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Default loft ventilation - fly screen or open holes in felt?

Following on from previous threads...
http://groups.google.com/group/uk.d-...ea?hl=en&tvc=1
....I'm putting 50mm celotex (equiv!) on the underside of the rafters.

As previously suggested, I'm going to cut some ventilation holes in
the (not breathable) felt near the ridge - a hole in between each pair
of rafters.

I thought that maybe I could buy some cheap fly screen (small meshed
plastic/fabric) and cut it to glue over these holes. This has the
obviously advantage of keeping insects out of the space between the
felt and celotex (unless the glue fails), but the likely disadvantage
of getting blocked up by dust/dirt eventually, so preventing the air
flow that's vital to stopping the timbers rotting.

I was also going to stick little 1cm wedges of wood into the overlap
between adjacent sheets of felt to get even more airflow.

There are no dedicated vents in the soffits into the felt/celotex
void, but the woodwork is quite loosely nailed together, so there's
plenty of breeze. If I ever have it replaced with plastic, I'll ensure
that suitable vents are included along the length. I _have_ put vents
in from soffit level directly into the loft space itself, which run
through, but don't vent into, the felt/celotex void.

What does the group think of the fly screen idea? And the wedges?

Cheers,
David.
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Default loft ventilation - fly screen or open holes in felt?

On 03/02/2011 13:01, David Robinson wrote:
Following on from previous threads...
http://groups.google.com/group/uk.d-...ea?hl=en&tvc=1
...I'm putting 50mm celotex (equiv!) on the underside of the rafters.

As previously suggested, I'm going to cut some ventilation holes in
the (not breathable) felt near the ridge - a hole in between each pair
of rafters.

I thought that maybe I could buy some cheap fly screen (small meshed
plastic/fabric) and cut it to glue over these holes. This has the
obviously advantage of keeping insects out of the space between the
felt and celotex (unless the glue fails), but the likely disadvantage
of getting blocked up by dust/dirt eventually, so preventing the air
flow that's vital to stopping the timbers rotting.

I was also going to stick little 1cm wedges of wood into the overlap
between adjacent sheets of felt to get even more airflow.

There are no dedicated vents in the soffits into the felt/celotex
void, but the woodwork is quite loosely nailed together, so there's
plenty of breeze. If I ever have it replaced with plastic, I'll ensure
that suitable vents are included along the length. I _have_ put vents
in from soffit level directly into the loft space itself, which run
through, but don't vent into, the felt/celotex void.

What does the group think of the fly screen idea? And the wedges?


I think I would go for fitting something like vented ridge tiles.

http://www.marleyeternit.co.uk/Roofing/Ventilation.aspx

Colin Bignell
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Default loft ventilation - fly screen or open holes in felt?

In article ,
David Robinson writes:
Following on from previous threads...
http://groups.google.com/group/uk.d-...ea?hl=en&tvc=1
...I'm putting 50mm celotex (equiv!) on the underside of the rafters.

As previously suggested, I'm going to cut some ventilation holes in
the (not breathable) felt near the ridge - a hole in between each pair
of rafters.


I would not deliberately cut the felt. It's your second line of
defence against a cracked or slipped tile, although a hole at the
top would not be as bad as a hole at the bottom. There's usually
an overlap very near the top, on account of the top sheet going
over the ridge timber, and effectively being less than half width
left to go down each side.

I thought that maybe I could buy some cheap fly screen (small meshed
plastic/fabric) and cut it to glue over these holes. This has the
obviously advantage of keeping insects out of the space between the
felt and celotex (unless the glue fails), but the likely disadvantage
of getting blocked up by dust/dirt eventually, so preventing the air
flow that's vital to stopping the timbers rotting.


I wouldn't bother. The larger insects aren't a problem for the
house, and the smaller ones like woodworm beetles, you won't
stop anyway.

I was also going to stick little 1cm wedges of wood into the overlap
between adjacent sheets of felt to get even more airflow.


Providing there's an overlap between layers (as there should be)
so that any water running down won't run in, that sounds OK.
Normally the water runs down centrally between the rafters due
to the droop in the felt, and this helps keep the rafters dry.
Make sure you aren't disturbing the felt enough to redirect any
water onto the rafters. I would not use anything which might rot
as a spacer. Either use offcuts of pressure treated battens, or
some plastic pieces.

The flet in my roof has enough droop between layers to allow a
little air flow along all the overlaps without adding anything,
and overall it probably adds up to more than the 10mm all round
vent generally used nowadays.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]
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Default loft ventilation - fly screen or open holes in felt?

On Feb 3, 7:59*pm, (Andrew Gabriel) wrote:
In article ,
* * * * David Robinson writes:

Following on from previous threads...
http://groups.google.com/group/uk.d-...ad/262ef95c259...
...I'm putting 50mm celotex (equiv!) on the underside of the rafters.


