Re-roofing workshop

[Adrian Brentnall]

HI Folks

My glass-making workshop here in Ireland has a torched-on felt roof (12' x 24'), which is, after 15 years or so, is showing its age and starting to leak, though the chipboard is basically sound.

The Plan is to clad it with some steel box-profile sheeting. While I'm at it, it seems like a good thing to introduce some additional insulation - so planning to screw treated battens to the existing roof, to allow 1" of sheet insulation to be laid between the old roof-surface and the underside of the new steel sheeting.

Which material would be best for this? My local builders' provider has a 'Kingspan'-type material (foam core, foil both sides) in 1" for €35 an 8' x 4' - so about €300 for the job. Is this an unnecessary extravagance - would expanded polystyrene be just as good, and a bit cheaper?

I know that pre-insulated steel sheets are available - but they are about 2.5x the price of the plain sheeting..

Thanks Adrian

[Tim Lamb]

In message [e-mail]>, Adrian Brentnall [e-mail] writes

Roof pitch? Felted roof can be very shallow pitch. You might need to consider a *drip edge* or risk rainwater wicking back under the roof.

Fixings. The pre-insulated sheets are normally fixed through the shallow section rather than the ribs. With compressible foam you risk issues with the neoprene sealing washers.

I have no idea if the foil contributes anything useful for your application.

The pre-insulated sheets creak loudly in intermittent sunshine!

Should you be considering suitability for eventual solar panel fixings?

[Theo]

According to ubakus.com, the stackup for a sunny roof:

Inside: reduced air circulation 20C 50%RH Chipboard 18mm Bitumen membrane 2mm Rigid foam PUR aluminium laminated 25mm (same as Celotex/PIR I think)

- between spruce battens 25mm high, 50mm wide, 400mm spacing Steel 1mm Outside air -5C 80%RH

has a U value of 0.967. If I change that to Extruded polystyrene the U value is 1.242. Without the foam or battens the U value is 3.606 The existing felt acts as a vapour barrier so moisture levels are OK with foam, but without any insulation the condensation will be raining inside.

By comparison, English building regs have just changed the maximum roof U-value for new builds from 0.13 to 0.11, and in existing dwellings from

0.18 to 0.15. That only applies to habitable rooms, not outbuildings.

To meet the retrofit U value for a habitable dwelling would need 100mm of PIR with 100mm battens, and then a 75mm layer of PIR over the top, getting you U=0.157.

To do that with XPS requires 200mm of XPS in the bottom layer plus 75mm on top, U=0.159.

On the other hand, you need to consider that if you have a super insulated roof it's no help if you have draughty doors and windows or uninsulated walls. At the end of the day it depends on what compromises you want to make, but the above suggests 25mm of anything is pretty minimal. Going to even 75mm would make a big difference.

Have a play with ubakus (use 'demo mode') and see if you can come up with the suitable stackup.

Theo

[Adrian Brentnall]

Grr - Thunderbird's playing silly bu&&ers again

- I saw that two kind souls replied to my original post, but now it won't show me those replies..

So - thanks if you did reply....

From memory - somebody asked about future solar panels. Not an issue, the workshop's on the North side of the house and more re less shadowed by the house and the ex-quarry rock-face into which it's been dug..

As to insulation - I wouldn't call it 'habitable' - the idea of adding a bit of insulation was an afterthought, the place does get cold in winter, and the tarred roof is a heat-magnet in summer for the few hours that the sun does ht it - so not looking to achieve huge amounts of insulation, but just toying with the idea, really.

Maybe it'd be just as effective to screw the steel sheet down onto battens, leaving an airspace (like they old LandRover tropical double-roof) - which might alleviate the summer overheating issue.

I know that there were some other points raised, so if you could please reply bk to your original comments then that might make Tbird show them to me...

Thanks Adrian

[Tim Lamb]

In message <O4l4+ [e-mail]>, Tim Lamb [e-mail]> writes

Copy as requested.

>

[Theo]

Replying with full quoting as requested :)

Leaving an air gap and having the steel suspended above the roof would help with solar shading, and it might help somewhat with condensation. But wouldn't do much for warmth.

