dormer bunglo insulation

[archie84]

hi. iv bought a dormer bunglo and im indulaying it at the moment. the void inbetween ground floor ceeling and upstairs floor is totaly empty and iv pushed and pulled rockwool 150 all the way through stopping just short of the soffit vent that i can see in the acces hatch. iv also insulated the upright walls before the pitch roof from the hatch side and also the centre flat loft roof.

now this has improved the house allot due to the whole ceeling and floor being in contact with outside air through the sofit vent.

my question is can i now insulate down with the pitch???

i can do this with either insulation boards or wool from the attic acces but i dont know if i should leave an air gap?

the roof is slate, built with trusses and the old bitumen type felt.

thanks

 

[wessex101]

If it is old non-breathable sarking felt you should technically leave a clear 50mm air gap between the top of the insulation and the sarking felt/underside of roof tiles.

A good quality rigid foam insulation like Kingspan will be better than a rockwool type quilt.

 

[freddiemercurystwin]

Is the ridge vented?

 

[garyo]

Is the inside plasterboard or lath&plaster? For rigid to work you need to be able to slide a board down the pitch and for it to be in good flat contact with the back of the plasterboard with no gaps due to gobs of plaster sticking up etc. Ideally you also need a good tight fit between the rafters which is easier said than done because your rafters are bound to wander a little.

I read a suggestion that a way of achieving this is to cut the board as you would have done anyway (to fit the gap exactly) then cut it down the middle at a very slight angle - say 5 degrees - to form a pair of sliding wedges. You can then put one side in, then push the other side in until they both press against the rafters. This will also help with fitting because there's invariably a collar truss joist at the top that would stop you being able to slide a whole piece in anyway. This isn't something I've actually tried, but may be soon

Rockwool could work if you could find a way of maintaining airflow... assuming you need the airflow...

 

[archie84]

hi. thanks for the response. its plasterboard so i was hoping to slide the boards down. its the old non breathable type with no ridge vent, just a sofit vent strip at the flatboard undearneath the facier.

what bit is meant to be vented. between the slate and the felt, or between the felt and the insulation or plasterboard.

 

[garyo]

If you imagine that water vapour can pentrate the plasterboard anywhere, and condensation can then form on the rafters and battens (i.e. between the felt and plasterboard), you have to provide a path for this to escape. Typically you have over fascia or soffit vents on both sides of the house and this promotes a cross-draft from one side of the roof to the other. How effective this is in a dormer roof scenario is dubious even with 100mm clearance between the plasterboard and felt even without you choking it up with insulation.

If you have ridge level ventilation then that's much better because you'll get a natural chimney effect. You can simulate the same effect by getting in the attic and sliding some wedges down the back of your felt laps to create air gaps, especially if you have naturally gappy tiles. People even sell proprietary wedges to do this, but a bit of wood works equally well.

 

[archie84]

hi. thanks for the advice. all insulation sorthe now with great results.
I hav howeaver noticed that the walls and ceilings now develope a hairline crack along the edgrs and corners. All the finishing is artex and not smooth pink finish. would this be just shrincage in the plasterboard?
I would have thought with the rooms being warmer now the liklehood would be for stuff to expand not shrink.

 

[garyo]

hi. thanks for the advice. all insulation sorthe now with great results.
I hav howeaver noticed that the walls and ceilings now develope a hairline crack along the edgrs and corners. All the finishing is artex and not smooth pink finish. would this be just shrincage in the plasterboard?
I would have thought with the rooms being warmer now the liklehood would be for stuff to expand not shrink.

What method did you use to get PIR fitted in to the pitch cavities?

 

[archie84]

hi. what do you mean by PIR.?

 

[garyo]

hi. what do you mean by PIR.?

Solid insulation board.

 

[archie84]

ah right. i glued 25mm boards with instastick for the part of the pitch that had plasterboard on and then topped it up with 150mm rockwool with roofing menbrain (tyvek) stapled to a piece of timbre at the bottom and then pulled it tight before stapeling it at the top in the attick area. this allowed a gap of arround 50mm from the felt and insulation. did the same with the small walls either side of the room in the storage areas. and used a rope to pull 200mm rockwool from one side of the floor joists to the other side.

 

[Nakajo]

(It's easier to pronounce than Polyisocyanurate)