What to use to insulate workshop roof.

[MikeG.]

Colin is spot on above the roof.........indeed, Tyvek also have a suggested fixing method without ply or boarded sarking where there product is pulled tight and joints taped....


....but I'll add to what he says about below the ply (or OSB) deck.

These sheeting boards is enormously resistant to the passage of vapour, and therefore there is always a danger of condensation forming on the underside of it. This means you should always provide a ventilated void below the ply. It also means that the more expensive breathable membranes, such as Tyvek, could, IMO, be a waste of money above the ply. You are effectively putting something vapour permeable directly over a vapour barrier. Save your money and use old fashioned felt instead.

Mike

 

[Dibs-h]

Colin is spot on above the roof.........indeed, Tyvek also have a suggested fixing method without ply or boarded sarking where there product is pulled tight and joints taped....


....but I'll add to what he says about below the ply (or OSB) deck.

These sheeting boards is enormously resistant to the passage of vapour, and therefore there is always a danger of condensation forming on the underside of it. This means you should always provide a ventilated void below the ply. It also means that the more expensive breathable membranes, such as Tyvek, could, IMO, be a waste of money above the ply. You are effectively putting something vapour permeable directly over a vapour barrier. Save your money and use old fashioned felt instead.

Mike

If the space between the rafters was insulated, then vapour barrier applied - then plasterboarded, would one not be minimising the risk of condensation forming on the underside of the ply\osb? To my mind if the underside of the roof has a vapour barrier - is that not minimising (perhaps to zero) the passage of vapour into the roof and therefore reducing the chance of condensation on the undeside of the osb\ply external deck? By vapour barrier, I mean something like Tyvek's SD2 product.

As pointed out the counterbattons would allow run-off and prevent water pooling, etc.

I recall seeing method statements from Tyvek relating to roofs with sarking board, breathable membrane stretched across and then counterbattoned. Is the ply\osb deck not analogous? Or is the fact that regular sarking (i.e. timber) is nowhere as resistant to the passage of moisture compared with ply\osb?

The reasoning behind the ply\osb was 2 fold - 1 to make the (raised tie) trussed roof more rigid and mimise the lateral thrust on the (essentially) single skin blockwork and as importantly reduce the chance of some toerag removing a few tiles and making their way in. There's a boundary wall along the back of the workshop, which could make life easy for the toerag as it's nearly 6 feet tall and the eaves are only 10 feet tall.

If the sheeting was fitted to the inside of the trusses - would that not achieve my aims (mostly) - i.e. make the roof more rigid and make breaking in just as difficult, but prevent condensation build up in the roof?

 

[MikeG.]

Careful here.........sheathing will stiffen up the roof, but won't do anything at all to obviate spreading. The roof will still try just as hard to push your walls over.

Look, in an intermittently occupied shed without great heating and fixed water-sources (sinks, toilets showers etc), you can get away with poorer design and construction standards. Make sure the ventilation within the shed is dealt with properly and I'm sure you'll be fine. However, this is not good building practice, and if I advocate unorthodox and questionable building details on an open forum, someone, somewhere will take it out of context and use it on, say, a lean-to outbuilding with a washing machine and tumble drier in it......the thing will fall apart within 5 years and my reputation, and possibly livelihood, would be under threat. I give advice here very freely and openly, but actually very carefully, mindful that not everyone will apply all of it, or will possibly apply it in the wrong circumstances.

Use Tyvek breathable sarking if you want. Fine. However, it is vastly more expensive than felt, and in these circumstances gives you no additional benefit.

Mike

 

[CWatters]

If the space between the rafters was insulated, then vapour barrier applied - then plasterboarded, would one not be minimising the risk of condensation forming on the underside of the ply\osb? To my mind if the underside of the roof has a vapour barrier - is that not minimising (perhaps to zero) the passage of vapour into the roof and therefore reducing the chance of condensation on the undeside of the osb\ply external deck? By vapour barrier, I mean something like Tyvek's SD2 product.

In theory the vapour barrier should do it's job yes but from what I read you need a belt and braces approach. Vapour barrier one side and means of escape (permeable or ventilated) on the other.

 

[CWatters]

The reasoning behind the ply\osb was 2 fold - 1 to make the (raised tie) trussed roof more rigid and mimise the lateral thrust on the (essentially) single skin blockwork

No it won't stop thrust. In fact the extra weight might increase it. You either need joists OR a structural ridge beam.

With a structural ridge beam the rafters "hang" from the ridge beam. The ridge beam transfers the roof load to the gable end walls which also need to stronger. The rafters need to be attached to the ridge with more than just the nails used on an ordinary joist roof - perhaps with galvanised straps over the top of the ridge beam to tie the rafters together.

I'm NOT a Structtural Engineer so best seek professional advice for this sort of roof.

 

[Dibs-h]

Mike\Colin

I really appreciate the advice.

If one ditches the ply\osb - as you say it won't alleviate any of the spreading. I'd quite happy live without it - however my concern in regard to security remains, I'm not to keen on a toerag being able to lift a few tiles out, tear the membrane\felt and drop through. Plasterboard doesn't really do anything. Any suggestions?

What about putting say 10-12mm ply (or osb) on the inside of the roof\room? That way I'd get my vapur barrier and make penetration more difficult.

I suppose the reason it keeps coming back to ply - is that for say a nominal 1/2 inch - it isn't easy to get through it.

Cheers.

p.s. in a situation where what appears to be T&G boards have been used as sarking - does the space behind the boards have to be vented?

Edit: Just come across this,

http://www.buildbase.co.uk/buildbase_ro ... mroof.html

"As Scottish practice is for the use of timber sarking, which has a high resistance to the transmission of water vapour, it is advisable to conduct a condensation risk analysis.

