# What to use to insulate workshop roof.



## garywayne (15 Jun 2009)

Hi all.

I have been attempting to use fibreglass type insulation to insulate between the roof joists. So far it has been a reel pain in the a**. I am now really pi**** off.

What in your opinion is the easiest (bearing in mind I am doing this on my own) and cheapest way to insulate my w\shop roof. The depth to be filled is 100mm.

A picture to show what the roof joists look like.

[img




[/img]

The shed is a lot further on than in the picture.

I really need to finish this w\shop. Its been going on far to long.


----------



## MikeG. (15 Jun 2009)

Where are you insulating, Gary.......between the joists or between the rafters?

Mike


----------



## garywayne (15 Jun 2009)

Rafters, I think?  The bits that hold up the roof at an angle.


----------



## MikeG. (15 Jun 2009)

Yep...the slopy ones are rafters!!

How deep are they?

Mike


----------



## garywayne (15 Jun 2009)

100mm


----------



## John C (15 Jun 2009)

Gary on site we use stick pins to stop insulation dropping in studded walls, your only using small pieces so maybe a can of spraytak or a roll of double sided carpet tape would help hold the insulation in place . cheers john c
50mm rips of thin ply cut tight to make a spring batten.


----------



## Shultzy (16 Jun 2009)

Gary, I used fine fishing line stapled to the rafters, cheap and it doesn't interfere with the boards.


----------



## MikeG. (16 Jun 2009)

Sorry Gary........I'm not ignoring you, but we had no power all evening. I'll respond in an hour or so when I get to work.

Mike


----------



## MikeG. (16 Jun 2009)

Right Gary..........I'm afraid it is a bit more complicated than just holding insulation up.

You shouldn't be insulating to the underside of your ply sheet. There should be a 50mm air gap between the top of the insulation and the underside of the ply, and this should be open to the outside to let air in.

Given the difficulty of trying to hold a floppy bit of mineral wool a set distance from the ply, whilst supporting it from the underside, I suggest that it is the wrong material. To do this job properly I think you are going to have to use one of the rigid board insulants.......Kingspan or Celotex..............or you are going to have to insulate at joist ("ceiling") level. Even then, you will need to admit air into the void.

Mike


----------



## CWatters (16 Jun 2009)

What Mike said..

Our house was insulated using Celotex which is a rigid foam. Each bit was cut to size with a hand saw and wedged in. If the sheet is a bit loose put a nail into the side of the rafter to hold it up. 

It looks like you are building is a so called "cold roof, warm loft" design. eg The insulation is between the rafters so top edge of the rafters and OSB are cold. (Aside: A proper "warm roof" has the insulation above the rafters. This is commonly done on new flat roofs and houses built using SIPS).

So you should allow a 25-50mm ventilated air gap above the insulation below the OSB. Should also install a vapour barrier below the insulation. The vapour barrier can be at rafter or ceiling joist level but all holes in it should be sealed. A proper job to modern building regs for a house would typically use 60mm celotex between the rafters with another layer (possibly thicker) under the rafters. 






You can sometimes despense with the ventilated cavity and fill the rafters IF the membrane is vapour permeable AND both the insulation manufacturer and the membrame manufacturer rate their products for that. If you fill the rafters without a ventilated cavity you risk condensation on the underside of the OSB. Particularly without a vapour barrier to stop humid air from the room below getting in there through gaps in the insulation. It's not guaranteed to occur but more likely. 

Google Interstitial condensation. eg..

http://www.siginsulations.co.uk/condensation.htm

Note: A lot of other web sites mix up warm and cold roof construction with warm and cold loft.


----------



## MikeG. (16 Jun 2009)

All I would add to what Colin has rightly said is that with a house the vapour barrier would be imperative. With a shed, you could argue that the foil on the Kingspan is good enough, assuming that a decent tight fit has been achieved between the insulation and the rafters. There is less vapour pressure in a shed than a house.

My workshop is insulated at joist level. It doesn't stop you using the loft as storage, and is much easier to insulate.

Mike


----------



## garywayne (16 Jun 2009)

I to had problems last night, hence no reply until now. 

*John C.* 
Double sided tape just might work. 

*Shultzy*. 
I have been using garden netting to hold up the insulation. The prolem has been keeping the insulation in place whilst positioning the net and stapling. Like John suggests, using double sided tape to hold the insulation in place whilst securing with the net might be the way. 

*Mike & Colin.* 
Due to the drugs i'm on, (for my leg) I sometimes find it hard to comprehend exactly what people are saying. 

*My roof construction so far is as follows:-* From outside in. 
i) Felt shingles. 
ii) Waterproof breathable membrane. (Waterproof from outside - in, breathable inside - out. 
iii) 2440mm x 1220mm x 18mm OSB sheeting. 
iv) 100mm x 50mm rafters. 

