I have a dormer build (badly in the 70's) with 3x3" timbers:

https://www.flickr.com/photos/timjwa...7645379974909/

2nd photo shows the depravity of the joinery.

When I have the roof retiled (inc dormer faces with hung tiles), I got
50mm celotex put between plus a breathable membrane to BCO's
satisfaction. Now I want to add some rigidity to the structure. And
another 50mm insulation which was always the plan.

---------


My initial idea was to plate across with 18mm ply, glued and screwed,
then celotex, then plasterboard. But I think interstitial condensation
could be a problem.


My second idea is to add 2x3" timbers as in picture 3, screwed through
with plastic DPC between new and old timbers. Fill in between with 50mm
celotex.

Then plate that with ply glued and screwed as in picture 4 (obviously
cutout around the windows - please excuse crappy drawing).


This will give 100% vapour barrier in the middle and the ply should
stiffen the structure to prevent shearing. Shearing would be bad as it
will eventually put strain on the windows and there's no diagonal bracing.


Are there any improvements I could make to this plan?

Main item is with plastic DPC between the timbers, that will have little
friction so I'm relying on lots of big screws to lock it together.


Cheers

Tim

On 20/07/2014 10:27, Tim Watts wrote:
I have a dormer build (badly in the 70's) with 3x3" timbers:

https://www.flickr.com/photos/timjwa...7645379974909/

2nd photo shows the depravity of the joinery.

When I have the roof retiled (inc dormer faces with hung tiles), I got
50mm celotex put between plus a breathable membrane to BCO's
satisfaction. Now I want to add some rigidity to the structure. And
another 50mm insulation which was always the plan.

---------


My initial idea was to plate across with 18mm ply, glued and screwed,
then celotex, then plasterboard. But I think interstitial condensation
could be a problem.


My second idea is to add 2x3" timbers as in picture 3, screwed through
with plastic DPC between new and old timbers. Fill in between with 50mm
celotex.

Then plate that with ply glued and screwed as in picture 4 (obviously
cutout around the windows - please excuse crappy drawing).


This will give 100% vapour barrier in the middle and the ply should
stiffen the structure to prevent shearing. Shearing would be bad as it
will eventually put strain on the windows and there's no diagonal bracing.


Are there any improvements I could make to this plan?

Main item is with plastic DPC between the timbers, that will have little
friction so I'm relying on lots of big screws to lock it together.


Make friends with your local BCO. They have seen most thing in their
careers and if approached politely they are usually very helpful.

--
Peter Crosland

Reply address is valid

On 20/07/2014 10:27, Tim Watts wrote:
I have a dormer build (badly in the 70's) with 3x3" timbers:

https://www.flickr.com/photos/timjwa...7645379974909/

2nd photo shows the depravity of the joinery.

When I have the roof retiled (inc dormer faces with hung tiles), I got
50mm celotex put between plus a breathable membrane to BCO's
satisfaction. Now I want to add some rigidity to the structure. And
another 50mm insulation which was always the plan.

---------


My initial idea was to plate across with 18mm ply, glued and screwed,
then celotex, then plasterboard. But I think interstitial condensation
could be a problem.


My second idea is to add 2x3" timbers as in picture 3, screwed through
with plastic DPC between new and old timbers. Fill in between with 50mm
celotex.

Then plate that with ply glued and screwed as in picture 4 (obviously
cutout around the windows - please excuse crappy drawing).


This will give 100% vapour barrier in the middle and the ply should
stiffen the structure to prevent shearing. Shearing would be bad as it
will eventually put strain on the windows and there's no diagonal bracing.


Are there any improvements I could make to this plan?

Main item is with plastic DPC between the timbers, that will have little
friction so I'm relying on lots of big screws to lock it together.

The vapour barrier should be on the hottest item, being the
plasterboard. So foil backed plasterboard is perhaps best. There should
also be some ventilation in the void on the colder side.

I don't think putting a DPM/DPC in the middle of the wall is a good
idea, as it will trap moisture precisely where you don't want it.

On 21/07/14 19:30, Fredxxx wrote:
On 20/07/2014 10:27, Tim Watts wrote:

The vapour barrier should be on the hottest item, being the
plasterboard. So foil backed plasterboard is perhaps best. There should
also be some ventilation in the void on the colder side.

