I'm about to start building an extension with a pitched roof. To keep the ridge height as low as possible whilst maximising the useful room in the roof (planning constraints...), I plan to put insulation between and below the rafters. I appreciate that from a purely thermal point of view it may be better to put the insulation over the rafters, but that takes more height. I'm planning to fully fill between the rafters, fit the membrane flat over the insulation and tape the joins, then fit counter battens, battens and tiles. My question is how one accesses the roof in practice to fit the membrane; you can't walk on the insulation and anything you put on the roof to stand on will be in the way.

Hope I'm not being stupid...

Thanks,

Mike.

The problem I see is that you can't fit the (850) battens on the rafters until all of the membrane is fitted and taped so you don't have the tiling battens to stand on while doing the membrane higher up the roof slope.

Why 850 battens?

Mike.

You can work your way up the roof as above but the counterbattens would have to be done in short lengths depending on the width of the membrane.

Get an extending forklift thing and work from a platform? Have two roofing ladders and use one to leapfrog the other with each run of membrane?

Ah, I was being a bit dim. I was assuming that the vertical battens should be rafter length, but I can see there's no need for that. Thanks!

Mike, What staging, if any, have you in place ?

how about OSB over your insulation followed by membrane and battens? OSB taped / glued / screwed onto gaskets must be a better air barrier than taped membrane. How are you going to tape the membrane at the eaves?

To try to answer the questions, we're not at the stage of doing the roof quite yet, I'm just thinking ahead. I was considering OSB if only because it might allow me to make faster and safer progress and as you say it would be a much more robust detail. I have not looked into its vapour permeability though. Wouldn't you also need a vapour barrier on the inside of the roof to prevent condensation at the OSB level if, as I assume, OSB is not very permeable compared to the fitted insulation (in the real world). I am sceptical that you can get a very good fit between that insulation and the rafters but would have a layer of insulation backed plasterboard on the inside which should be airtight.

I'd use one of the rigid sheet PUR foams the full depth of the rafters by preference, either Celotex, Kingspan or equivalent.

Mike, I found using OSB as a airtight barrier a very satisfactory way of detailing. I had total control over insuring each joint was sealed and you can over detail to you hearts content, depending on how "belt and braces" you wish to be. How many surfaces you apply sealant/mastic to etc. You are able to carry out a very comprehensive visual check on completion. I also enjoyed constructing and being able to work on a very stable platform, 18mm OSB3 (The OSB was a structural engineers requirement so I just made the most of it) A WARNING there are two different faces to OSB3, use to the outside, the face which has a slight grid pattern impressed into it, as it provides grip, the other face can be unforgiving. I achieved our roof build, with a friend, (roof joists, OSB boards) with the minimum of Scaffold, which was why I was asking about staging. I am not suggesting it is advisable, sensible or goes anywhere near meeting H&S requirements and I would not have anyone employed in this situation but it is doable. The one element I would say is an absolute necessity is a kick board at eves level This saved a gas nailgun, driver and on one occasion myself from contact with the ground. We actually also had a single handrail but it was the board that was the important element. Other than that we had a access tower for lifting materials up to roof. I am NOT advising this is the way to do it just the way I did it and we are working on a single storey building but because of the site on one face this equates to double height.
I am reasonably comfortable at heights but we have since attempted to start the installation of the membrane without the kick board being in place and had to stop due to feeling far to vulnerable. I am now trying to work out if I can utlise my abseiling harness in some way!
In respect of the internal vapour barrier, due to engineers requirements we are again facing with OSB/ply, so can seal joints but I am trying to assess whether to install a barrier, Intello or similar, or try painting on a vapour barrier product before fermacell boarding.
As I say this is the way I constructed my roof assessing the risk to myself and for most of the time I felt perfectly comfortable. I am not in anyway advising anyone else to build in this dangerous and foolhardy manner

''I am sceptical that you can get a very good fit between that insulation and the rafters but would have a layer of insulation backed plasterboard on the inside which should be airtight.''

