Roof condensation question...

LordSutch

We recently had a concrete garage built. Its got a concrete floor, a damp course, and its built with hollow concrete blocks, its a very solid construction. So far so good.

Now to the roof...
The sloping roof is made with thick strong beams supporting some type of plywood (waterproof variety), then covered with two layers of felt, which was torched on with a burner. Excellent.

Then my worries begin; The garage has zero ventallation, its a sealed box, which leads me to wonder might it suffer from condensation next winter? Cold air on the outside/ warm air on the inside.

So I plan to fit Kingspan insulation between the beams and then to plasterboard the underside and install LED spots in the ceiling. But what about condensation? Would I even know if it existed up there? hidden as it were above the plasterboard & Kingspan.

Only to discover that the roof is rotten in about five years time!

Having encountered condensation problems on a past build, I was just wondering if I should install some kind of ventillation between the underside of the Ply and the kingspan, or should I just put vents in the garage itself?

Dimensions. 21ft L/ 11ft W/ 7ft H.

LordSutch

Come on you 'manshed' folk, somebody must know about such things?

Calahonda52

Sutch impatience! Are you landed gentry in a time warp perhaps:D

some bedtime reading
http://www.environ.ie/housing/buildi...ance-documents
and
https://www.nsai.ie/S-R-54-2014-Code-of-Practice.aspx




The essential ideas here are:
when vapour gets past the barrier from inside out, it has to be able to get out so the construction needs to cater for that. the ply and felt is impermeable, period.

Could you put in a flat ceiling and insulate it and then just ventilate the roof at the two apexes
Its not clear where the 7 feet is to.

sky6

is there no Eves and Guttering on the long side of the Garage. If there is you should be able to vent it there. If not then you will have to do as Calahonda has said. Put vents in both Gables in the Attic area. You should still put a vent in the Room also.

LordSutch

You are both good eggs, and tata for your input.

I will vent through the eaves at the front of the Garage (the highest point) by drilling multiple holes to correspond with each section between beams. I will then do the same at the rear of the building (the lowesest point) by drilling similar holes for the air to be vented front and rear!

Only problem is the 'noggins', that separate the beams (each one bout 16" wide) which prevents the air travelling from front to rear, so I will have to drill holes in these too I guess, for a proper air flow . . . . .

Many thanks for the advice,
Cheers.

Czhornet

As the walls are built with hollow blocks you most certainly will have condensation, because of the cold bridging in the wall, then dampness will eventually make its way from the outside to the inside leaving you with "sweaty walls"
Maybe you should look into some mechanical means to get the dampness out, a fan on some sort maybe.

sky6

If you have a pvc Soffit you can get a length of vent grill. Then just fix it in place. No need to go drilling loads of holes.
Then do the same on the low side. You shouldn't need to go drilling the Noggins either.
If the Rafters are not treated timber then it wouldn't do any harm to paint on some wood preserver before you seal them in with the new Ceiling.

LordSutch

The soffit is ply . . .

The hollow block walls have a thick layer of plaster on the outside if that helps?
After six weeks I will shortly be painting on a 'mist' coat of exterior paint before the real coat.

Getting back to the condensation. Amazing that the builders didn't give this question a thought. I gather condensation is a very common problem with such designs (sealed concrete garage with felted ply sloped roof)!

We currently have a dehumidifier running to suck out the moisture before I decorate the inside.

Can't believe the hollow block design will make the walls sweat !!
... otherwise the whole design is a disaster.

whizbang

You won't have any problems with hollow block construction. Its been used for years. If you are still worried, look at exterior waterproofing paints, but really...

Build in the insulation so that its as airtight as possible, and as above, ventilate the space between the timber and insulation.

sky6

You can use a few 9 x 3 Aluminium Grills in the Ply so. Just make sure you cut it between the Rafters. " will do for a 12 to 15 ft Garage.
That's all you need to ventilate the Rafters.
As said there should be no problem with Hollow block walls there's plenty of Houses built with them. Although you would have been a lot better to build a Cavity leaf wall. Don't rush to paint it as the Block work and plaster needs to dry fully, Otherwise the paint will peel and Blister. I'd give it a couple of Months personally.