*To be constructed:-* From outside in. 
i) Insulation. 
ii) Waterproof membrane. 
iii) 2440mm x 1220mm x 12mm OSB sheeting. 

I apologize for sounding a bit dance. 

Thank you all for your help. I really need it.


----------



## MikeG. (16 Jun 2009)

Gary,

very simply........you must not insulate directly under the OSB (which is on top of your rafters). There must be an air gap. Therefore, mineral wool ("fibreglass") will not be a suitable material, unless you insulate at your joist level.

Colin and I are telling you that your proposed construction is wrong.

Mike


----------



## garywayne (16 Jun 2009)

So, if I use Wickes's General purpose insulation slabs which are 50mm thick in place of my 100mm insulation and leave a 50mm gap between insulation and the OSB, then the waterproof sheet, then the thin OSB, everything should be fine.


----------



## MikeG. (16 Jun 2009)

Almost Gary..........except that 50mm of that insulation isn't enough. Which is why I suggested Kingspan or Celotex, which are much more efficient as insulators.

Don't forget that it isn't sufficient to have an air gap...........the gap must be ventilated to the outside (and that vent must be insect proof).

Mike


----------



## mahking51 (17 Jun 2009)

Hi All,
Watching this one with great interest to avoid asking the same questions twice!
Mike
I cannot find the membrane anywhere except in great big wide rolls I can barely lift; can you give me a brand name and possible source please/ The stuff I had before was about 1m wide by 50m a roll and worked well.
Also, the likes of Travis Perkins, Wickes B&Q and Build Center near me have no idea what I am talking about whenI ask for insect mesh! Again a, any brand names and supply sources please.
Cheers,
Martin


----------



## garywayne (17 Jun 2009)

Thanks Mike for your perseverance. You have been a great help.
One last question. As Martin requests, what would you recommend to make the vent insect proof?


----------



## MikeG. (17 Jun 2009)

Stainless steel insect mesh is sold in 2 inch wide rolls at my local builders mechant..........Ridgeons, and here is a link to someone that google found for me.........

http://www.meshdirect.co.uk/Insect-Fly-Screen-c-30.html

Martin, what sort of membrane are you after? If it is a timber frame breathable membrane the Tyvek Housewrap (or is it Homewrap) is the standard one, but there lots of alternatives. Could you describe the build up of your wall, from say the inside to the outside.

Mike

PS Mike's definitive guide to shed building can surely only be a matter of time!!!


----------



## Dibs-h (17 Jun 2009)

Mike Garnham":yndo5rrr said:


> Gary,
> 
> very simply........you must not insulate directly under the OSB (which is on top of your rafters). There must be an air gap. Therefore, mineral wool ("fibreglass") will not be a suitable material, unless you insulate at your joist level.
> 
> ...



Mike

On my workshop - I'm looking to do something similar - 

tiles,
tiling battons,
*counter battons*,
tyvek,
ply or OSB,
rafters,
insulation in between the rafters,
vapour barrier,
plasterboard.

Now beacuse of the counter battons - the space behind the tile but above the membrane will be vented - but the space under the ply\osb won't. Because of the counter battoning I'm under the belief that this construction method is fine.

Or is it because the ply\osb is effectively a vapour barrier, that I could have issues with the roof at a later point in time?

Cheers.


----------



## CWatters (17 Jun 2009)

The counter battens are needed because of the ply. Ply makes the roof flat so water could pool above regular battens. Counter battens raise the regular horizontal battens off the ply allowing the water underneath. On a roof without the ply the membrane sags and this allows the water to pass underneath and down the roof to the gutter.

Tyvek say you can use it directly over "boards"..
http://construction.tyvek.co.uk/Tyvek_C ... d_roof.pdf

..but it's not clear if they mean their product is ok used that way or if they mean your roof design as a whole is ok that way. What I mean is that the tyvek might be fine, waterproof and have a long life... but you might get horrible condensation on the underside of the plywood if it's not ventilated. I'm not sure plywood is vapour permeable enough for it to escape.


----------



## MikeG. (17 Jun 2009)

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 (17 Jun 2009)

Mike Garnham":1p4jehn5 said:


> 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.
> ...



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. (18 Jun 2009)

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 (18 Jun 2009)

Dibs-h":2csqrng7 said:


> 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 (18 Jun 2009)

Dibs-h":lk8um60t said:


> 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 (18 Jun 2009)

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 (18 Jun 2009)

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. (19 Jun 2009)

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 (19 Jun 2009)

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 (19 Jun 2009)

Mike Garnham":2a9s9pqo said:


> Dibs,
> 
> This job of yours woke me up this morning.
> 
> ...



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. (19 Jun 2009)

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 (22 Jun 2009)

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 (22 Jun 2009)

Paddywack":1ryfr8tr said:


> 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.


----------

