dry wall screws

scalerb

Hi All
I will be insulating roof soon, plan is to use
100mm rafter-lock between rafters (50mm air gap above)
140mm taped polyurethane insulation board under this followed by
12.5mm plaster board

Can i get a dry wall screw to hold up the insulation and plasterboard it would need to be around 200mm long or is there a better way.

Also do i need to install a vapor barrier between pb and insulation or will the taped insulation be ok.

RKQ

I think you will have to re-think your insulation structure / make-up.
I've never seen 200mm long dry-wall screws.
If they exist they will be expensive and hard to use.

Imagine screwing in a slab, that is over your head, with 200mm screws! You would be holding that slab a long time!
200mm screw into a 50mm wide joist / rafter would be very difficult to keep straight. A crooked screw could split the timber... nightmare!

You'd need a great drill with lots of spare batteries!

scalerb

or is there a better way

RKQ

I would suggest that you look at our European cousins.
They insulate between the rafters and on top of the rafters. There are a number of special fixings (long) on the market to fix the insulation onto the rafters.
It may be possible to insulate above, between and below the rafters!
Sometimes it can be more practical to ply over the rafters, then apply high density insulation. ( The Foil backed insulation is stronger if supported on 9mm / 12mm ply - easier & safer for roofer!)

Slig

alternatively, cross batten perpendicular and below the rafters and insulate between. you are increasing the bridging factor but the intersections between them will only be small. you will also have a more structural backing for your slabs.

Villain

What size rafters are best to use to allow for good roof insulation, 7x2 or 9x2 or would 9x2 be ott?

Slig

9x2 would give you 175mm of insulation between but also allow for insulation below, maybe 50mm cozyboard. 7x2 will only allow for 125mm

Villain

How do you mean allow for insulation below also?

Slig

When I learned detailing first, timber wasent considered a cold bridge.
Now however, if you are putting up to 175mm high performance insulation then the timber will not perform as well, this can be reduced by fixing 50mm composite board (35mm insulation 12.5mm plasterboard) to the underside of the rafters.

topcatcbr

This should have got a new thread i think. came across by accident

Villain wrote: »

What size rafters are best to use to allow for good roof insulation, 7x2 or 9x2 or would 9x2 be ott?

I would consider a warm deck type construction
see link http://www.insulation.kingspan.com/uk/pdf/k7.pdf

I would consider the unventilated (Unventilated – Insulation Between & Over Rafters)
Option.

I know its not standard practice at moment but it has definate advantages.

Villain

I was considering a new thread but thought it was related.

I have been looking at warn roof details and it looks good but the only thing any Carpenter I know has never done it and I have fears that the timber may not respond well over time but that could be totally wrong.

The main reason I'm looking insulating between and under rather than over is that I can do it myself where as my Carpenter's would have to do a lot more work in a warm roof. I am currently looking at doing what is on page 492 of the new homebond book, which shows 50 mm air space under felt then 100mm of insulation between rafters and then 50 mm below and across rafters but I will up that to 125mm between with a 7x2 rafter.

So that will give me a 175mm of insulation I will also put a vapour control layer bwteen the 50mm and rafter, what you think?

topcatcbr

Villain wrote: »

I was considering a new thread but thought it was related.

I have been looking at warn roof details and it looks good but the only thing any Carpenter I know has never done it and I have fears that the timber may not respond well over time but that could be totally wrong.

The main reason I'm looking insulating between and under rather than over is that I can do it myself where as my Carpenter's would have to do a lot more work in a warm roof. I am currently looking at doing what is on page 492 of the new homebond book, which shows 50 mm air space under felt then 100mm of insulation between rafters and then 50 mm below and across rafters but I will up that to 125mm between with a 7x2 rafter.

So that will give me a 175mm of insulation I will also put a vapour control layer bwteen the 50mm and rafter, what you think?

I havnt seen the new HB manual yet. They usually give good advise and I would be slow to go against anything they suggest. (who am I to argue).

Having said that it will depend alot on the design of your roof.

I have begun leaning to warm deck type construction as it is difficult at times to guarentee the ventilation is adequate. Therefore with the additional insulation requirements i would be inclined to design out the need for any significant ventilation in the roof structure. This also has the advantage of reducing the need for oversizing your rafter timbers. i have also noticed a trend of sheathing the top of the rafters in osb or ply then breather membrane and batten and tile (I think this will become the standard). As for timber frame walls.

I was a carpenter and do not think it would be too difficult to adjust too.

RKQ

I put insulation on top of my rafters in 2003. It works fine.

Its difficult to get carpenters to change. We didn't ply the roof but this detail in standard in Scotland.

The ply is good from a practical point of view - health & safety! As it supports the insulation. Makes installation safer.

50mm insulation to underside is difficult - long screws ( hard to keep straight)
and difficult to work over your head! Much easier to lay over rafters and work on incline.

Important to counter-batten, as per standard manufacturers details, but rarely seen in practice!

Also remember that all breather felts are UVA sensitive! So you must install bitumen felt at eaves.

Villain

A mate is a rep for a well known company that supplies dry wall screws and he reckons there is no problem getting them to do what I want. I do appreciate that a warm is better but its not suitable for me for cost reasons.

I am still considering sheeting the roof and counter battening so I won't need the 50 mm vent space below the slates, would that help to make the insulation easier and give me the full 7" to insulate??

topcatcbr

Villain wrote: »

I am still considering sheeting the roof and counter battening so I won't need the 50 mm vent space below the slates, would that help to make the insulation easier and give me the full 7" to insulate??

Be carefull here unless the timber is inside the insulated envelope it will require an min of 50mm continuous ventilation. Placing ply or osb on the rafters does not change this requirement.

Villain

ah ok so I can't use Ply unless I insulate above the rafter?

Slig

I'm not too sure about that.
I dont see a problem in filling the cavity with insulation and sheeting with ply. then fixing your breather membrane, battening and cross battening.
From a practical point of view it is exactly the same as a timber frame wall as long as there is a vapour check on the inside.

Villain

New thread open: http://www.boards.ie/vbulletin/showthread.php?p=56436052#post56436052