If your budget is tight, you might think about at the very least two layers: one with battens and one without. You then use thermally-broken fixings to secure the top layer. For example, in my stackup above, 25mm of XPS with battens plus 25mm without gets you U=0.689. Nowhere near English regs, but avoids the thermal bridge through the battens. Making that PUR gets you to U=0.473.

A way to think of these numbers is that if the current state is U=3.6, a U=0.473 has 7.5x less heat loss (or gain). U=0.11 has 32.7x less heat loss.

If you use the calculator to work out the U-value for your walls and floor as well, and then multiply the U-values [which are in W/(m^2*K) ] by the m^2 surface area of each and sum all the products, that gives you a figure in W/K. That tells you how many watts you're losing overall for every degree temperature difference from inside to outside. In the calculator it might be 20C inside and -5C outside, so multiply by 25 and you get a figure in watts, which is how much heating it would take to keep the building at 20C when it's -5C outside. Then you can work out how much difference various roof U-values are going to make to the whole building.

If you have draughts, however, it is a good idea to plug them first - no amount of insulation is going to help if there's a howling gale coming through.

Theo

[Adrian Brentnall]

HI Tim, Thanks for that....

The roof drops about 18" - 2ft over the 12ft width, so it's not terribly flat, but I think a bit of felt under the far end to direct rain into the gutter would be a good precaution, thanks.

As to solar, the house roof was fitted with solar pv a few months back - the slope on the workshop and the shading of it mean that it wouldn't be a lot of use for solar...

I think the 'kingspan' foam is fairly rigid, but I'm imagining that the fixings will be into the timber battens - so if I get the thicknesses of those right then they should be fairly firm..

Thanks for the comments - it's appreciated Adrian

[Adrian Brentnall]

HI Theo

Thanks for the requote - ironically, Thunderbird is now showing all of the original replies!

As I said - the original aim is to waterproof the roof. Twas the lad who is going to do it who suggested adding insulation...

The workshop is over-warm in summer, and can get chilly in winter, but draughtproof it isn't (by design) as there's an amount of soldering etc that goes on in there, as well as some kiln-work, so a draught isn't all a bad thing, at times.

I think the major benefit of any insulation would be to reduce the heating from the sun beating down on the tar roof in the summer...

The kingspan 1" will cost me about 300 euro - so I think that's probably a reasonable cost - especially if it means that we can keep the workshop a bit cooler in the summer...

Here in South-West Ireland we don't get terribly cold winters...

Anyway - thanks for your comments - very useful Adrian

[Animal]

For a given amt of insulating value, polystyrene is much cheaper. So I'd go with that. 2" of EPS will undercost & outinsulate the 1" of PIR.

[fred]

You might consider eaves closure pieces suited to the profiles mtal sheeting. Should help combat back wicking

[Adrian Brentnall]

Thanks - yes - the sheeting comes complete with the foam 'sealing' that matches the profile, and adding a bit of flashing tape will help the rain find its way into the gutters.

[Theo]

I came across these Kingspan panels, not sure if they're what you're talking about:

some youtubers: ?v=2OcEd9I9ZhU I can see the advantage in terms of time and effort installing: it's a single panel that does weatherproofing, insulation, vapour barrier and interior finish all in one.

For my single garage (roughly 3x5m) I calculated about £1200 for 50mm celotex, boarding, fascias and EPDM. For this (minus fascias) it's £500-600, and it doesn't need a complex roof structure to be built up.

Not sure about fitting it over my existing deck+felt, but I might ponder that...

Since it's Kingspan it might be made in Ireland and you can get it without too much transport costs. That distributor sells to Ireland:

Theo

[Adrian Brentnall]

Thanks for that. I've a local supplier who quoted about €2700 for the insulated panels, and €1200 for the standard ones....

If I was starting from scratch I'd probably go the pre-insulated route, but as it's a 'reasonably-quick&dirty' job, then I think the vanilla sheets plus 1" of either poly or kingspan will have to do the job..

Will call to the builders' providers this afternoon, and see how we can achieve some battens that are the same thickness as the insulation - just to make life easier... They have planer/thicknessers, big circular saws etc, so I might as well get everything as 'ready' as possible...

Thanks Adrian