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Author: Subject: Insulating a garage roof.
omega 24 v6

posted on 21/12/16 at 11:59 PM Reply With Quote
Insulating a garage roof.

Good evening all and Merry xmass to you all .
Its been a while, as my latest project has been a house refurb and now here come the question/discussion

I replaced the garage roof in the summer, with a new plywood base and fibreglass overlay. The old one was rotten to the core and i also replaced two of the joists.

However, now that the cold weather has set in,the condensation has began to turn the inside of the roof plywood black with mould.
Ideally, i'd want to use up the spare glass wool insulation that I have left, but there seems to be a bit of a debate on whether this is the way to go.
I was going to bleach the mould, and then gloss paint or use garage floor paint, on the plywood prior to filling to joist height with glass wool.(200mm)
Some say i need an air gap or condensation will still form.
I have a couple of bits on the roof that are ply sheeted and insulated already. Its ply, with glass wool behind for carrying pipes/wiring. On a frosty day, the external part of the roof where they are, is still white with frost while the rest becomes wet. I assume this is why the plywood is becoming mouldy as its the cold face / warm face barrier.
So should I just insulate and plasterboard over the top of it and forget all about it. Or something else.
Discuss.





If it looks wrong it probably is wrong.

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Nickp

posted on 22/12/16 at 06:45 AM Reply With Quote
I've got a coated tin roof. I just filled the joist gaps with insulation and boarded it in with thin plywood. Had no issues with condensation for 4yrs now
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AvonJas

posted on 22/12/16 at 07:25 AM Reply With Quote
Google cold roof system, the ventilation needs to be in the roof void between the insulation and the plywood. Drilling circular vent holes in the fascia might work.
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cliftyhanger

posted on 22/12/16 at 07:41 AM Reply With Quote
You need to ventilate where the condensation forms, so insulation and then ventilate above that.
OR put a vapour barrier below the insulation.

Definitely get the mould killed off, not easy, and seal with thinned oil based paint when dried out. Then ideally a second coat.

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nick205

posted on 22/12/16 at 10:21 AM Reply With Quote
What dimensions are the space?

Could you install 4x2 straight joists insulate above with glass wool then cover below with plasterboard or plywood?

I've fitted an extra layer of glass wool in my house loft taking it to nearly 300mm thick overall. I kept it back slightly from the eaves (which have ventilation) so air can circulate above the glass wool. No condensation issues in the house or loft space and dramatically improved heat retention inside the house. Minimal cost plus some of my time - job done.

I'm sure you would, but best to wear suitable clothing, gloves and a face mask when dealing with glass wool - saves itching and coughing afterwards.

[Edited on 22/12/16 by nick205]






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v8kid

posted on 22/12/16 at 10:34 AM Reply With Quote
I can vouch for B&Q own make mould and algae killer it is exceptionally effective and unusually for these type of products did not make me wheeze after applying it. My neighbor sprayed the date with it on a mould covered outside concrete apron in his farm and it is still readable 3 years hence. I have extensive external wooden decking and this keeps it slip free all year round with an annual application.

WRT insulation it is standard practice to use a 600 gauge vapor barrier sealed at the edges with the cold side of the insulation continuously vented. Without knowing exact details a 50mm gap should be adequate and all 4 sides of the void should be vented. Use the largest practicable thickness of insulation not less than 150mm fibreglass or 100mm polyurethane. You need to make sure the air gap is not blocked by the fiberglass and as its hassle to do most people use solid insulation jammed between the joists.

Cheers!





You'd be surprised how quickly the sales people at B&Q try and assist you after ignoring you for the past 15 minutes when you try and start a chainsaw

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omega 24 v6

posted on 22/12/16 at 12:46 PM Reply With Quote
space is 6.5 by 4.5 mtrs roof joists are 8 by 2's with a 4 by 2 cut to give run over the 4.5mtrs.
I had made the mistake of leaving a fully loaded trailer of branches etc in the garage with no ventilation a think this is whats caused the mould.
the air bricks are now all cleared and space ventilated.
my plan was to pack between the joists with glass wool and finish with duplex foil backed plasterboard. But doubts were placed in my mind.
surely if i leave ventilation we have the same issue of warm air condensing on the plywood sheets.
Confused now.





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wilkingj

posted on 22/12/16 at 02:50 PM Reply With Quote
I joined the Self Build Motor Caravan Club 3 years ago, and have built my own Camper. They are very similar to Locostbuilders, except its for Campervans etc Not kit cars!

Having just built a campervan, I would NOT use rockwool or similar "loose" insulation.
Once it gets moisture / damp its a sod to get dry, and then becomes a useless insulator. Hence all the talk about ventilation. But ventilation = Draughts!

I would use polystyrene builders sheet (check the fire risk), or Kingspan/Extratherm type insulation. This is very efficient stuff. and comes in various thicknesses.
An 8x4ft sheet of 25mm thick is approx 25, but 50mm is like 27, so dont put two layers on one thickness, just get the ticker stuff to start with.
Shop around and use trader type discounts etc. I found Travis Perkins were good.

With the kingspan type stuff I would seal the edges with Alluminium tape, and seal between boards with the same tape.
I got my Ally tape from Screwfix. If you can find it cheaper, that's fine.
With Kingspan (or similar) if you ally tape between the panels you will give yourself a moisture barrier automatically.
Otherwise use B&Q green plastic vapour barrier, various makes about but B&Q were cheap.

Bottom line is any decent insulation is not that cheap. But its worth it to have a warn and dry workspace / storage area.

I learned a lot of new things and skills building my camper. Its not a "Gin Palace" on wheels but it is functional!

Happy Xmas to you all
Geoff





1. The point of a journey is not to arrive.
2. Never take life seriously. Nobody gets out alive anyway.

Best Regards
Geoff
http://www.v8viento.co.uk

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v8kid

posted on 22/12/16 at 09:58 PM Reply With Quote
quote:
Originally posted by omega 24 v6
space is 6.5 by 4.5 mtrs roof joists are 8 by 2's with a 4 by 2 cut to give run over the 4.5mtrs.
I had made the mistake of leaving a fully loaded trailer of branches etc in the garage with no ventilation a think this is whats caused the mould.
the air bricks are now all cleared and space ventilated.
my plan was to pack between the joists with glass wool and finish with duplex foil backed plasterboard. But doubts were placed in my mind.
surely if i leave ventilation we have the same issue of warm air condensing on the plywood sheets.
Confused now.

No the ventilation of the insulation is to the outside it will be cold dry air absorbing the moisture and transporting it to outside. It ain't rocket science and is well covered in building regs





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jacko

posted on 23/12/16 at 04:19 PM Reply With Quote
http://www.foregale.co.uk/insulated-roofing/

This is what i have put on my garage its great the insulation looks like kingspan
jacko

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