Loft boards insulation in roof trusses

[theDIYkid]

Hi there
I have an old detached bungalow and want to put a music room in the loft as well as use for storage. The gap between the felt and edge of the trusses is 75mm, can I put kingspan or polystyrene in between the joists to insulate the roof then plasterboard. If so what is the max thickness of insulation boards I can use, 50mm? leaving a 25mm gap between the felt and tiles from the boards?
thanks

 

[foxhole]

Yes , you can gain additional insulation by lining with insulated plasterboard.

 

[theDIYkid]

so 25mm then insulated boards

 

[plastic_peanut]

The insulation should sit flush with the rafters, the thickness of the boards is then down to personal preference and budget.

 

[theDIYkid]

Yes but I only have 75mm from tiles/felt to edge of rafters, so I am reading I need to leave a 50mm gap for air flow between the felt and insulation which means I can only use 25mm kingspan to edges which would give me the 50mm behind...then as you say use insulated plasterboard as oppose to standard plasterboard for better insulation, is all that correct?will 25mm be enough thanks

 

[theDIYkid]

Yes , you can gain additional insulation by lining with insulated plasterboard.

Thats good but can you tell me if the mx kingspan I can use is 25mm to leave the 50mm gap behind for the air flow? thanks

 

[foxhole]

Thats good but can you tell me if the mx kingspan I can use is 25mm to leave the 50mm gap behind for the air flow? thanks

Yes , but you could add 25mm baton to current to give yourself room for 50mm insulation [its cheaper than buying thicker insulated plasterboard and easier to install due to the small sections required].Buy thickest insulated plaserboardboard you can afford.

 

[theDIYkid]

Agh cool, I really dont want to lower the ceiling anymore but I will take a look at that mate...otherwise its the 25mm and good insulated plaster. Thx

 

[theDIYkid]

Hi, If I use the 25mm and leave the 50mm gap behind, do I need to add vents to the centre top ridge beam that runs along the whole of the house rafters, does the air flow need to go from one side and over the top to the other side, if so that would mean adding a vent in between all all the top end of the insulation points. I attached a pick to illustrate, hope you can help, hoping I dont need to to this...thanks

 

[Wayneox]

The felt is a water barrier, not vapour. The damp air should escape through the felt.. the ventilation gap is so you have enough storage space to hold the damp air without it condensing on the insulation before it can escape through the felt. Remember that this is the cold side so the air will be traveling down too (warm air rises , cold air sinks)

 

[Wayneox]

You could also compartmentalise the space by a simple (insulated) stud wall. This will mean you only insulate the roof space you need to use. (It's cheaper to heat up a smaller space). And the sound will echo in a larger space too... Which may lead to neighbourly disputes.

 

[theDIYkid]

Hi thanks for that, yes I am making a room in the loft so less than half the loft for a music room, the rest of the loft will remain as is...I am going for Rockwool for its acoustic properties and they told me I could use 50mm leaving a 25mm air gap as the 25mm would not do much to stop sound...also they said at the ridge leave an inch for the air to be able to travel from one side to the other, that sound right? of course that would mean I have an inch all way along the ridge for sound to escape and come in so I am not keen on that...do I need to make sure the air can travel one side of the roof to the other? is a pitched roof. thx

 

[Wayneox]

You can build a stud frame onto the rafters if you need more. But it's not the sound going up. It's the sound between walls you should worry about

 

[Wayneox]

What is the setup at the ridge tiles. Is it a cement or dry ridge or...

Have you considered a flat roof instead of them joining .. also... After insulation... There will be a gap at the top... As you'll lose about 8-15cm either side.... So plaster board will intersect lower that your. Ridge beam...

 

[Leofric]

You need a 50mm continuous air gap over the top of the insulation where it follows the pitch of the roof ,with ventilation at eaves and ridge.
Incidentally,your underfelt looks to be the 'old ' type that isn't breathable.