# Garage Roof Insulation



## Shadowfax (16 Apr 2006)

For anyone who has insulated the garage/workshop roof. How did you do it?
I have an attached garage with a pitched roof which is felted and tiled but there is no ceiling under it. I have floored over most of the joists so that I can use the roofspace for storage. This is great but because of the lack of insulation the garage/workshop is very cold in the winter and warm in the summer. I will be dealing with the door soon but at the moment I just need ideas for insulating the roof itself. I am thinking of using Celotex or Kingspan between the rafters but I am a little concerned about ventilation above it.
I don't really want to go the whole hog and fix two layers of Celotex (between the rafters and across the rafters) but I am wondering if 40 or 50mm of insulation will make a reasonable difference and save a bit on heating the space.
Any ideas, guys?
Cheers.

SF


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## cambournepete (16 Apr 2006)

I'm hoping that 50mm sheffield insulation (the solid polystyrene stuff) will help as I've got a stack of it on the drive that I acquired for free from work awaiting installation.
The main problem is fitting it - it's just wider than the gap between the rafters and so each piece needs cutting lengthways, leaving lots of blue "snow" on the floor...


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## OLD (17 Apr 2006)

Shadofax your proposal is fine no ventilation problems definite improvement look for other areas to insulate and stop drafts.
Cambournepete you can cut with a thin japanese saw i used the type you flush cut dowels etc (no stiffener ) very little snow, if you get it correct it self supports.


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## Shadowfax (17 Apr 2006)

Thanks Old. Just one question, though. 
Can I infill the space between the rafters completely, i.e. insulation goes right up to the felt or must I leave a gap of 50mm or something? It seems that for a cold roof there should be an air gap and proper airflow but this assumes a ceiling and a room below. What have is not like that so I wondered if I can just infill the rafters with insulation and hope that there will be no condensation above the insulation because there is no gap for it to form in.
If I leave an air gap there is only the overlap between each sheet of felt where any air could get in or out, so what kind of ventilation would that be anyway?
This has got me stumped at the moment.
Thanks.

SF


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## RogerS (17 Apr 2006)

Shadowfax":1z9449yw said:


> Can I infill the space between the rafters completely
> 
> SF



I think your concerns only apply when there is the likelihood of condensation in a room (as in cooking or breathing).


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## Shadowfax (17 Apr 2006)

Roger Sinden":1p7ba7wl said:


> Shadowfax":1p7ba7wl said:
> 
> 
> > Can I infill the space between the rafters completely
> ...



I'm certainly planning on breathing out there, Roger!
You make a good point, though.
Maybe I just have a fixation about condensation forming above the insulation and it is not worth worrying about it.
Thanks.

SF


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## Nick W (17 Apr 2006)

If there is any timber in the roof then you should leave space for 'cross ventilation' to keep things dry. You should also ensure that air can get in, at the bottom of the roof, and out at the top (solar heating will provide a pump effect). The best way to achieve this will depend on the construction of your roof, but do put plastic insect grille over any spaces unless you want wasp nests in the roof.

In my workshop I installed insulation between the rafters, and then pasterboarded over the top (under the bottom?) of that.


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## Jake (17 Apr 2006)

I would leave a 50mm gap between insulation and felt, and staple a dpm under the rafters to stop the warm damp air getting through to the cold side of the insulation - they are cheap enough and it wouldn't take long. Foil backed plasterboard would look posher. A couple of vent tiles near the ridge will ventilate the gap.


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## OLD (17 Apr 2006)

The felt under the tiles will allow water vapour through the overlaps so to do the job to the regs you could stop w/vap. by useing foil backed insulation to full depth or staple a vapour controll to rafters and cover as required .Result no condensation and any w/vap. in structure passes out through roof.


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## Jake (17 Apr 2006)

If you are not going to leave an air gap you need a vapour permeable membrane instead of the felt - to meet "regs" at least. In practice you might get away with it, but balancing the hassle of replacing the whole roof if a condensation problem does arise versus just how warm this workshop needs to be, I'd say leaving an air gap between the felt and insulation is a good idea (as is ventilating that gap). If you use foil backed insulation between the rafters, you'll need to tape over gaps where the rafters break that barrier, otherwise you might as well not bother. I think it would be quicker and simpler to use a seperate sheet of dpm, or foil-backed plasterboard (which would give a brighter workshop).


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## Shadowfax (17 Apr 2006)

I'm thinking the best route will be to ensure that the overlaps in the felt are "open" enough to allow air to move in and out between the rafters and use full depth Celotex right up to the felt. I have no plans to plasterboard the underside. I just want to prevent heat loss in the winter and stop the workshop becoming too warm in the summer. There will be no ceiling so cross-ventilation of the roof space will be fine but unless I take the tiles off (no chance of that!) I can't get any counter battons on as Celotex recommend. However, their advice is intended for an enclosed roof space.
Does that sound like a good or bad idea?
Thanks for all the replies so far.

