Re: Timber garden office vapour barrier

[iamSteve]

Hi,

I need some advise please. I am building a garden office, the walls are built from 4x2 timber and the roof is supported by 9x2 joists and 4 to 0 furrings.

My question relates to the wall and ceiling vapour barrier... I guess I will need one in an insulated building to allow the condensation vapour to escape, however where does is go. The current wall construction is a 4x2 frame with 12mm plywood on the outside and soon to be fitted 19mm T&G cladding, I will then be filling the cavity with insulation (prob kinspan or equivalent) then 12mm plaster board on the inside to finish.

Where does the vapour barrier go, between exterior ply and T&G or between ply and insulation, or insulation and plasterboard. I would guess it would go on the warm inside between plaster and insulation but i am unsure?

Regards,
Steve.

 

[Alastairreid]

duplex foil backed p/b will suffice.

Builders paper stapled to external ply.

ext cladding fixed to vertical battens.

 

[iamSteve]

Thank you for your reply!

So I should not fit the cladding flush/direct to the ply then, but use some battens to allow air flow?

...and as you suggest builders paper or similar to prevent damp in the ply board.

I will see what my local BM's have in the morning.

Steve.

 

[Alastairreid]

So I should not fit the cladding flush/direct to the ply then, but use some battens to allow air flow?

Yes, minimum 22mm @ 600c/c

View media item 54006

 

[iamSteve]

Thanks for the information.

I have purchased 50mmx25mm battens and also WebUV25 Membrane, to complete the task.

 

[iamSteve]

Thank you for your assistance alastairreid

This is the garden office wall construction thanks to your assistance.

External Wall

• iamSteve
• 28 Dec 2012
• 1

Ply, covered in WEBUV25 Membrane and then 25x50 battens and Cladding.

Just in case anyone wonders why the battens stop short of the top of the wall, it is because some work needs to be undertaken to finish the roof section before the cladding is taken to the top and I wanted decent lengths overlapping the wall, joist, and roof rather than a 300mm piece of batten wobbling about on the top!

 

[iamSteve]

Same subject but different area of the office....

I have the joists and furrings in place with 18mm ply as the roof deck, once complete I plan to cover the deck with Firestone EPDM Heavy Duty 1.52mm Membrane stuck directly to the ply.
Like the walls I intend to use foil backed plasterboard over the insulation.

Where in the roof should I fit a vapour barrier? If at all?

Internal Roof Structure

• iamSteve
• 28 Dec 2012
• 1

The small noggins are just to support the ply joints, I still need to install the 'proper' joist noggins, that will also support the plasterboard joints.

 

[Alastairreid]

Looking good Steve, well done.

There are 2 different types of flat roof construction cold deck where the insulation is below the deck.
And warm deck where the insulation is fitted above the deck.

Your construction will be a cold deck! rigid insulation boards would be fitted between the rafters with a 50mm airgap between the insulation and deck, this method also requires ventilation at the facing and ideally at ridge/abutment.
Either that omit the airgap and pack the joists solid with rigid boards and use expanding foam any gaps.

Foil backed boards ..duplex.. incorporate a vapour barrier.

 

[Alastairreid]

I still need to install the 'proper' joist noggins, that will also support the plasterboard joints.

You dont need noggins for the p/b run across the joist staggered is sufficient.

Run 1 line of noggins mid span.

 

[iamSteve]

Thanks once again.

Yes my roof is a cold deck design and will have a 50mm air gap at one end and 150mm air gap at the other end, so a minimum of 50mm gap between the ply and the insulation anywhere on the roof.

The joists overhang the outer ply wall by 100mm at the rear (bottom of the slope) and 500mm in the front. and the deck slopes from 100mm to 0mm over the 4.8m joist length.

I am still trying to find a nice way to install vents, especially at the front. I was looking at the round 70mm plastic vents, but was thinking of maybe as the battens for the cladding will now be fitted length ways in the front overhang would I be able to fit fewer, say 4 rectangular stainless steal ones over 6 meters as the air can flow freely between joists in the space the battens create.... if that makes sense?

The rear of the building I am looking for a 50mm vent to run along the entire length, probably plastic so this should not cause a problem.

 

[iamSteve]

I still need to install the 'proper' joist noggins, that will also support the plasterboard joints.

You dont need noggins for the p/b run across the joist staggered is sufficient.

Run 1 line of noggins mid span.

Mid span is roughly where the PB will join anyway.

 

[iamSteve]

Just to assist with my explanation regarding roof ventilation above.

Front Joist over hang, I assume I will need to trim out the membrane to allow free air flow under roof deck, was thinking of fixing batons along the front and using the cladding offcuts (opposite way to the wall)

Roof Front

• iamSteve
• 28 Dec 2012
• 1

Front overhang, for ventilation and LED lighting

Rear Joist over hang, was originally 100mm, but after the battens and cladding it will be about a 60mm gap to fill with some sort of vent.

Roof Rear

• iamSteve
• 28 Dec 2012
• 1