Roof insulation advice.

steve22

Hi, im currently working on my attic conversion and was hoping to get some advice on insulating my roof.
I plan to insulate all the way out to the eaves and im wondering what is the recommended insulation for roofs and how do people ensure the required ventilation gap remains?
The rafters are 100mm deep and i believe you need 50mm for ventilation. Is it possible to insulate the rafters horizontally? I mean could you fix 100mm insulation boards across the rafters giving the full depth of 100mm for ventilation while still having 100mm insulation?
Or, is it better just to insulate between rafters?
Any advice would be appreciated, thanks.

Philip82

Hi. you could nail 2x1 lath on rafters up against the felt so that when you stick a 60mm kingspan or equivelant sheet you will be flush with the slab surface and you will ensure your 50mm gap is kept. I have even seen people pull a chalk line 50mm from the felt and bang in a few nails just to keep the boards from getting shoved up to the felt.
Another option is spray foam insulation which would be a much better job for air tightness and u values.Before the spray foam is sprayed they put in a grid which maintains a ventilation gap before its sprayed in.
Too be honest 4 inch rafter are very skimpy so if you have the head room I would then do as you said put another insulated slab on it horizontely or add two inches to your rafters with 2x2's and add 100mm boards between them.

steve22

Thanks Philip. I looked into the spray foam too and it seems better for airtightness like you said... Its probably easier to insulate those awkward areas by foam too but its something i wouldnt be able to apply myself- then again that mightnt be a bad thing.
What would most people recommend for insulating rafters? Foam or insulation boards etc?

DaveSave

Well Steve

I would go for a full fill with spray foam 2 reasons, its cheaper and performs way better. When you spray it from wall plate right up to ridge you stop all air movement unlike cutting in rigid insulation. The spray foam takes about 4 hours and there is no waste, unlike the rigid which takes ages then there is loads off waste labour and 99% of the time its not sealed and then the air movement effects its performance. it also saves you on the air tightness system siga or the likes. I would go for a full fill with the vent card then trim it level and then fix a thermally lined plaster board.
We use to do rigid insulation installs but since we purchased the spray foam rig its just a much higher quality jib and it costs less.

Dave

Edmond_Dantes

spray foam insulation has worked well for us in a similar situation

paddy147

Come on now lads.......do I have to be the one to post the pics?????:pac::pac::D

TheLastMohican

paddy147 wrote: »

Come on now lads.......do I have to be the one to post the pics?????:pac::pac::D

Paddy, the pics look like a commercial building. And what looks like baffles. If on a house, you sprayed on the bare slates/tiles, would you not be dicing with interstitial condensation? In some of the ads for spray insulation, they recommend it for keeping loose slates/tiles in situ.

paddy147

TheLastMohican wrote: »

Paddy, the pics look like a commercial building. And what looks like baffles. If on a house, you sprayed on the bare slates/tiles, would you not be dicing with interstitial condensation? In some of the ads for spray insulation, they recommend it for keeping loose slates/tiles in situ.

Eh,its our house and those are vent cards installed in the attic....just before spray foaming commenced

The foam is open cell foam,as its for use in a domestic house.

TheLastMohican

paddy147 wrote: »

Eh,its our house and those are vent cards installed in the attic....just before spray foaming commenced

The foam is open cell foam,as its for use in a domestic house.

Gotcha Paddy. If you hadn't put in the vent cards/baffles, and had sprayed on sheer slate/tile, would that cause interstitial condensation? In much the same way that insulation board can when mushroomed to an open north-facing wall?

steve22

Thanks for all the replies/photos - It looks like spray foam is the future so.
If it is, as people say, - cheaper and better performance it make sence to go with this...
Would people advice that i install the roof window before insulation so that insulation could be sprayed right up to the window or could the window be installed afterwards?

Philip82

Its not going to be any addition having the window in place because the foam wont be touching the window because the velux sits between rafters ie. you will have 2 inches of timber all around the frame of the window. You will however need to have the frame size cut out of the rafters with any additional supports in place.
Best of luck with it!!!

steve22

Philip82 wrote: »

Its not going to be any addition having the window in place because the foam wont be touching the window because the velux sits between rafters ie. you will have 2 inches of timber all around the frame of the window. You will however need to have the frame size cut out of the rafters with any additional supports in place.
Best of luck with it!!!

That makes sense... I completely forgot you'd have the rafters/supports around the window anyway.

DaveSave

TheLastMohican wrote: »

Paddy, , they recommend it for keeping loose slates/tiles in situ.

That's the closed cell spray foam it's seriously strong. It's a plastic in effect and complete moisture barrier . You can achieve great u values with it and its mainly used on metals,
Then open cell on timbers.

BobDaley

Spray foam insulation will definitely help; it's airtight and doesn't degrade over time the way that foam slabs will. If you have questions about anything to do with the product I am happy to try and help!

filbert the fox

Great advice so far BUT may I piggyback on your question with the following?

1. Is breathable roof felt the solution to interstatial condensation and if one has it how does one recognise it?
2. I'm living in a house which incorporated the attic room as a bedroom in the design. Trouble is it's draughty and freezing in winter. there's fibreglass insulation between rafters and visqueen "air tightening " membrane supposedly creating a seal. The house is ventilated and heated through a warm air system with solar in summer and gas boiler in winter. Do I need to worry about condensation when there is a continuous air circulation system in operation?
3. Is roof ventilation solely concerned with moisture and therefore wood rot/mould? If so why does it not apply to all wood in the house e.g. partitions?

MiketheMechanic

Philip82 wrote: »

Hi. you could nail 2x1 lath on rafters up against the felt so that when you stick a 60mm kingspan or equivelant sheet you will be flush with the slab surface and you will ensure your 50mm gap is kept. I have even seen people pull a chalk line 50mm from the felt and bang in a few nails just to keep the boards from getting shoved up to the felt.
Another option is spray foam insulation which would be a much better job for air tightness and u values.Before the spray foam is sprayed they put in a grid which maintains a ventilation gap before its sprayed in.
Too be honest 4 inch rafter are very skimpy so if you have the head room I would then do as you said put another insulated slab on it horizontely or add two inches to your rafters with 2x2's and add 100mm boards between them.

Sorry to bring this up, but I'm thinking of doing something similar in my own attic. i.e. fix 2 x 2" laths to the rafters to accomodate 100mm Kingspan TP10 or Xtratherm rigid insulation, leaving a 50mm airgap, and then to fix plasterboard over this. A builder friend seemed to think that this would provide a bridge for the cold to get in, as the cold could travel through the rafters and on to the plaster board. Should an additional insulated slab be added horizontally across the rafters? Does the rigid insulation need to be sealed with tape to stop air movement?

Thanks in advance

Philip82

Hi Mike, ye ur friend is right air would travel down onto the slab no matter how tight you try get the board in. You can get tape but a air tight membrane is on the market that will cover all the rafters before slabbing. Insulated slabs would increase the value of the insulation aswell, however spray foam might be the easiest and cheapest option.

MiketheMechanic

Philip82 wrote: »

Hi Mike, ye ur friend is right air would travel down onto the slab no matter how tight you try get the board in. You can get tape but a air tight membrane is on the market that will cover all the rafters before slabbing. Insulated slabs would increase the value of the insulation aswell, however spray foam might be the easiest and cheapest option.

Thanks Philip - any idea what the air tight membrane is called, or where I would get it? How does it fit around Velux windows I wonder?

Thanks
MtM