Advice on Insulating under rafter instead of between

[MarkH395]

Hi all

I have started to convert an old barn into a 2 bed bungalow that will have vaulted ceilings through the majority of the building. I’m getting towards the point of needing to insulate the roof.

 

The plans say in put 50x50mm lengths of wood up the existing 100x50mm rafters then insulate between and then overboard with insulated backed plasterboard.
 

It wouldn’t be bad in a new build to do it this way but the old rafters are all slightly off spacing, are twisted and not square. (200 year old building, nothing is square)

 

my idea was to just to screw the 100mm insulation directly to the rafters, maybe add a layer of OSB board below that then insulation backed plasterboard below that. 
the osb board would be there to provide a fixing point for the plasterboard and light fittings attached to the ceiling. 
 

any advice or ideas would be appreciated 

[Thorfun]

put glass wool batts between the rafters as they'll 'squash' to fit the wonkyness to some extent and then put PIR internally. would mean you could reduce the amount of internal PIR and still get the U-value you want.

[Carrerahill]

13 minutes ago, MarkH395 said:

Hi all

I have started to convert an old barn into a 2 bed bungalow that will have vaulted ceilings through the majority of the building. I’m getting towards the point of needing to insulate the roof.

 

The plans say in put 50x50mm lengths of wood up the existing 100x50mm rafters then insulate between and then overboard with insulated backed plasterboard.
 

It wouldn’t be bad in a new build to do it this way but the old rafters are all slightly off spacing, are twisted and not square. (200 year old building, nothing is square)

 

my idea was to just to screw the 100mm insulation directly to the rafters, maybe add a layer of OSB board below that then insulation backed plasterboard below that. 
the osb board would be there to provide a fixing point for the plasterboard and light fittings attached to the ceiling. 
 

any advice or ideas would be appreciated.

 

I have done both methods, a combination of both and various others. 

 

First off, I would highly recommend insulating it now, as you suggest you will do, because it will make your life so much easier and healthier, PIR in a enclosed room is no fun, working with an open roof structure will be good.

 

I would probably work from the outside and fit, as best I can, PIR of 75% depth of the rafters, cut the sheets to max width, then with the help of a handsaw encourage the PIR down between the rafters using the rafter as a guide for the saw, then, a small bead of expanding foam to seal. Once complete, internally fix PIR to the underside and foil tape to create a insulated diaphragm. I would screw them up with long screws and washers and pull them in flush, you only need 4 or 6 if your going to be PBing over, more if left along and less important to get flush fit. If plasterboard needs to be screwed up make sure to mark timber locations or you will have a fight to find them. 

 

Depending on how thick the underslung PIR is will impact how I would proceed next, 2 inches (because you have more in between the rafters), I would just fix the PB up through it with 100mm board screws, if you go thicker I would put the PIR in, couple of screws to hold, then add battens fixed up into the rafters or you may have screw pop issues in the future as the PB is sort of floating on a PIR base and the screws are acting more like suspension points - ask how I know.

 

There are of course many other ways too, but this is my methods. It also all depends if that is going to form a boarded ceiling or not and if headroom is important. Mean, you could also put 200mm PIR in underslung, I'd then use lag screws and battens to clamp the boards in.

[Thorfun]

just to add, as you've got to re-tile the roof have a look at a breathable roofing membrane and batten/counter batten build up above the rafters as then you can fully fill the rafters with 100mm mineral/glass wool. obviously check with your BCO that they're ok with that method. ours was but sometimes they differ in their opinions!

[Thorfun]

we used Roofshield. that web page has examples of what i'm talking about. 🙂

[MarkH395]

1 hour ago, Carrerahill said:

I have done both methods, a combination of both and various others. 

 

First off, I would highly recommend insulating it now, as you suggest you will do, because it will make your life so much easier and healthier, PIR in a enclosed room is no fun, working with an open roof structure will be good.

 

I would probably work from the outside and fit, as best I can, PIR of 75% depth of the rafters, cut the sheets to max width, then with the help of a handsaw encourage the PIR down between the rafters using the rafter as a guide for the saw, then, a small bead of expanding foam to seal. Once complete, internally fix PIR to the underside and foil tape to create a insulated diaphragm. I would screw them up with long screws and washers and pull them in flush, you only need 4 or 6 if your going to be PBing over, more if left along and less important to get flush fit. If plasterboard needs to be screwed up make sure to mark timber locations or you will have a fight to find them. 

