Cut roof or trusses?

[flanagaj]

I am currently trying to weigh up whether to buy trusses or to cut the roof myself.  It's a very straightforward dual pitch roof with 4.2m rafter length, 7m span and a low 1.2m ridge height from plate.  Although cutting the roof will be easy, I do wonder what the actual cost difference is likely to be between the materials for cutting it myself and buying in trusses.  I like trusses for the simple reason, that fitting PIR insulation between the rafters is nice and simple as the ceiling joist is joined to the rafter with a plate so no need to notch out like you do when fitting PIR into a cut roof rafter.

 

[JohnMo]

28 minutes ago, flanagaj said:

trying to weigh up whether to buy trusses or to cut the roof myself

Very different beasts from a structural perspective.

 

A cut roof needs a ridge beam, trusses don't.

 

34 minutes ago, flanagaj said:

fitting PIR insulation between the rafter

Wouldn't you fill with mineral wool - perfect fit every time. Use something like Rockwool Flexi.  Better decrement delay than PIR - keeps heat out better.  Then under-draw with PIR?

[flanagaj]

43 minutes ago, JohnMo said:

Very different beasts from a structural perspective.

 

A cut roof needs a ridge beam, trusses don't.

 

Wouldn't you fill with mineral wool - perfect fit every time. Use something like Rockwool Flexi.  Better decrement delay than PIR - keeps heat out better.  Then under-draw with PIR?

Ok, thanks.  Good question regarding the insulation.  The technical architect has spec'd the PIR between the rafters.  I didn't question that.

[JohnMo]

Just now, flanagaj said:

technical architect has spec'd the PIR between the rafters.

In theory better u value, in practice a pain to seal against rafters to get perfect seal. If you don't get a very good fit, you get bridging. So a softer material like mineral wool fits tight and no bridging. Real U value is better generally for way less work.

[nod]

You will probably save around 30% 

Presuming your time is free 

Many of the jobs that I’m working on are using the multi foil That comes in a consetina form Just pushes Hes between the rafters Hardly any cutting or waste 

 

I’d did ours in 50% trusses and 50% loose rafters 

There was quite a bit of work birds mouthing the trusses But even more work doing the same plus the angles on the loose 

[sgt_woulds]

Also, PIR shrinks over time and when exposed to the high temperatures typical under a roof.  Even if by some miracle you get a perfect fit when installed, as it shrinks, you will get thermal bypass.

 

I agree that flexible insulation is better in this situation.  Flexible woodfibre would be better still as the decrement delay will help with overheating issues we are likely to experience ever more often.

 

On my retrofit, I used woodfibre between cut rafters (allowing minimum 40mm ventilated airspace) and PIR underneath, fully taped and sealed, and acting as the VCL and airtightness.  This was a compromise to meet the U-value requirements and stay within the structural capacity of the existing timbers.  

 

Has worked very well through the heatwaves so far, and no sign of PIR shrinkage as it is protected by the woodfibre above.

[JohnMo]

32 minutes ago, nod said:

multi foil

As a side note to others: On its own the decrement delay is tiny, plus if you do not respect the minimum air gaps either side of the product - you may as well not bother buying or installing it. It needs the same amount of depth as any other insulation once the required air gaps are accounted for.