Following on from earlier (related!) queries...

I'm insulating an existing dormer window, having pulled down the lath
and plaster; this has left exposed the 4" horizonal joists which form
the roof of the dormer (roof is about 5'x3'). Dormer roof is composed
of timber boards with lead(?) covering.

Plan is to insert 2" Kingspan insulation between the joists, and overlay
with vapour barrier and insulation-backed plasterboard. However, this
will leave a completely sealed cavity above the insulation and if there
is any ingress of damp in the future sounds like a recipe for dry rot,
no? The dormer is dry currently, but there's staining on the underside
of the now-exposed timbers so obviously it hasn't always been so.

So what to do? I could drill ventilation holes in the joists to connect
the separated cavities, not sure if that's worthwhile though? Moe
significantly I was wondering about drilling a row of upward slanting
0.5" holes in the front of the dormer above the window and into the
cavity - would that be sensible? (would need to stop insect ingress I
suppose...) And as the room will be completely vapour-sealed from
below, any future leaks to the dormer roof would probably go completely
undetected until really serious damage had been done.

Difficulty seems to be the merging of 1912 and 2006 building practices!

Thanks for any pointers
David

Lobster wrote:
Following on from earlier (related!) queries...

Plan is to insert 2" Kingspan insulation between the joists, and overlay
with vapour barrier and insulation-backed plasterboard. However, this
will leave a completely sealed cavity above the insulation and if there
is any ingress of damp in the future sounds like a recipe for dry rot,
no? The dormer is dry currently, but there's staining on the underside
of the now-exposed timbers so obviously it hasn't always been so.

On the underside of T&G floor boarding there are slots cut to prevent
forming a sealed box between the joists. You could cut similar shallow
slots in the beams/joists. When I built my Dormer I used 100m. thick
Rockwool Slab instead of Kingspan. It seems to absorb more sound.

Chris.

wrote:
Lobster wrote:
Following on from earlier (related!) queries...

Plan is to insert 2" Kingspan insulation between the joists, and overlay
with vapour barrier and insulation-backed plasterboard. However, this
will leave a completely sealed cavity above the insulation and if there
is any ingress of damp in the future sounds like a recipe for dry rot,
no? The dormer is dry currently, but there's staining on the underside
of the now-exposed timbers so obviously it hasn't always been so.

On the underside of T&G floor boarding there are slots cut to prevent
forming a sealed box between the joists. You could cut similar shallow
slots in the beams/joists.

Makes sense, except that this is an existing dormer which I ain't
dismantling!

Avoiding the 'sealed box' between joists sounds good; but that just
generates a larger 'sealed box' comprising the space between several
joists?? Hence the inquiry about venting that to the outside...

David

On Tue, 08 Aug 2006 12:50:19 GMT, Lobster
wrote:

Following on from earlier (related!) queries...

I'm insulating an existing dormer window, having pulled down the lath
and plaster; this has left exposed the 4" horizonal joists which form
the roof of the dormer (roof is about 5'x3'). Dormer roof is composed
of timber boards with lead(?) covering.

Plan is to insert 2" Kingspan insulation between the joists, and overlay
with vapour barrier and insulation-backed plasterboard. However, this
will leave a completely sealed cavity above the insulation and if there
is any ingress of damp in the future sounds like a recipe for dry rot,
no? The dormer is dry currently, but there's staining on the underside
of the now-exposed timbers so obviously it hasn't always been so.

So what to do? I could drill ventilation holes in the joists to connect
the separated cavities, not sure if that's worthwhile though? Moe
significantly I was wondering about drilling a row of upward slanting
0.5" holes in the front of the dormer above the window and into the
cavity - would that be sensible? (would need to stop insect ingress I
suppose...) And as the room will be completely vapour-sealed from
below, any future leaks to the dormer roof would probably go completely
undetected until really serious damage had been done.

Difficulty seems to be the merging of 1912 and 2006 building practices!

Thanks for any pointers
David

Hi,

Try asking on the forums on www.periodproperty.co.uk

cheers,
Pete.

Pete C wrote:
On Tue, 08 Aug 2006 12:50:19 GMT, Lobster
wrote:

Following on from earlier (related!) queries...

I'm insulating an existing dormer window, having pulled down the lath
and plaster; this has left exposed the 4" horizonal joists which form
the roof of the dormer (roof is about 5'x3'). Dormer roof is composed
of timber boards with lead(?) covering.

Plan is to insert 2" Kingspan insulation between the joists, and overlay
with vapour barrier and insulation-backed plasterboard. However, this
will leave a completely sealed cavity above the insulation and if there
is any ingress of damp in the future sounds like a recipe for dry rot,
no? The dormer is dry currently, but there's staining on the underside
of the now-exposed timbers so obviously it hasn't always been so.