SF


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## Jake (17 Apr 2006)

I wouldn't use celotex, 50mm polystyrene would be good enough for these purposes, and a lot cheaper. You know what I think about maintaining an air gap, and that is based on the standard schemes for insulating roofs as you might have seen from Celotex's site.


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## Shadowfax (17 Apr 2006)

Looks like I have conflicting advice here.
The question is do I need an air gap or not? What is its purpose if there is no top and bottom ventilation to achieve an air flow between the rafters. I only have the gaps between the sheets of felt.

Jake, thank you, I appreciate your thoughts but as a fireman there is no way I would stuff polystyrene in a roofspace. Have you seen what that stuff does when it burns? However, I understand why you are recommending it. To me it is just one of the most horrendous products out there. Propogates flame faster than anything else I can think of as a building material but they never tell you that, do they? That is why I mentioned Celotex. It is much more efficiant at insulating and is not a hazard in itself. Your idea is still appreciated, though so please don't get me wrong.
It's just this air gap thing that is worrying me and how it is supposed to work without a breather membrane and cross-ventilation or top to bottom ventilation. 
I suppose what I need to know is if I leave a 50mm gap will this be sufficient to prevent condensation on the hidden side of the insulation? Or, if I fill the space between the rafters with no air gap will there be a problem? There is no ceiling, just a partly boarded roofspace that is used for storage but there are large holes left for access to the space. Plaster boarding the roof would just be overkill as far as the looks of the thing go. It would certainly not make things any brighter at floor level. This is all being contemplated purely to keep things comfortable.
Thanks for all your support here.

SF


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## Jake (17 Apr 2006)

Shadowfax":2i8k1o3d said:


> The question is do I need an air gap or not? What is its purpose if there is no top and bottom ventilation to achieve an air flow between the rafters. I only have the gaps between the sheets of felt.



If it isn't ventilated then it is pretty pointless. The idea is to allow the condensation to evaporate. The gaps between the felt may or may not be enough in practice. That is why Nick and I suggested adding ventilation - simply a matter of replacing a couple of tiles with vents.

However, the idea of replacing an possibly inadequately ventilated air gap with no air gap at all cannot improve matters, and can only make it more likely that the condensation will not evaporate and will rot your rafters.



> Jake, thank you, I appreciate your thoughts but as a fireman there is no way I would stuff polystyrene in a roofspace. Have you seen what that stuff does when it burns? However, I understand why you are recommending it. To me it is just one of the most horrendous products out there. Propogates flame faster than anything else I can think of as a building material but they never tell you that, do they?



Fair point - I thought this was a shed.

If you are going to risk it by not adding ventilation to the cavity, why not sling a layer of celotex under the rafters. You aren't going to need all that much thickness in the celotex to give you good enough performance for a workshop, and then you can easily tape the joints with alu tape to make sure that the warm damp air can't get through to the cold side of the insulation - that being the part of the battle that you can win without ventilating the cavity.


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## Shadowfax (17 Apr 2006)

Jake, are you saying that there would need to be a vent between each pair of rafters on each side of the roof? That is the only way I can see that each gap would be fully ventilated. There is no real connection between each pair of neighbouring rafters otherwise so how would just a couple of vents do the trick. Or am I being thick?
Old's idea seems to be that the gaps at the overlaps of the felt would be sufficient.
Cheers.

SF


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## Jake (17 Apr 2006)

Yes, I guess you would need a vent for every rafter. Or I suppose you could counterbatten underneath the rafters instead of over them - same difference.

Do what Old suggests then if you prefer his advice. He may well be right, and in practice you may get away with it.

Celotex (whom you seem to trust) say this:



Celotex":1hep5qqq said:


> Ventilation must be provided above an insulated ceiling directly through the cold
> void. Failure to do so could result in serious condensation problems that may lead to
> decay and possible failure.



http://www.celotex.co.uk/appl/PDF/SOL_BUR.pdf

If you aren't going to ventilate, I would at least leave a void so that the gaps in the roofing felt have a chance to ventilate the whole cavity rather than just a few cm around the gap.


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## Shadowfax (17 Apr 2006)

Thanks very much, Jake. You have been more helpful than you perhaps realise. Because the roof was constructed with pre-made trusses there is a lot of timber to insulate around if the insulation does not go between the rafters. I read the detail, that you also highlighted, on the Celotex site and it did make me think. I am wondering if the whole thing will be worth the effort to be honest. 
Whatever happens I will ensure there is a 50mm air gap above any kind of insulation I might use. I think I will go for a warm door first. That will be easier anyway. A steel door is just like a radiator in reverse in the dead of winter!
Many thanks to all of you who replied to this thread. It has been very interesting.
Back to the planning now, then!
Cheers.

