Garage Roof Insulation

[Shadowfax]

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

 

[cambournepete]

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...

 

[OLD]

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.

 

[Shadowfax]

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

 

[RogerS]

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).

 

[Shadowfax]

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).

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

 

[Nick W]

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.

 

[Jake]

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.

 

[OLD]

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.

 

[Jake]

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).

 

[Shadowfax]

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

 

[Jake]

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.

 

[Shadowfax]

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

 

[Jake]

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.

 

[Shadowfax]

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

 

[Jake]

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:

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.

 

[Shadowfax]

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

 

[les chicken]

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

 

[Jake]

...and the tiles have breathable felt under them

There lies the rub, though.

 

[Shadowfax]

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