What to use to insulate workshop roof.

[garywayne]

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.

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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.]

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

Mike

 

[garywayne]

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

 

[MikeG.]

Yep...the slopy ones are rafters!!

How deep are they?

Mike

 

[garywayne]

100mm

 

[John C]

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]

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

 

[MikeG.]

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.]

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]

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.]

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]

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.]

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]

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.]

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]

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]

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.]

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]

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

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]

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.