Dormer timber revisited - strengthening

[Tim Watts]

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

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

[Peter Crosland]

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

[Fredxxx]

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.

[Tim Watts]

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 bollocks please feel free to say so :) I find condensation a hard thing to full understand.

[Martin Bonner]

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.

No. The absolute humidity is higher *inside* remember

- water will be diffusing out.

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.

[Tim Watts]

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

[Andy Burns]

Or even 'Martin' :-)

[Tim Watts]

Bugger - sorry - got lost in the follow ups.

Thank you Martin :)

[meow2222]

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