I don't think putting a DPM/DPC in the middle of the wall is a good
idea, as it will trap moisture precisely where you don't want it.

Yes - I thought about that.

Thanks for thinking about it - it's a hard problem.

The reason for me thinking it should be in the middle is:

I have 3x3" studwork exposed on the outside to cold and damp.
This is fine as these can breath in and out the same way through the
external tyvek membrane under the tiles.

However, if I sheet these with ply, then cover with a vapour barrier (my
original and structurally best idea) then I am worried the follwing will
happen:

Looking top down through a section:


outside

1 2
TTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTT
SSS SSS SSS
SSSCCCCCCCCCCCCSSSCCCCCCCCCCCCCSSS
PPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPP
----------------------------------
CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC
BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB

room side



T = tiles on battens
S=3x3" stud
P = 18mm ply
- = vapour barrier (in practice the foil faced celotex)
C = celotex
B = plasterBoard

I *think* that moisture will travel though the studs then through the
ply to the midpoints 1 and 2 inside the ply.

Then the temperature will drop at night and the water will condense.

Water in the studs SSS will quickly find a way out but water at points 1
and 2 will find it fairly hard to get out again.


So my revised idea as originally described:

outside

1 2
TTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTT
SSS SSS SSS
SSSCCCCCCCCCCCCSSSCCCCCCCCCCCCCSSS
----------------------------------
SSSCCCCCCCCCCCCSSSCCCCCCCCCCCCCSSS
PPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPP
BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB

room side


Allows the outside studs to breath. The ply is on the warm side and the
inside studs next to the ply can breath back into the room.




Now, if you think my concerns are ******** please feel free to say so
I find condensation a hard thing to full understand.

On Monday, 21 July 2014 20:32:15 UTC+1, Tim Watts wrote:
On 21/07/14 19:30, Fredxxx wrote:
On 20/07/2014 10:27, Tim Watts wrote:

The vapour barrier should be on the hottest item, being the
plasterboard. So foil backed plasterboard is perhaps best.
There should also be some ventilation in the void on the colder side.

Yup.

I don't think putting a DPM/DPC in the middle of the wall is a good
idea, as it will trap moisture precisely where you don't want it.

+1

I have 3x3" studwork exposed on the outside to cold and damp.

This is the mistake you are making. The outside is cold, but it
is not actually damper than the inside - not in the important way
at least: grams of water / cubic m (g/m3). That will almost
certainly be *higher* on the inside. The *relative* humidity
is higher outside - but that doesn't matter.

Interstitial condensation happens when air with a high absolute
humidity cools down below its dew point, as it diffuses out
through the structure. As such, the vapour barrier needs to be
immediately behind the plaster board.

Looking top down through a section:

outside
1 2
TTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTT
SSS SSS SSS
SSSCCCCCCCCCCCCSSSCCCCCCCCCCCCCSSS
PPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPP
----------------------------------
CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC
BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB

room side


T = tiles on battens
S=3x3" stud
P = 18mm ply
- = vapour barrier (in practice the foil faced celotex)
C = celotex
B = plasterBoard

I *think* that moisture will travel though the studs then through the
ply to the midpoints 1 and 2 inside the ply.

No. The absolute humidity is higher *inside* remember
- water will be diffusing out.

Then the temperature will drop at night and the water will condense.
I doubt it. Because the ply is inside the first layer of celotex, it
will be appreciably warmer, so there won't be condensation there.

On 30/07/14 13:05, Martin Bonner wrote:
On Monday, 21 July 2014 20:32:15 UTC+1, Tim Watts wrote:
On 21/07/14 19:30, Fredxxx wrote:
On 20/07/2014 10:27, Tim Watts wrote:

The vapour barrier should be on the hottest item, being the
plasterboard. So foil backed plasterboard is perhaps best.
There should also be some ventilation in the void on the colder side.

Yup.

I don't think putting a DPM/DPC in the middle of the wall is a good
idea, as it will trap moisture precisely where you don't want it.

+1

I have 3x3" studwork exposed on the outside to cold and damp.

This is the mistake you are making. The outside is cold, but it
is not actually damper than the inside - not in the important way
at least: grams of water / cubic m (g/m3). That will almost
certainly be *higher* on the inside. The *relative* humidity
is higher outside - but that doesn't matter.