Where using proper timber sarking planks (usually 150mm wide) laid with a 2mm gap between the boards, there is no need to provide additional ventilation below the membrane. However, where using sarking boards comprising sheets of plywood or OSB, this type of warm unventilated roof configuration is not recommended. In a warm roof where the membrane is supported on a plywood or OSB sarking board, ventilation of the mandatory 50mm airspace below the sarking board is required, in accordance with BS 5534 (BS 5250: 2002). Under these circumstances there is no requirement for further ventilation above the membrane.

Use of counterbattens above the membrane (installed on a sarking board) is required where tiles or slates are to be installed on battens. In some instances tiles or slates can be installed by nailing through the membrane directly into the sarking board. However, the tile manufacturer’s instructions must be strictly adhered to."

 

[CWatters]

Speaking of condensation risk analysis I posted the question here about the need for ventilation under the plywood.

http://www.greenbuildingforum.co.uk/for ... 167&page=1

Basically it seems it's a marginal call. In theory the risk analysis says it should be ok without ventilation but ... well best read the replies.

Plywood on the inside would work eg

Tiles
battens
counter battens
Permeable membrane (one approved for no gap)
Insulation between rafters
Plywood
Vapour control layer
Plasterboard

As would a warm roof construction with the insulation above the rafters..

Tiles
battens
counter battens
Permeable membrane (one approved for no gap)
Insulation
Plywood
Rafters (with no insulation between)
Vapour control layer
Plasterboard

The issue with this is the need for long screws to fix the battens through the insulation into the rafters. I believe special screw systems exist for this and some include aligning tools/jigs to get the screws in straight but I've not tried tried this method. The condensation risk is eliminated with this approach because the rafters and plywood are totally on the warm side of the insulation.

The other issue is the thickness of the roof at the gutters. Needs a little care.

A warm roof is great for a barn conversion because it allows the rafters to be exposed. In that case you would use a foil backed plasterboard between the beams.

 

[MikeG.]

Dibs,

This job of yours woke me up this morning.

It is a while since I looked at this, but I realised at 5.30 this morning that I had got your roof wrong. Please disregard all of my earlier advice. I had conflated in my mind your roof description with the photos at the start of the thread, which clearly had OSB on the top of the rafters. When you said your roof was to be lines with OSB under the rafters, all my advice assumed that this was in addition to lining the top!!!! It was thinking about you saying you need the sheathing for burglar-resistance that actually woke me up.

I have re-read your description.......and what you are doing is fine.

Can I just suggest that in future we have new threads for new projects? Here we have 2 different rooves under one thread......and if it confused me who was paying some sort of attention, how much will this have confused casual browsers?

So humble apologies to all misled by the nonsense I spouted earlier, and Dibs.........carry on!

Mike

 

[mahking51]

Mike,
Proposed build of wall from inside to out is:

18mm OSB, painted white
50mm polystyrene fitted tightly between the 4x2 treated studding. KINGSPAN way too expensive!
Breathable membrane (TYVEK HOME WRAP?) laid to fall, 1m wide rolls, stapled to outside of studding.
2x1 treated battens
15mm T&G treated
Insect mesh at bottom of battened space

Roof will be OSB with fibreglass 600 gram layup and top coat, side and front flashings with grooved flashing kit to return wall.
Underside of OSB there will be 50mm gap then 100mm insulation then OSB or plasterborard ceiling.

Sound OK?
Cheers and thanks for the time you put in on these problems on our behalf, much appreciated.
Martin

 

[Dibs-h]

Dibs,

This job of yours woke me up this morning.

It is a while since I looked at this, but I realised at 5.30 this morning that I had got your roof wrong. Please disregard all of my earlier advice. I had conflated in my mind your roof description with the photos at the start of the thread, which clearly had OSB on the top of the rafters. When you said your roof was to be lines with OSB under the rafters, all my advice assumed that this was in addition to lining the top!!!! It was thinking about you saying you need the sheathing for burglar-resistance that actually woke me up.

I have re-read your description.......and what you are doing is fine.

Can I just suggest that in future we have new threads for new projects? Here we have 2 different rooves under one thread......and if it confused me who was paying some sort of attention, how much will this have confused casual browsers?

So humble apologies to all misled by the nonsense I spouted earlier, and Dibs.........carry on!

Mike

Mike

Sorry about the sleep disturbance and many thanks for the reply, and the advice - I'll start a new thread, should someone else's spark off a question or two.

Cheers.

Dibs

p.s. Colin - Cheers for the link and info.

 

[MikeG.]

Martin,

that's fine. You really don't need 18mm OSB on the inside, though..........that'll be really really heavy, and more expensive than necessary. 10 or 12mm will be fine.

Secondly, can't you fully fill the studs with polystyrene? If not, then make sure the insulation is hard up to the inside of the wall....ie, up against the OSB inner lining.

Just be aware that polystyrene is a real fire hazard. If that ever goes up in flames, keep very well away.........don't try and rescue your favourite plane!

Mike

 

[Paddywack]

Hi,
I have a ready build roof on a building that I am going to use for my workshop, is it ok for me to use Spray Foam and have no air gap and then Plaster board on the ceiling up tight to the foam?

 

[Dibs-h]

Hi,
I have a ready build roof on a building that I am going to use for my workshop, is it ok for me to use Spray Foam and have no air gap and then Plaster board on the ceiling up tight to the foam?

I wouldn't have thought so to be honest. I've seen a few "companies" offering to do the same for domestic properties - backed apparently by Insurance guarantees.

All that means to me is that the insurance company see little chance in liability going their way.

Without more details on your particular roof construction - the short answer would be no.