SF


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## les chicken (18 Apr 2006)

SF
Missed the beginning of this post doing other things over Easter. I extended my garage by 12ft last year, it is a pitched tiled roof with 4x2 rafters and the tiles have breathable felt under them.

I used 50mm kingspan cut to the gap and jammed in between the rafters flush with the bottom, if the fit was loose a couple of nails into the rafter for the kingspan to sit on. Look out for factory seconds, I paid £9 a sheet for mine    

During the snow there was no evidence between the part with the ceiling and the open extension of the snow melting, and that was with the hotspot stove in the extension.

Les


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## Jake (18 Apr 2006)

les chicken":ula4lmuq said:


> ...and the tiles have breathable felt under them



There lies the rub, though.


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## Shadowfax (18 Apr 2006)

You will like this!
As I was in Building Control this morning I asked how should this job be done. Told them all about the tiles, felt, air flow, gaps etc. etc.
Answer was to butt the insulation right up to the roofing felt if it was the type with foil on it. This should be fine I was told if there is plenty of air volume in the space below the roof, i.e. the workshop/garage.
I said surely there should be a ventilated air gap above the insulation but was told this will be fine. The reason there is no information about this sort of thing for garages is that normally garages do not require insulation.
The recommendation was to tape up the joints, though.
Interesting, but I am going to concentrate on changing the up and over door to a pair of insulated timber doors first.
Cheers all.

SF


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## Jake (18 Apr 2006)

Were they assuming it is an unheated space, perhaps?

In which case, no temperature gradient to cause condensation. Otherwise, they are just plain wrong, but there you go.


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## Shadowfax (18 Apr 2006)

Hi Jake
I think you are right but I did press the point and was told that because of the single skin wall and the potential for heat loss there, as well as through the door, there would be nothing to worry about. Any minor interstitial condensation would simply run out of the bottom of the slope because of the foil backing. The guy felt (sorry!) that as roofing felt tends to "sweat" anyway there would be no real difference and it would not be worth worrying about it. All based on experience rather than tests and specifications, of course, and not covered by the Building Regs as they do not apply to garages.
Like I said, interesting. Still going to concentrate on the door, though.
Cheers.

SF


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## Jake (18 Apr 2006)

He's brave, sticking his neck out like that. He may well be right in practice. I wouldn't risk it.


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## Waka (18 Apr 2006)

I have been reading this thread with great interest, really surprizing what you can learn on the forum.

Can anyone enlighten me regarding an air gap btween insulation and roof where the roof is of the flat type?
I intend to do mine before next winter, floor ans walls are already done.


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## Jake (18 Apr 2006)

The exact same principles apply. 

Vapour barrier on the hot side of the insulation to stop the warm damp air getting through to the cold side in the first place. Either a ventilated cavity or a vapour permeable membrane on the cold side to let any condensed vapour escape. 

Just the same principle applies if you are insulating a loft - a dpm should go below the insulation to stop the warm damp house air condensing on the top of the insulation when it hits cold attic air, and dripping back down. 

Loads of stuff on Celotex's website on types of flat roof, have a poke around.


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## Shadowfax (18 Apr 2006)

Just to continue the saga. I had to go back to Building Control again this afternoon so I asked the question again - this time of one of the Senior BCOs. No connection with the other guy from this morning. He was still out on site. I got the same answer though. But the assumption is that the space (workshop) is basically unheated except for a radiator which produces no moisture and not a high temperature such as would be found in the house.
He also suggested that it would be sensible to use semi-rigid batts made from mineral wool such as would normally be used to insulate a wall. They are made to be in contact with the outside skin of a wall and do not allow condensation to generate. His advice was aimed at allowing for a more balanced environment between winter and summer. In other words to control solar gain to an extent.
Like I said I am going to concentrate on the door but I am starting to enjoy this thread so keep your ideas and questions coming. We will be able to start our own advisory site soon!
Any the wiser, Waka?
Building the workshop from scratch is obviously the best move but it seems that changing things afterwards is a problem.

SF


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## Jake (18 Apr 2006)

If you don't heat it, there isn't a problem, same as if you have draughts everywhere. The atmosphere will be the same in and out, and on both sides of the insulation. And if you are going to heat it an hour or so here and there, that isn't going to make a huge amount of difference. That wasn't clear, all you said was that you were trying to save on heating costs! Use a dry heat source when you do heat it, and insulate how you like - but no point in using celotex in that case 'cos it would be way over the top for the requirements.


