Insulating problem

[iandandav]

I wish to add loft insulation in an old terrace. The pitched roof is such that the lower part of the rafters forms part of the sloping ceilings in the bedrooms. See pic (first diagram). The roof has an old style non breathable bitumen type membrane. At the moment there is ventilation between the rafters (red arrow). I wish to add insulation between the lower parts of the rafter (pink line) as well as horizontally over the plaster board ceiling (purple on diagram) to ensure the ceiling area marked with the blue arrow (second diagram) is insulated. Trouble is if the insulation is fitted here it stops the ventilation. What is the solution besides fitting breathable membrane. Would ventilation elsewhere in the attic space suffice or must it be down at the eves?

 

[DAZB]

An option may be to cut and fit a bevelled section of kingspan to the bedroom side then lay lining paper and paint it, assuming it would match your decor. That would create a warm surface from below rather than above allowing you to maintain airflow above.

 

[freddiemercurystwin]

Your roof relies on the eaves being open on both sides in order for it to breathe therefore you cannot block these junctions up. If you were to block them up and provide additional tile vents just above the ceiling level in your drawings to retain cross roof ventilation you will end up with a section of roof that is unventilated leaving the timbers prone to rot in these locations.

Ideally you would fit insulation into the void and retain an air gap between it and the underside of the roof thus retaining a ventilation route. This is though, as you can imagine, somewhat easier said than done especially if you are not considering removing he plasterboard which you no doubt wish to avoid!

How deep are the rafters?

 

[freddiemercurystwin]

An option may be to cut and fit a bevelled section of kingspan to the bedroom side then lay lining paper and paint it, assuming it would match your decor. That would create a warm surface from below rather than above allowing you to maintain airflow above.

Or fit some Kingspan and then some plasterboard beneath it would be sensible? Again though this will involve getting a plasterer in to finish it off unless you can plaster yourself.

 

[iandandav]

Your roof relies on the eaves being open on both sides in order for it to breathe therefore you cannot block these junctions up. If you were to block them up and provide additional tile vents just above the ceiling level in your drawings to retain cross roof ventilation you will end up with a section of roof that is unventilated leaving the timbers prone to rot in these locations.

Ideally you would fit insulation into the void and retain an air gap between it and the underside of the roof thus retaining a ventilation route. This is though, as you can imagine, somewhat easier said than done especially if you are not considering removing he plasterboard which you no doubt wish to avoid!

How deep are the rafters?

Plaster board is mostly off at the moment so complete access.

 

[iandandav]

DAZB freddy, good suggestion never thought of insulating from below. Ive never used kingspan. How is it attached to the rafters, special nails/screws. My plastering is slow but very good. Do you just skim it?
What type, thickness do I use. Ive seen a variety of them

 

[DAZB]

You can use 2" if you like as depth of kingspan is not too much of an issue in this case and if you install plasterboard over the top then just go with drywall screws that are long enough to accomodate the kingspan you use and the plasterboard, so if you use 2" kingspan and 12.5mm board then drywall screws of 90mm are available.

 

[freddiemercurystwin]

Keep a 50mm gap between the underside of the felt and the insulation. If you've got the obligatory 3 layers of rockwool insulation at ceiling level you'd be better off fitting about 100+mm of Kingspan though. A proper job would be to friction fit Kingspan between the rafters (leaving the air gap) then fit a continuous layer beneath the rafters then fit your board beneath that.

Bit like this:

Untitled

• freddiemercurystwin
• 8 Jun 2010
• 20

 

[iandandav]

Thanks for your help.
Kingspan onto the rafters it is.
The rafters are only about 3in thick so probably couldnt fit kingspan between them and have sufficient gap above.