Interstitial condensation happens when air with a high absolute
humidity cools down below its dew point, as it diffuses out
through the structure. As such, the vapour barrier needs to be
immediately behind the plaster board.

Looking top down through a section:

outside
1 2
TTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTT
SSS SSS SSS
SSSCCCCCCCCCCCCSSSCCCCCCCCCCCCCSSS
PPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPP
----------------------------------
CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC
BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB

room side


T = tiles on battens
S=3x3" stud
P = 18mm ply
- = vapour barrier (in practice the foil faced celotex)
C = celotex
B = plasterBoard

I *think* that moisture will travel though the studs then through the
ply to the midpoints 1 and 2 inside the ply.

No. The absolute humidity is higher *inside* remember
- water will be diffusing out.

Then the temperature will drop at night and the water will condense.
I doubt it. Because the ply is inside the first layer of celotex, it
will be appreciably warmer, so there won't be condensation there.


Wow. It seems so much clearer when you explain it like that...

John, you have the gift of clear and ration explanation.


So my original original idea of ply sheeting across the beams, glued and
screwed is probably the best one then?

Followed by 50mm celotex and PB. The celotex will also provide a good
vapour barrier if taped.

It would certainly make a very stiff structure for only 18mm additional
loss of space (each side). I'm happy losing 70mm over both ends and both
sides.

Cheers!


Tim

Tim Watts wrote:

Martin Bonner wrote:

The outside is cold, but it is not actually damper
than the inside - not in the important way

Wow. It seems so much clearer when you explain it like that...
John, you have the gift of clear and ration explanation.

Or even 'Martin' :-)

On 30/07/14 15:10, Andy Burns wrote:
Tim Watts wrote:

Martin Bonner wrote:

The outside is cold, but it is not actually damper
than the inside - not in the important way

Wow. It seems so much clearer when you explain it like that...
John, you have the gift of clear and ration explanation.

Or even 'Martin' :-)


Bugger - sorry - got lost in the follow ups.

Thank you Martin

On Monday, July 21, 2014 8:32:15 PM UTC+1, Tim Watts wrote:
On 21/07/14 19:30, Fredxxx wrote:
On 20/07/2014 10:27, Tim Watts wrote:

The vapour barrier should be on the hottest item, being the
plasterboard. So foil backed plasterboard is perhaps best. There should
also be some ventilation in the void on the colder side.

I don't think putting a DPM/DPC in the middle of the wall is a good
idea, as it will trap moisture precisely where you don't want it.

Yes - I thought about that.
Thanks for thinking about it - it's a hard problem.
The reason for me thinking it should be in the middle is:
I have 3x3" studwork exposed on the outside to cold and damp.
This is fine as these can breath in and out the same way through the
external tyvek membrane under the tiles.
However, if I sheet these with ply, then cover with a vapour barrier (my
original and structurally best idea) then I am worried the follwing will
happen:
Looking top down through a section:
outside
1 2
TTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTT
SSS SSS SSS
SSSCCCCCCCCCCCCSSSCCCCCCCCCCCCCSSS
PPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPP
----------------------------------
CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC
BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB
room side
T = tiles on battens
S=3x3" stud
P = 18mm ply
- = vapour barrier (in practice the foil faced celotex)
C = celotex
B = plasterBoard
I *think* that moisture will travel though the studs then through the
ply to the midpoints 1 and 2 inside the ply.
Then the temperature will drop at night and the water will condense.
Water in the studs SSS will quickly find a way out but water at points 1
and 2 will find it fairly hard to get out again.
So my revised idea as originally described:
outside
1 2
TTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTT
SSS SSS SSS
SSSCCCCCCCCCCCCSSSCCCCCCCCCCCCCSSS
----------------------------------
SSSCCCCCCCCCCCCSSSCCCCCCCCCCCCCSSS
PPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPP
BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB
room side
Allows the outside studs to breath. The ply is on the warm side and the
inside studs next to the ply can breath back into the room.
Now, if you think my concerns are ******** please feel free to say so
I find condensation a hard thing to full understand.

Both those diagrams are no-go. Your vapour barrier needs to be on the warmest side of all insulation. Having a vb further out is asking for trouble. I'd probably peel the foil face off both sides of the existing whatsittherm.


NT