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## DaveL (18 Apr 2006)

Well I have bought a roll of the metalised bubble wrap type stuff that I am going to fix on the bottom of the rafters. I have a staple gun for fixing it and some of the ali foil tape to seal the joints. It looked a good deal from ebay. 
Of course I do need to move all of the timber that is stored in the roof space to be able to fix it up. 

I also have lots of packs of laminate floor, along with DPM and insulating pads to go on the floor, its only the first line of the instructions I have a problem with: 

_*Clear the room.*_ 

Just who are they kidding! 

But I hope that once I have done this over the summer then next winter will be much more comfortable in the shop. Having a neighbour who brings me old pallets for fire wood will help as well.


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## Waka (19 Apr 2006)

Thanks guys, I get the picture.


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## Shadowfax (19 Apr 2006)

Sorry, Jake. I moved the goal posts didn't I? I knew what I meant but that was not what I wrote! I will heat the workshop, sometimes, but only enough to keep it from freezing the glue etc. That sort of thing.
One thing I would urge all of you who think expanded polystyrene is a cheap alternative to proper insulation - please think very carefully before you use it at all indoors. It is highly hazardous when it burns. It produces tremendous heat and thick, black noxious smoke that contains icocyanates and cyanide amongst other things. It also drips burning droplets so one fire quickly turns into several more and it burns very quickly. In short, it is horrible stuff! It kills in short order!
I won't have it in the house in anything but small quantities because I have seen what it can do. 
More stories about this if you like!
Thanks to all who have contributed to this thread, especially Jake.
Best wishes .

SF


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## Jake (19 Apr 2006)

No problem, my misconception really.

Building regs will allow poly as long as it is behind plasterboard to give half-hour fire resistance. Interested to know whether you think that is enough to deal with it? 

What does poly-iso do in a proper fire then? I know it kind of goes brown and smoulders rather than doing the dripping balls of fire act to start with, but at full-blown house fire temperatures, does that still hold true?


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## Shadowfax (19 Apr 2006)

Hi Jake
The big trouble with polystyrene is that it burns very hot and the smoke and fumes from it are toxic. It melts and spreads while it burns and the heat it gives off is extreme. If there is an air gap around or above it the effect is even worse and the flame propogation is much quicker.
When polystyrene tiles were really popular, in the 70s and early 80s people used to ignore the fixing advice and use blobs of glue to install them on their ceilings. The air gap, though small, behind the tiles was enough to spread fire at an almost unbelievable rate. I have seen fire travel the line of a poly "picture rail" around a room faster than it could be followed!
The cyanide and stuff that it gives off make the smoke highly dangerous. It is said that one good breath of the stuff will knock you out and the second will kill you. This is why we have always been very wary of the product in all its forms.
As to the temperature in a fire situation - 600 degrees C at ceiling height could easily be increased to 1000 degrees C when this stuff is in the room. At those sorts of temperatures the chance of flashover, where everything in the room self-ignites, is high. The backdraught potential is even better - where the temperature is raised enough to mean that any inrush of air will lead to super expansion of the gases and create a smoke explosion. Once the cover for it is broken or burned away, melted or whatever, the fire can race away. Way past time to be getting out!
This is why I actually hate expanded polystyrene! It can be bad for your health in a major way. 
It is a marvellous material but people do not understand its bad points. For me they mostly outweigh the good points.
Horrible stuff!!

Cheers.

SF


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## Jake (19 Apr 2006)

Shadowfax":20u33q3y said:


> polystyrene tiles



Yes, notorious. I saw someone buying some the other day in a B&Q, thought they would have been banned. 



> The cyanide and stuff that it gives off make the smoke highly dangerous. It is said that one good breath of the stuff will knock you out and the second will kill you. This is why we have always been very wary of the product in all its forms.



Are the polyisocyanurates (eg celotex) any better in this respect though? I'm no chemist, but from the name they sound like they might give off cyanide too!

Celotex waffle on about how much better it is at fire resistance, but I've seen it burn, it just smoulders more rather than racing away. I suspect that at 600C, that might be a different story though - not that I have any reason to do so, which is why I'm interested.


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## Shadowfax (19 Apr 2006)

Hi Jake
They are made from a different kind of foam. The chemical structure is different. Something to do with the polymer chains. Anything at all will burn if it gets hot enough but where man-made stuff is concerned we have tended to learn the hard way, I think.
The new foams burn differently, tending to seal up with the heat rather than freely releasing more material to burn even more. Even the look is different. Celotex is very dense whereas expanded foam is just loads of little balls joined together with a lot of air. 
We will need a chemist to explain the theoretical differences but in practice my vote goes for the slower-to-react products. Anything that helps to prevent the building falling on top of me in a fire is all right with me!
Water+heat=steam=knock-down and cooling. Lovely!

SF


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