roof ventilation

[CluelessIncompetent]

I have a single skin brick shed that I'm planning to convert into a liveable room. The roof is flat 18mm ply on top of ceiling joists. Felt covering. How should I go about insulating the ceiling? If I could lay the insulation sheets on top of the joists I understand this would qualify as a warm roof. However that is not possible for me. I could fix the insulation sheets to the underside of the ceiling joists or I could fit the insulation between the joists or I could do both - fitting insulation between and to the underside of the joists. This would make it a cold roof would it not? Are there any ventilation requirements to consider? I have a uPVC fascia but no soffit. It has been suggested to me that I should fit Celotex insulation boards to the underside of the ceiling joists and install vents into the front and rear fascia board to allow free air to pass between each of the ceiling joists. Is this correct?

 

[freddiemercurystwin]

This is a very common scenario when converting garages on the side of houses and lean to's like this. Its a 'fit the insulation between the joist and hope for the best affair'. A hybrid if you like. And most BCO's seem OK with this as the alternative will involve re-roofing and/or major reworking th the various roof junctions. When the roof eventually gives up the ghost, it can become a proper warm roof or vents fitted as required.

 

[CluelessIncompetent]

I think I'll just fix the insulation sheets to the underside of the ceiling joists.
I won't put anything between the joists but I will put vents in the fascia front and back. So free air should blow across the roof between each of the joist gaps.

I just wonder how well this vent idea of mine will work, the length of the joists are 14ft, will air circulate that far or will it stagnate in the middle?

I can't even find a professional builder who seems to know what to do.

 

[CluelessIncompetent]

Boom!


http://www.londonflatroofing.co.uk/cold-flat-roof-explained/

 

[catlad]

I would counter your joists with 3x1" before fitting insulation it will help the air flow better.

 

[freddiemercurystwin]

Well if its no issue to retrofit some vents then yes proper cold roof is best.

 

[CluelessIncompetent]

I would counter your joists with 3x1" before fitting insulation it will help the air flow better.

I'm sorry but what do you mean by that?

 

[catlad]

If you fix insulation panels to the bottom of your joists then you will create individual voids that need to be vented, if you put some timber say 3x1" perpendicular to your joist then you won't create those individual voids.

 

[CluelessIncompetent]

So are you saying that I should fill the cavity between the joists with timber?

 

[freddiemercurystwin]

No, what he is saying that if you were to fit only a couple of vents along the verges, you would not be venting each void between each joist. And if that was the case then you could fit some 1" battens underneath your existing joists at right angles to them and then fit your insulation underneath those, then you will ensure that cross ventilation is maintained.

Or alternatively just stick a vent in between each joist which I kinda assumed you would do anyway.

 

[CluelessIncompetent]

Ok, got it. Thanks.

I fitted my vents today. I fitted one in the fascia in between each joist cavity, front and rear.

The front vents are clear and unobstructed but the rear vents have a brick in front of them. There is 30mm gap between the vents and the bricks however. And a 40mm gap over the top of the bricks to the roof space beyond.

Is this enough space to allow free air flow or should I remove the top course of bricks behind my rear fascia board and install air bricks behind the fascia vents instead?

PS, my ceiling joists are 100mm thick at the lowest point. I was thinking I could :

A. fit 30mm insulation inside the joist cavities (leaving 70mm free air flow above) and then screw 30mm insulation sheets to the underside of the joists giving me 60mm of insulation overall or I could just fit 72.5mm Celotex PL4060 insulation sheets60mm insulation with a 12.5 plasterboard laminate) to the underside of the joists.

Would would be better, particularly from a thermal perpective? IS there any difference? Both would give me 60mm insulation overall...