As previously suggested, I'm going to cut some ventilation holes in
the (not breathable) felt near the ridge - a hole in between each pair
of rafters.


I would not deliberately cut the felt. It's your second line of
defence against a cracked or slipped tile, although a hole at the
top would not be as bad as a hole at the bottom. There's usually
an overlap very near the top, on account of the top sheet going
over the ridge timber, and effectively being less than half width
left to go down each side.

I thought that maybe I could buy some cheap fly screen (small meshed
plastic/fabric) and cut it to glue over these holes. This has the
obviously advantage of keeping insects out of the space between the
felt and celotex (unless the glue fails), but the likely disadvantage
of getting blocked up by dust/dirt eventually, so preventing the air
flow that's vital to stopping the timbers rotting.


I wouldn't bother. The larger insects aren't a problem for the
house, and the smaller ones like woodworm beetles, you won't
stop anyway.

I was also going to stick little 1cm wedges of wood into the overlap
between adjacent sheets of felt to get even more airflow.


Providing there's an overlap between layers (as there should be)
so that any water running down won't run in, that sounds OK.
Normally the water runs down centrally between the rafters due
to the droop in the felt, and this helps keep the rafters dry.
Make sure you aren't disturbing the felt enough to redirect any
water onto the rafters. I would not use anything which might rot
as a spacer. Either use offcuts of pressure treated battens, or
some plastic pieces.

The flet in my roof has enough droop between layers to allow a
little air flow along all the overlaps without adding anything,
and overall it probably adds up to more than the 10mm all round
vent generally used nowadays.


Thanks for the feedback Andrew. Yes, the felt droops a little, but not
by much in most places. Will use plastic spaces though - good idea!
I'll see how much air I can get in at the top with that, without
cutting. I'm not sure there is a join within 1 foot or so of the top
though.

It amazes me how different the responses are to this question. It
seems that opinion hasn't quite settled to a single standard answer
when it comes to loft ventilation and felt.

Given that pros often leave a loft in a state of permanent
condensation, while Victorian properties with none of the modern
practices fair well enough, I suspect we still have something to
learn.

Cheers,
David.
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Default loft ventilation - fly screen or open holes in felt?

On Feb 3, 5:09*pm, "Nightjar \"cpb\"@" "insertmysurnamehere wrote:
On 03/02/2011 13:01, David Robinson wrote:

Following on from previous threads...
http://groups.google.com/group/uk.d-...ad/262ef95c259...
...I'm putting 50mm celotex (equiv!) on the underside of the rafters.


As previously suggested, I'm going to cut some ventilation holes in
the (not breathable) felt near the ridge - a hole in between each pair
of rafters.


I thought that maybe I could buy some cheap fly screen (small meshed
plastic/fabric) and cut it to glue over these holes. This has the
obviously advantage of keeping insects out of the space between the
felt and celotex (unless the glue fails), but the likely disadvantage
of getting blocked up by dust/dirt eventually, so preventing the air
flow that's vital to stopping the timbers rotting.


I was also going to stick little 1cm wedges of wood into the overlap
between adjacent sheets of felt to get even more airflow.


There are no dedicated vents in the soffits into the felt/celotex
void, but the woodwork is quite loosely nailed together, so there's
plenty of breeze. If I ever have it replaced with plastic, I'll ensure
that suitable vents are included along the length. I _have_ put vents
in from soffit level directly into the loft space itself, which run
through, but don't vent into, the felt/celotex void.


What does the group think of the fly screen idea? And the wedges?


I think I would go for fitting something like vented ridge tiles.

http://www.marleyeternit.co.uk/Roofing/Ventilation.aspx


Thank you. Looks very good, but well beyond my budget (and climbing
ability!).

Cheers,
David.



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Default loft ventilation - fly screen or open holes in felt?

In article ,
David Robinson writes:
It amazes me how different the responses are to this question. It
seems that opinion hasn't quite settled to a single standard answer
when it comes to loft ventilation and felt.
Given that pros often leave a loft in a state of permanent
condensation, while Victorian properties with none of the modern
practices fair well enough, I suspect we still have something to
learn.


Yes, there's this incorrect belief that loads of insulation on the
ceiling and plenty of ventilation will somehow stop condensation,
whereas it actually causes it. You have to have some heat leaking
into the loft space to stop condensation, because on a clear night
the loft will otherwise be colder that the outside air, so the
circulation of outside air into the colder loft will cause
condensation.

There's also a potential problem with highly insulating some walls,
in that the brickwork originally used some leaking heat from the
house to dry out, and without that it will be constantly holding more
water, hence spall and fall to bits faster (for similar reasons a
garden wall built from the same bricks as your house will only last
a fraction of the time).

I have a feeling that some of the super insulation initiatives we're
seeing now will result in some serious building damage in 20 or so
years time.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]
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