50mm roof insulation enough?

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Hi,

Would appreciate your advice on the amount of insulation i should be installing in the roof.
Long story but its a flat roof and is constructed using 4x2 with 4 - 0inch firings ontop and osb3 ontop of that and finished with resin.

I'm not sure if 50mm insulation is enough or should i be installing 100mm?

My only concern is at the lower end when the firring starts to narrow down... if i used 100mm insulation it may block up air circulation .

Your advice would be most appreacited.

Thanks in advance.
 
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Hi,

I take it that this is insulation for the outbuilding you have mentioned in a previous post?
So, I believe it doesn't have to conform to BC standards?

Personally, I would put 50mm between the joists, and at least 50mm below the joists.
This would maintain the ventilation gap and avoid cold bridging, whilst not taking too much away from the ceiling height.

I would also cover the insulation board joints with foil tape to create a vapour barrier.

If ceiling height will be an issue, insulated plasterboard could be used an alternative, below the joists.

I hope that makes sense! :)
 
Hi yes it is.

So you saying there should be minimum 100mm insulation?
Ideally i dont want the ceiling height lowered.
 
Generally with insulation, the more the better, until a point of uneconomical returns.
The recommendations for homes is 270mm rockwool, or (according to some!) around 150mm celotex.
As this is an out building, the amount of insulation may depend upon its use.
Is the room going to be heated for instance?
If un-heated and a store, stick with 50mm.
If you can't lose ceiling space, stick with 50mm and some insulated plasterboard.

If the room is going to be used and heated, you could look at alternatives, like full-fill insulation and forgetting about ventilation!
 
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Hi thats again for the reply... yes the room will be used and heated.
Ive never heard of full fill can you please tell me more about this option?

I always thought it would need to be ventilated hence space left inbetween insulation and top of roof sheet
 
Hi thats again for the reply... yes the room will be used and heated.
Ive never heard of full fill can you please tell me more about this option?

I always thought it would need to be ventilated hence space left inbetween insulation and top of roof sheet

a cold roof needs a void directly below the roof deck to allow air to circulate. Insulation is fitted below throw roof joists.

a warm roof has insulation directly under the roof deck and there must be no gaps and there has to be a complete vapour barrier between room and cold roof, fitted on the warm side of the insulation.

if your firrings are inline with the joists and insulation can be pushed up tight directly under the roof deck, you could insulate as a warm roof, but you would have to use foil tape around every joist, connecting the celetex insulation foil together to act as a vapour barrier.

please note, this is a hybrid roof, there is still risk of interstitial condensation. But if you seal it really well - cut all the insulation 8mm smaller and foam up the joints, then foil tape, the risk is small.

if you do a cold roof and don’t have good ventilation and air flow you will again have condensation risk.
 
My firrings run perpendicular to the joists unfortunately.

I would imagine that at the end where the firrings are 4ich i shouldn't have a problems with installing a decent amount on insulation but at the lower end where the firrings decrease down to 0 i might have a problem.

Given my roof structure as it is, and with out lowering 5he ceiling height, can i effectly insulate ot to keep the place nice and warm in the winter?
 
You'll have to compromise somewhere! :D
If it's not on ceiling height, or ventilation, it's got to be thickness of insulation.
Whether that will be good enough - only time will tell, but anythings better than nothing! ;)
 
Firstly, 50mm of what?

Secondly, keeping a room "nice and warm" in winter is not solely dependent on the roof insulation.

Generally you need as thick an amount of insulation as you can fit, of the most efficient type you can find. And this includes fitting to avoid thermal bridges and gaps and voids.

And it's no good just concentrating on the roof and having poorly insulated walls and floor and windows and door
 
With 75mm insulation baords i would imagine will should leave plenty room for ventilation at the lower end of the firrings.

But I'm just not sure if 75mm will be enough to keep the heat in..

I know theres many other factors to consider when calculating the heat rentention, but assuming walls floor are well insulated and windows are good.... how good will 75mm insualtion hold up in the roof?
 
Firstly, 50mm of what?

Secondly, keeping a room "nice and warm" in winter is not solely dependent on the roof insulation.

Generally you need as thick an amount of insulation as you can fit, of the most efficient type you can find. And this includes fitting to avoid thermal bridges and gaps and voids.

And it's no good just concentrating on the roof and having poorly insulated walls and floor and windows and door
Snap... i was writing the same thing just as you sent the above.

I agree totally, windows are argon filled and walls 100mm insulation, floor has 50mm celotex
 
I'd fit quilt between the furrings up against the deck, no voids. 100mm PIR between joists. At least 25mm PIR under the joists. Tape all joints and seal all gaps to form a vapour check layer. And likewise seal the roof perimeter externally.
 

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