So what to do? I could drill ventilation holes in the joists to connect
the separated cavities, not sure if that's worthwhile though? Moe
significantly I was wondering about drilling a row of upward slanting
0.5" holes in the front of the dormer above the window and into the
cavity - would that be sensible? (would need to stop insect ingress I
suppose...) And as the room will be completely vapour-sealed from
below, any future leaks to the dormer roof would probably go completely
undetected until really serious damage had been done.

Difficulty seems to be the merging of 1912 and 2006 building practices!


Try asking on the forums on www.periodproperty.co.uk

Hmm, they're a bit scary on the if you use phrases like "pulling down
lath and plaster" and "inserting Kingspan" they think you're a troll...

David

Lobster wrote:

I'm insulating an existing dormer window, having pulled down the lath
and plaster; this has left exposed the 4" horizonal joists which form
the roof of the dormer (roof is about 5'x3'). Dormer roof is composed
of timber boards with lead(?) covering.

Plan is to insert 2" Kingspan insulation between the joists, and overlay
with vapour barrier and insulation-backed plasterboard. However, this
will leave a completely sealed cavity above the insulation and if there
is any ingress of damp in the future sounds like a recipe for dry rot,
no? The dormer is dry currently, but there's staining on the underside
of the now-exposed timbers so obviously it hasn't always been so.

So what to do? I could drill ventilation holes in the joists to connect
the separated cavities, not sure if that's worthwhile though? Moe
significantly I was wondering about drilling a row of upward slanting
0.5" holes in the front of the dormer above the window and into the
cavity - would that be sensible? (would need to stop insect ingress I
suppose...)

Further to the above enquiry... tomorrow is the point-of-no-return day
so I've decided I'd better go for it and drill the holes as above.

These will be through the front of a decorative moulding, so difficult
to fit any form of grille over the front - so, anyone got any smart
ideas for a reliable and totally maintenance-free solution for
preventing insects entering a hole of that size... what could I shove up
which would stay put and prevent bugs while still permitting air ingress?

David

Lobster wrote:
Lobster wrote:

I'm insulating an existing dormer window, having pulled down the lath
and plaster; this has left exposed the 4" horizonal joists which form
the roof of the dormer (roof is about 5'x3'). Dormer roof is composed
of timber boards with lead(?) covering.

Plan is to insert 2" Kingspan insulation between the joists, and
overlay with vapour barrier and insulation-backed plasterboard.
However, this will leave a completely sealed cavity above the
insulation and if there is any ingress of damp in the future sounds
like a recipe for dry rot, no? The dormer is dry currently, but
there's staining on the underside of the now-exposed timbers so
obviously it hasn't always been so.

So what to do? I could drill ventilation holes in the joists to
connect the separated cavities, not sure if that's worthwhile though?
Moe significantly I was wondering about drilling a row of upward
slanting 0.5" holes in the front of the dormer above the window and
into the cavity - would that be sensible? (would need to stop insect
ingress I suppose...)

Further to the above enquiry... tomorrow is the point-of-no-return day
so I've decided I'd better go for it and drill the holes as above.

These will be through the front of a decorative moulding, so difficult
to fit any form of grille over the front - so, anyone got any smart
ideas for a reliable and totally maintenance-free solution for
preventing insects entering a hole of that size... what could I shove up
which would stay put and prevent bugs while still permitting air ingress?

David
You should have some kind of venting between the joist and either the
rest of the loft space or the outside ideally.

Holes is no bad thing, but really there is probably some leakage anyway.
The staining is probably due to failure of the lead at some time past.

The Natural Philosopher wrote:
Lobster wrote:
Lobster wrote:

I'm insulating an existing dormer window, having pulled down the lath
and plaster; this has left exposed the 4" horizonal joists which form
the roof of the dormer (roof is about 5'x3'). Dormer roof is
composed of timber boards with lead(?) covering.

Plan is to insert 2" Kingspan insulation between the joists, and
overlay with vapour barrier and insulation-backed plasterboard.
However, this will leave a completely sealed cavity above the
insulation and if there is any ingress of damp in the future sounds
like a recipe for dry rot, no? The dormer is dry currently, but
there's staining on the underside of the now-exposed timbers so
obviously it hasn't always been so.

So what to do? I could drill ventilation holes in the joists to
connect the separated cavities, not sure if that's worthwhile
though? Moe significantly I was wondering about drilling a row of
upward slanting 0.5" holes in the front of the dormer above the
window and into the cavity - would that be sensible? (would need to
stop insect ingress I suppose...)

Further to the above enquiry... tomorrow is the point-of-no-return day
so I've decided I'd better go for it and drill the holes as above.

These will be through the front of a decorative moulding, so difficult
to fit any form of grille over the front - so, anyone got any smart
ideas for a reliable and totally maintenance-free solution for
preventing insects entering a hole of that size... what could I shove
up which would stay put and prevent bugs while still permitting air
ingress?


You should have some kind of venting between the joist and either the
rest of the loft space or the outside ideally.

