lagging or insulation material is specifically designed to trap squillions of pockets of still air. So if a gap is entirely filled with lagging material, I'm not sure that it could still be regarded as permeable, or that it could effectively allow the passage of ventilating air.

I would humbly question your non-expert opinion with my own even less expert musings and wonder if the type of lagging material makes a difference? Surely something like rockwall 'slows down' rather than 'traps' pockets of air, thus allowing moisture to permeate though it rather than become trapped? Otherwise, wouldn't laying 8 inches of rockwool over a typical ceiling risk trapping moisture against the joists?

(Please, let's have a real expert view to put us out of misery!)

 

[Penners]

Please, let's have a real expert view to put us out of misery!

Hear, hear!

wouldn't laying 8 inches of rockwool over a typical ceiling risk trapping moisture against the joists

As I understand it in my non-expert and not-very-humble way, insulation laid on the "floor" of your loft (ie: "cold roof") doesn't risk trapping condensation, because no condensation occurs. The insulation keeps the cold air above and the warm air below apart, thus preventing condensation.

It's all to do with e=mc^2. You wouldn't understand.

 

[Flyfisher]

I think E=mc2 only applies to moisture travelling near the speed of light.

At normal velocities, typical of most period properties, wouldn't there be a temperature gradient across the insulation such that the dew point for any condensation might be within the insulation?

(where is that expert?!)

 

[Ruffles]

I think E=mc2 only applies to moisture travelling near the speed of light.

At normal velocities, typical of most period properties, wouldn't there be a temperature gradient across the insulation such that the dew point for any condensation might be within the insulation?

(where is that expert?!)

Actually I crawled into both loft spaces last night to measure up. The old loft is a hideously cramped space - hands a knees and you can't turn round.

Anyway, the 'new' extension loft has only 2" of rockwool. It contains the CH header tank so there is some source of warmth. A single layer of rockwool is laid over the sloping ceiling but since it is so thin there is an air gap above right down to the eaves. There is no sign of condensation though the felt is torn in places. Maybe deliberately to increase ventilation? The air feels fresh.

The 'old' loft has fairly brown looking rockwool but a full 8" deep. There is visible condensation on the felt. There is the odd drip onto the rockwool but if you squeeze it it doesn't feel wet. So it doesn't look like condensation is occurring on the wool - just the felt. The rockwool is stuffed into the sloping part of the roof between the plasterboard and the felt with no air gap at all. This may not matter that much since the sloping bit is only 1.5' at most. I'm fairly sure the eaves will be closed off anyway. The atmosphere is definitely stuffy in there! I reckon this loft needs ventilation urgently. Is it worth replacing the rockwool? Is more modern stuff a better insulator?

Of course if I ventilate the loft the air temperature will be lower and then, perhaps, I may have problems with condensation on the rockwool?

 

[Lime]

If there is already a problem with condensation then any additional insulation will make matters worse.
The problem that further insulation brings to a loft space is that it insulates the loft from the house more efficiently.
On cold days the air in the loft space will be nearly as cold as outside.
The nett result is that condensation is more likely to occur within the loft space particularly if some moisture is escaping into the loft from the house...and in most older houses that happens all the time.
So condensation occurs and if the ventilation is not sufficient it remains soaking into the fabric of the roof.

The older roof space with the felt and thicker insulation demonstrates the point very well.
The felt is wet and the insulation is damp.
If the wood was to be examined it would be found to hold a large amount of moisture too.
The older roof space urgently needs ventilation before damage occurs.
If the house is well insulated it will not matter if there is a gale blowing through the loft, all the better for removing damp.
There are many different tile ventilation systems on the market.
When they are fitted ensure that there is a cross-flow of air through and across the entire loft space.

The old insulation is perfectly adequate and does not need to be replaced.

Remember to insulate the water tank well but leaving no isulation directly under it.
Also ensure it is sealed as it will fill the loft space with moisture otherwise.

 

[Ruffles]

The older roof space urgently needs ventilation before damage occurs.
If the house is well insulated it will not matter if there is a gale blowing through the loft, all the better for removing damp.
There are many different tile ventilation systems on the market.
When they are fitted ensure that there is a cross-flow of air through and across the entire loft space.

Excellent. I'll deal with the old loft first. See how much the lap vent comes to. I'm just hoping it doesn't disturb the tiles. They are clay and quite fragile.

 

[Penners]

I think E=mc2 only applies to moisture travelling near the speed of light.

You should see the way it comes up our study wall.

I think Lime has explained things very well. If there's condensation on the loft insulation, then the loft is insufficiently ventilated and the insulation is also probably insufficient.

 

[Flyfisher]

Not as fast as through my roof, I bet (not the recently repaired one of course).

Yes, Lime's explanation was very helpful.

 

[Penners]

Not as fast as through my roof, I bet

I'll concede that!

 

[chuckey]

The easy way of increasing the ventilation is to open up the overlap in the slaters felt, so a decent amount of air can get in. Push some thin (6mm diam?) garden canes down the overlap joint from the inside. On the first one get a buddy on his mobile phone to view the roof so you don't end up with a foot of cane appearing from under the tiles. The problem is that the felt will be extremely hard and rigid so working the canes down will not be easy.
Looking at the "lapvent" it needs a deep trough in the felt for it to be fixed properly, i.e. it is not for a retro fit.
Frank

 

[Ruffles]

Looking at the "lapvent" it needs a deep trough in the felt for it to be fixed properly, i.e. it is not for a retro fit.
Frank

If you rummage around the website there's an animation on the fitting.
You actually cut the felt then insert a new piece of folded felt between the rafters.