 

Depending on how thick the underslung PIR is will impact how I would proceed next, 2 inches (because you have more in between the rafters), I would just fix the PB up through it with 100mm board screws, if you go thicker I would put the PIR in, couple of screws to hold, then add battens fixed up into the rafters or you may have screw pop issues in the future as the PB is sort of floating on a PIR base and the screws are acting more like suspension points - ask how I know.

 

There are of course many other ways too, but this is my methods. It also all depends if that is going to form a boarded ceiling or not and if headroom is important. Mean, you could also put 200mm PIR in underslung, I'd then use lag screws and battens to clamp the boards in.

Is underslung the correct term for attaching the insulation board below the rafters? Headroom won’t be an issue as it’s a vaulted ceiling. The insulation I have is kingspan therma pitch 100mm and 42.5 insulated backed plasterboard as specked by the architect. Would you use 25x50mm battens to hold up the insulation then screw the plasterboard to the battens?

[Carrerahill]

45 minutes ago, MarkH395 said:

Is underslung the correct term for attaching the insulation board below the rafters? Headroom won’t be an issue as it’s a vaulted ceiling. The insulation I have is kingspan therma pitch 100mm and 42.5 insulated backed plasterboard as specked by the architect. Would you use 25x50mm battens to hold up the insulation then screw the plasterboard to the battens?

Underslung insulation is a term I use, and hear used, so I am going to say yes, even if not, I guess the term is technically 100% accurate even if it is not an industry standard term. I also hear face fixed, but I always think of a "face" being vertical. 

 

25*50 would be nice, make sure the battening out is within the warm zone and cannot in itself create a draft zone diminishing the effectiveness of your main boards, you can do this by ensuring the insulation envelope encloses any open areas.

 

If I had the chance to build some my building projects again, the single biggest thing I would increase was my insulation, I just don't think you can get enough, so anything you can do to make it the best, I think is money well spent, we don't heat our extension really, I never installed the radiators, the pipe tails are under the floor if I ever desire, but the insulation is good for 350 days of the year, so that tells me for a small % increase in cost at the time, I could have made it 365 day proof!

 

 

 

 

[MarkH395]

1 hour ago, Carrerahill said:

Underslung insulation is a term I use, and hear used, so I am going to say yes, even if not, I guess the term is technically 100% accurate even if it is not an industry standard term. I also hear face fixed, but I always think of a "face" being vertical. 

 

25*50 would be nice, make sure the battening out is within the warm zone and cannot in itself create a draft zone diminishing the effectiveness of your main boards, you can do this by ensuring the insulation envelope encloses any open areas.

 

If I had the chance to build some my building projects again, the single biggest thing I would increase was my insulation, I just don't think you can get enough, so anything you can do to make it the best, I think is money well spent, we don't heat our extension really, I never installed the radiators, the pipe tails are under the floor if I ever desire, but the insulation is good for 350 days of the year, so that tells me for a small % increase in cost at the time, I could have made it 365 day proof!

 

 

 

 

I currently have about £18k in insulation for the conversion so hopefully that will be enough to keep me warm!

The next thought is do I put the stud walls up first or insulate the roof first. Will insulating the roof make it difficult to get a good fixing at the top of the stud walls?

[Temp]

I'd get the membrane on at least in case it rains. Then 75mm glass wool batts (allowing 25mm for the membrane drape. Then as much PIR below the rafters as you can afford. Expanding foam where the PIR meets the purlins. Tape any joints.

[Nickfromwales]

Rack the outside of the roof structure with 11mm osb, then membrane, then roofing battens. Create a false ceiling internally, bridging over the purlins etc, and then install an airtight membrane right through the interior. 

Make the new interior surface so that the distance from OSB to membrane is 300mm, and pump the resultant void full of cellulose. It'll be quiet as a mouse when it rains, will be insulated well , and will allow you to lose the purlins (cold bridges) which can be replicated with faux timberwork for character, if so desired. This then negates tedious cutting and fitting of rigid PIR boards and near zero waste. It also means you can get rain-proof PDQ.

This allows you the option to go for a more contemporary option for the interior, eg to achieve uber-flat / straight ceilings and some uniformity, again, if so desired. The caveat is you may need to cap the purlins 3 sides around with 30mm PIR to stave off cold bridging and guarantee zero interstitial condensation risk.

A better end result IMO, and guaranteed airtight to boot.

The battens which hold the membrane on become service voids for cables etc, and MVHR ducts can be lost in the cellulose, as long as they are kept tight to the inner 'chords'.