My approach would be to get every layer of the 'sandwich' as air-tight as I possibly could. If one layer 'leaks' it isn't enough that the next one doesn't, ideally. When fitting Pu between rafters, at very least I cut small bevel when the Pu is level with the rafter face, and silicone it and all joints before taping. I also tape across te rafters from board to board, and if any big enough joints were open, I'd squirty-foam them.

If I am feeling particularly obsessive I splurge mastic over the side faces of the rafters before I push the (tight) Pu board in. I always cut the boards slighly over-size and scribe them in in situ.

Tom, no choice re. the 18mm OSB, Structural engineers specific requirement for racking, as is the inner skin. I think she may have lost the plot at some point, thinking I was building an out of town shopping unit, but I wasn't going to win that one. Having said that not sure what the cost difference would be and as the joist centres are at 600 I was much happier in my circumstances working on a v rigid platform of 18mm. To be honest given the choice to do it again I would still choose the method we used but I guess its what you feel comfortable with.
Nick, I agree with the methodology of making each layer airtight hoping that at least one will work/sustain but I put my trust in the OSB, as you say slathered with sealant (or bubble glue) on joints and joists. I am quite fastidious about the detailing but I can see trouble ahead with the membrane.

Tom, I can imagine that once the Warmcell is blown in you also have a pretty rigid construction. I did manage to get my engineer to recant on the blocking at every board joint when I pointed out that I could find no detail/spec anywhere that required this, I wanted to save unnecessary work but mainly remove more bridging.

Why would you need a membrane taped at joins and touching top of rafters and insulation to make it windtight?
if the rafter was fully filled with PUR all gaps foamed it would be windtight anyway.
if you dont allow felt/membrane to bag in the traditional way between rafters/counter battons then any water getting through to felt/membrane may run through nail holes.
dont we want a bit of wind to flow round this area anyway to remove any mositure forming on cold side of insulation ?
Pulling it tight and sitting in flat on top as discribed seems pointless.

Posted By: Mike.Ban additional vapour barrier under the plasterboard

Best to leave the pbd out of the picture, to be punctured at will in the future (rely on that).

Posted By: jamesingramWhy would you need a membrane taped at joins and touching top of rafters and insulation to make it windtight?
if the rafter was fully filled with PUR all gaps foamed it would be windtight anyway.
if you dont allow felt/membrane to bag in the traditional way between rafters/counter battons then any water getting through to felt/membrane may run through nail holes.
dont we want a bit of wind to flow round this area anyway to remove any mositure forming on cold side of insulation ?
Pulling it tight and sitting in flat on top as discribed seems pointless.

There are lots of papers shared by Viking House & others regarding the risks of thermal bypass. If you're to get the advertised performance from the insulation then you need to make sure that it isn't exposed to air movement & that air isn't allowed to bypass it. Roof timbers will twist over time & any seal between insulation & rafters cannot be trusted long term.

The whole point of breather membranes is that they allow water vapour to escape without requiring ventilation or air movement below them. Most breather membrane BBA certificates allow use in contact with insulation &/or sarking boards, i.e. they are "non-tenting" & can be used "fully-supported".

Only a small amount of water will make it onto the sarking membrane & most of this will flow down the roof. If a counter batten becomes twisted then small amounts of water may collect under the up-turned edge &, if there is a counter batten fixing at this point, there is a risk that the area around the fixing may get damp. However, this isn't greatly reduced by draping the membrane & the membrane should seal around the fixing. If you're concerned about it you can always run double sided butyl tape under the counter battens. More important, I think, is that the counter battens are at least 38mm deep so that the tile batten fixings don't penetrate the membrane.

Something to note is that draping the breather membrane over the counter battens or over the rafters without counter battens can result in more nail penetrations through the membrane. Depending upon the roof covering, there are often more tile batten fixings than counter batten fixings.

David