Unfortunately there's no way through to the rest of the loft space -
complicated to explain, but trust me on that! So yes, it's vent holes
through to the outside, but through this moulding not a flat face, so
the followup question was how to prevent bugs from getting in...

David





Holes is no bad thing, but really there is probably some leakage anyway.
The staining is probably due to failure of the lead at some time past.

On Mon, 14 Aug 2006 23:50:17 GMT, Lobster
wrote:

Lobster wrote:

I'm insulating an existing dormer window, having pulled down the lath
and plaster; this has left exposed the 4" horizonal joists which form
the roof of the dormer (roof is about 5'x3'). Dormer roof is composed
of timber boards with lead(?) covering.

Plan is to insert 2" Kingspan insulation between the joists, and overlay
with vapour barrier and insulation-backed plasterboard. However, this
will leave a completely sealed cavity above the insulation and if there
is any ingress of damp in the future sounds like a recipe for dry rot,
no? The dormer is dry currently, but there's staining on the underside
of the now-exposed timbers so obviously it hasn't always been so.

So what to do? I could drill ventilation holes in the joists to connect
the separated cavities, not sure if that's worthwhile though? Moe
significantly I was wondering about drilling a row of upward slanting
0.5" holes in the front of the dormer above the window and into the
cavity - would that be sensible? (would need to stop insect ingress I
suppose...)

Further to the above enquiry... tomorrow is the point-of-no-return day
so I've decided I'd better go for it and drill the holes as above.

These will be through the front of a decorative moulding, so difficult
to fit any form of grille over the front - so, anyone got any smart
ideas for a reliable and totally maintenance-free solution for
preventing insects entering a hole of that size... what could I shove up
which would stay put and prevent bugs while still permitting air ingress?

David

I'd be more worried by rain being blown in than insects.

When I did what you are planning the dormer ceiling void was open to
the attic. After I'd completed it, it wasnt nice to see all the dew
drops forming on the t&g over the dormer when it turned cold. I put it
down to damp from some new plastering and I've put an air brick in the
attic and I'll see what happens this winter.

marvelus wrote:
On Mon, 14 Aug 2006 23:50:17 GMT, Lobster
wrote:

Lobster wrote:

I'm insulating an existing dormer window, having pulled down the lath
and plaster; this has left exposed the 4" horizonal joists which form
the roof of the dormer (roof is about 5'x3'). Dormer roof is composed
of timber boards with lead(?) covering.

Plan is to insert 2" Kingspan insulation between the joists, and overlay
with vapour barrier and insulation-backed plasterboard. However, this
will leave a completely sealed cavity above the insulation and if there
is any ingress of damp in the future sounds like a recipe for dry rot,
no? The dormer is dry currently, but there's staining on the underside
of the now-exposed timbers so obviously it hasn't always been so.

So what to do? I could drill ventilation holes in the joists to connect
the separated cavities, not sure if that's worthwhile though? Moe
significantly I was wondering about drilling a row of upward slanting
0.5" holes in the front of the dormer above the window and into the
cavity - would that be sensible? (would need to stop insect ingress I
suppose...)
Further to the above enquiry... tomorrow is the point-of-no-return day
so I've decided I'd better go for it and drill the holes as above.

These will be through the front of a decorative moulding, so difficult
to fit any form of grille over the front - so, anyone got any smart
ideas for a reliable and totally maintenance-free solution for
preventing insects entering a hole of that size... what could I shove up
which would stay put and prevent bugs while still permitting air ingress?

I'd be more worried by rain being blown in than insects.

Point taken - I'd hoped that angling the holes downwards would help on
that front but let's add that problem to the criteria then!

David

Lobster wrote:


You should have some kind of venting between the joist and either the
rest of the loft space or the outside ideally.

Unfortunately there's no way through to the rest of the loft space -
complicated to explain, but trust me on that! So yes, it's vent holes
through to the outside, but through this moulding not a flat face, so
the followup question was how to prevent bugs from getting in...

Oh. ask at the builders merchant. There are either grilles or mesh
available for that. Or staple perf. zinc etc. over the holes.

The Natural Philosopher wrote:
Lobster wrote:


You should have some kind of venting between the joist and either the
rest of the loft space or the outside ideally.

Unfortunately there's no way through to the rest of the loft space -
complicated to explain, but trust me on that! So yes, it's vent holes
through to the outside, but through this moulding not a flat face, so
the followup question was how to prevent bugs from getting in...

Oh. ask at the builders merchant. There are either grilles or mesh
available for that. Or staple perf. zinc etc. over the holes.

Thanks, I've been and done just that at TP. Doesn't seem to be anything
to suit my purpose really but I bought a length of soffit vent grille
stuff, and with some judicious carving with a Stanley knife have
produced a length of L-section uPVC grille which doesn't look too awful
from ground level! And will hopefully stave off my rot worries.

(Got the undivided attention of a Very Nice Man at the hated TP for 20
mins, who sorted me out for 3 GBP - so that's a first!)

David