roof rafter insulation and ventilation

[ch4chi]

Hi,

First post so please be gentle, here goes in my home i have an already functioning attic room which had a fair sized space to the side which i have recently knocked into. The issue is the room is freezing cold during winters, after some investigation i have found that there is absolutely no insulation in the ceiling or roof, there was just plaster boards put on to the rafters and stud walls had a little old style glass wool insulation put in and then rolls of insulation rolled over the floor space where i have knoccked into.

With regards to ventilation: At the moment there are vents or meshing at the eves where i can see daylight through, behind the rafters there is some very old heavy looking felt above which there are the roof tiles. I am planning on purchasing some 1000mm celotex/kinspan to put between the rafters before plater boarding over, at the moment i only have 3x2 for rafters and will have to double them up to provide enough depth.

The issue is is that i have been told i need to leave enough space to ensure enough space is left for ventiltion, how do i do this? is there some kind of spacer or other device/tool i need to use to maintain a gap? and how much of a gap is needed?

On a side note is it worth me reinsulating under the already existing floor as at the moment there is a upto 5-6 inches of old style glass wool which looks very worse for wear, is it worth taking up the floor boards and replacing it with new stuff?

Any advice would be much appreciated, and thanks in advance.

 

[freddiemercurystwin]

Assume you are making the existing room/rooms larger rather than creating an additional separate room? Otherwise there could be fire escape ramifications. If the floor where you are breaking into is not actually a floor but just a ceiling at the moment then the structure will need upgrading. Its a bit of a grey area as to whether these works are notifiable ie Building Regs required or not. If the floor needs upgrading it definitely needs Building Regs.

You need to leave a 50mm gap to allow for airflow and the airgap needs to be linked from the eaves all the way to the ridge. You need to put in a tile vent under the ridge between each rafter or change the ridge tiles for a continually vented system. If you don't do this you will not achieve any cross vent up the roof and condensation and consequently damp timbers will result leading to rotten rafters eventually.

As you have 3" deep rafters you should be fitting say 40mm battens to the underside of your existing, then fitting 65mm Celotex/Kingspan between the rafters (using small nails/pins or small battens to prevent the insulation being pushed up against the felt and maintaining the 50mm air gap) the insulation can be friction fitted or held with glue or nails (think small pins rather than 4" nails) and any gaps filled with expanding foam. Then fit a continuous layer of 50mm foil faced Celotex or Kingspan beneath the rafters, tape all the joints with foil tape and then fit your plasterboard under the insulation, screwing through into the rafters.

Whilst mineral wool insulation is equivalent to about 70mm or thereabouts of Celotex and whilst its thermal qualities do not really deteriorate over time it may look a bit knackered but it will still work. That said mineral wool is not very effective when used in a vertical situation as it sags over time so it would be worth whacking 100mm Celotex in the dwarf walls.

Mineral wool in the new area of floor is sometimes required, not for heat though but for fire resistance; it is usually required when you have a lathe and plaster ceiling and then it ought to be suspended with chicken wire or equivalent.

You ought to be upgrading the insulation on the existing party wall too or that will be a magnet for heat loss unless your neighbours have converted their loft too.

The existing mineral wool insulation in your already converted floor can be left alone there are no advantages to replacing it.

 

[ch4chi]

thanks for the quick and detailed response, the space knocked into will require joints and floor boards to be laid?

" You need to leave a 50mm gap to allow for airflow and the airgap needs to be linked from the eaves all the way to the ridge. You need to put in a tile vent under the ridge between each rafter or change the ridge tiles for a continually vented system. If you don't do this you will not achieve any cross vent up the roof and condensation and consequently damp timbers will result leading to rotten rafters eventually."

please clarify the above, can I not add the insulation and just using tacks/nails/batonns ensure there is 50mm space behind the I insulation so the air flow from the eves is able to flow around behind the insulation panels?

 

[ch4chi]

thanks for the quick and detailed response, the space knocked into will require joints and floor boards to be laid?

" You need to leave a 50mm gap to allow for airflow and the airgap needs to be linked from the eaves all the way to the ridge. You need to put in a tile vent under the ridge between each rafter or change the ridge tiles for a continually vented system. If you don't do this you will not achieve any cross vent up the roof and condensation and consequently damp timbers will result leading to rotten rafters eventually."

please clarify the above, can I not add the insulation and just using tacks/nails/batonns ensure there is 50mm space behind the I insulation so the air flow from the eves is able to flow around behind the insulation panels?

 

[freddiemercurystwin]

There will be a ridge board at the top of the roof that the rafters fix to that prevents the air from blowing from one side to the other so no air travel will occur without ventilation at the ridge. Avoid the high level ventilation at your own risk.

 

[ch4chi]

Thanks again, and please bare with me- i think i understand now, so the ventilation needs to cross from one side to the other but with the ridge (wood going acroos the top at which point rafters meet) will prevent the air travelling from one side to the other.

Padon my ignorance but doesnt the fact that air is going up through one side mean it is being ventilated already via the vent at the eves? as long as both sides are being ventilated from one side, why does it need to pass from one through to the other?

Also is there no other means of ventilation without me messing with the roof tiles by putting in tile vents?

 

[freddiemercurystwin]

It should really be called cross-venting as it needs a vent on each end otherwise yes I would acknowledge that the void is vented to the open air but you are trying to achieve a breeze within the air gap that draws any moisture with it.

You could maybe fashion a bit of a bodge-up in the lap of the felt with a timber fillet shoved in the gap depending where the lap occurs if you get my drift? Won't get past Building Regs but would work in theory.

 

[ch4chi]

sorry but I don't understand what you mean by the alternative to the vents, I'm generally very competent in DIY but this is all new to me, can you please dumb down your explanation as you lost me... in terms of the lap of the felt and timber fillet???

 

[freddiemercurystwin]

Untitled

• freddiemercurystwin
• 21 Aug 2012
• 1

Untitled

• freddiemercurystwin
• 21 Aug 2012
• 2

Gettit?

 

[ch4chi]

now I do, thanks for that illustration. ok, I have another question (sorry for doing your head in), how many of these would be required, just 1 on each opposing sides of the roof? I'm in a semi so do I need to put the same amount of these in the 3rd side of the roof??

 

[freddiemercurystwin]

As air cannot pass through the rafters you need one between each rafter so you'll have a lot to do.

 

[ch4chi]

I've had what sounds like a good quote for the installation of the roof tile vents, as such just wanted to ask how many you think I would need to install, the roof is on a bigger then your average semi home, and again would I need them on just the 2 opposing sides of the roof or on all 3 sides?

 

[freddiemercurystwin]

Gahhhh!

Its the same as the fillet in the felt bodge, you need one between each rafter just below the ridge.

 

[ch4chi]

Thanks again mate, youve been a great help, appreciate it!

 

[ch4chi]

Hi hoping you could help, again. I'm now ready to put the inaction in the rafters, the question is with regards to your suggestion of placing a bung in between the felt lap to encourage air flow ventilation. Question is in each rafter I have a total of three felt raps so where do I place the bung, top middle or bottom????

Please help, your advice is greatly appreciated, thanks in advance.