Sleeper12

All rooms need to be vented

LordSutch

So I guess what were really saying is that there are two seperate climates!

One vented climate in the 50mil roof space under the ply/felt, and then the climate in the garage below which is seperastely vented through wall vents.

PS; When putting the insulation board between the beams can I seal it in with silicone? also ... do I need to put up a vapour barrier before I put up the plasterboard ceiling?

(what is a vapour barrier made from, and is it porous)?

10-10-20

Here you are:
http://www.kingspaninsulation.ie/getattachment/78f5cba0-5188-4b8a-a9ec-a0708ccc61be/Kooltherm-K7-Pitched-Roof-Board.aspx?disposition=attachment

Enjoy!

whizbang

Yes, treat the spaces as 2 separate climates.

You dont need a vapour barrier. The insulation quoted above is as good as.
In a perfect world, the timber beams would be kept in the warm space, but isolated from the warm vapour laden air... but thats almost impossible!

Dont use silicon, get the cheapest tubs of acrylic caulk, decorators filler, etc.

Skatedude

You need some ventilation and air flow or you get condensation,

LordSutch

I had a crazy thought the other day which consisted of installing a solar powered fan connected to a venting pipe 20ft long with offshoots into and between each each set of rafters! Nice idea but it might not be 100% ifficient, so I think I'll stick with individual 3" round grills placed along the outside of the (felt covered/ply) fascia board.

PMBC

LordSutch wrote: »

We recently had a concrete garage built. Its got a concrete floor, a damp course, and its built with hollow concrete blocks, its a very solid construction. So far so good.

Now to the roof...
The sloping roof is made with thick strong beams supporting some type of plywood (waterproof variety), then covered with two layers of felt, which was torched on with a burner. Excellent.

Then my worries begin; The garage has zero ventallation, its a sealed box, which leads me to wonder might it suffer from condensation next winter? Cold air on the outside/ warm air on the inside.

So I plan to fit Kingspan insulation between the beams and then to plasterboard the underside and install LED spots in the ceiling. But what about condensation? Would I even know if it existed up there? hidden as it were above the plasterboard & Kingspan.

Only to discover that the roof is rotten in about five years time!

Having encountered condensation problems on a past build, I was just wondering if I should install some kind of ventillation between the underside of the Ply and the kingspan, or should I just put vents in the garage itself?

Dimensions. 21ft L/ 11ft W/ 7ft H.

For condensation to occur a few things are necessary the first of which is a source of water in vapour form e.g. cooking, washing/drying clothes, normal human activity such as respiration. If you don't have that source, and depending on the connectivity between garage and house, you wont have condensation. If cavity block wall is properly plastered externally you wont have dampness through the walls - it more frequently occurs via faulty downpipes, gutters and roofs. While you are right to be concerned about condensation I think insulation is OTT. But IF you are considering a future use as something else e.g. playroom, gym now is the cheapest time to insulate for that purpose.
Vapour barriers should always be as close as possible to the 'warmer' side to stop movement of vapour into insulation or structure to stop interstitial condensation and timber rot. The insulation prevents the air temperature dropping below the Dew Point temp. so that condensation doesn't occur, as well as keeping colder air from penetrating to the warm side. Remember that the moisture bearing air has to go somewhere so that it will find the coldest surface and this is prevented by adequate ventilation. I don't see that you need wall vents in a garage.
Hope this helps. Perhaps also google 'Warm Flat Roofs' and 'Cold Flat Roofs'.

PMBC

Calahonda52 wrote: »

Sutch impatience! Are you landed gentry in a time warp perhaps:D

some bedtime reading
http://www.environ.ie/housing/buildi...ance-documents
and
https://www.nsai.ie/S-R-54-2014-Code-of-Practice.aspx




The essential ideas here are:
when vapour gets past the barrier from inside out, it has to be able to get out so the construction needs to cater for that. the ply and felt is impermeable, period.

Could you put in a flat ceiling and insulate it and then just ventilate the roof at the two apexes
Its not clear where the 7 feet is to.

The barrier is a vapour barrier and if installed properly with laps and seals will not allow vapour to 'get by'. Sorry if this seems pedantic but its one of the principles.

PMBC

LordSutch wrote: »

The soffit is ply . . .

The hollow block walls have a thick layer of plaster on the outside if that helps?
After six weeks I will shortly be painting on a 'mist' coat of exterior paint before the real coat.

Getting back to the condensation. Amazing that the builders didn't give this question a thought. I gather condensation is a very common problem with such designs (sealed concrete garage with felted ply sloped roof)!

We currently have a dehumidifier running to suck out the moisture before I decorate the inside.

Can't believe the hollow block design will make the walls sweat !!
... otherwise the whole design is a disaster.

you will have a lot of moisture after 'wet construction' i.e. traditional plastering. In normal summer weather this dries naturally, unassisted. In wet, damp, humid weather like here it can take longer. Take care with dehumidifier as the room needs to be dried slowly. The water component of the plaster is part of a chemical reaction - its not just to make it liquid for easy application.
Best of luck with your project.

LordSutch

PMBC wrote: »

For condensation to occur a few things are necessary the first of which is a source of water in vapour form e.g. cooking, washing/drying clothes, normal human activity such as respiration. If you don't have that source, and depending on the connectivity between garage and house, you wont have condensation. If cavity block wall is properly plastered externally you wont have dampness through the walls - it more frequently occurs via faulty downpipes, gutters and roofs. While you are right to be concerned about condensation I think insulation is OTT. But IF you are considering a future use as something else e.g. playroom, gym now is the cheapest time to insulate for that purpose.
Vapour barriers should always be as close as possible to the 'warmer' side to stop movement of vapour into insulation or structure to stop interstitial condensation and timber rot. The insulation prevents the air temperature dropping below the Dew Point temp. so that condensation doesn't occur, as well as keeping colder air from penetrating to the warm side. Remember that the moisture bearing air has to go somewhere so that it will find the coldest surface and this is prevented by adequate ventilation. I don't see that you need wall vents in a garage.
Hope this helps. Perhaps also google 'Warm Flat Roofs' and 'Cold Flat Roofs'.

OK, lets start at the beginning again as I am now a little confused.

Garage/Storage space/Workshop.

The sloping roof is made with thick strong beams supporting a waterproof plywood, which is then covered with two layers of felt, torched on with a burner. Excellent.

The 'cold roof' space will have zero ventallation once I install a plasterboard ceiling, (like a sealed box), which leads me to wonder might it suffer from condensation inside the roof space between the beams? Cold air on the outside/ warm air inside the sealed roof space!

So I guess what were really saying is that there are two seperate climates!

One vented climate in the 50mil roof space under the ply/felt, and then the climate in the garage below which is seperastely vented through wall vents.

Dimensions. 21ft L/ 11ft W/ 7ft H.

Am I wasting my time doing all this roof ventillation carry on?
Should I just pack the roof space with Rockwool and seal it, air tight?

MicktheMan

LordSutch wrote: »

Am I wasting my time doing all this roof ventillation carry on?
Should I just pack the roof space with Rockwool and seal it, air tight?

There is no such thing as air tight (in the true sense) where moisture is concerned. If you do not vent the timber structure you have a very real risk (I would say real certainty) that water vapour will get in, will condense on colder surfaces, will get trapped and will rot your roof.

LordSutch

Thanks for that, so I'm back on track and ready to put 11 fascia vents along the back of the roof (1 vent between each set of roof beams) allowing a 50 mill gap above the insulation for the air to circulate from the front of the garage, to the rear. This will prevent fungus, mould and rot from getting hold in the roof.

Thanks for all your contributions, we've covered everything, so I guess that's about it.

Cheers & adios.

THREAD CLOSED.