False Pitch Roof - Insulation Approaches

Evening All,

I have an extension to be built with a warm flat roof, likely format, 18mm deck, VCL, 120mm Roof Board / PIR type insulation like Kingspan TR27 and a built up high performance felt membrane, probably 3 layer.

There is also a false pitch where things look a bit more tricky to try and get continuity of insulation.

What is the normal accepted approach to try to get similar levels of insulation / air tightness in that perimeter corridor, beneath the false pitch, around 3 of 4 sides of the extension area?

The false pitch is pitched off struts from the flat roof joists 400mm back from the walls.

The struts and the false pitch roof joists are 100mm deep.

The designer appears to be showing insulation bats between rafters, nothing actually written in text and in addition, 165mm insulation [edit: boards] sitting just above plaster ceiling level between flat roof joists just in that false pitch corridor. Not in rest of flat roof due to warm roof being present.

Worries me that corridor area, (the outer ceiling / roof cavity), could become trapped with warm / moist air with tight insulation between rafters below the warm deck level.

The ceiling plaster board is specified foil backed but won't stop everything, that is what the VCL is for?

Would it be better to fit insulation board between false pitch rafters, like 50mm or 75mm between the 100mm deep rafters of false pitch and then another 50mm or 75mm underneath the false pitch rafters? Aim would be to try and get similar performance to the 120mm insulation on rest of roof to avoid issues, edit: Cold spots, condensation, drafts...

If I am on right track, could you or would you put a vapour control layer on the underside of that insulation below rafters to help seal for airtightness or better let moisture vent through the breather membrane and plain tiles?

Image attached.

Appreciate any help. Thanks

 

Looks exactly the same as my extension. Basically a warm deck flat roof hidden behind a tiled pitched roof. I treated the 2 parts as separate entities. Warm deck as specified except I used GRP on top, then for the pitch, I treated it like a loft. Used 270mm of loft insulation. No vapour barrier as the eaves are vented so along with the breathable roof membrane under the tiles, should deal with any condensation.

Caveat: I’m not a builder.

 

Thanks. It makes sense to look at them as separate entities. I'm totally with you there.

I've got to say, I hadn't thought about loft insulation. I was thinking more about going between the false pitch rafters and forgetting the ceiling level insulation..... Worth thinking about for sure.

No idea what I'd need for comparable U values to 120mm PIR type board. Some googling might tell me. Joists are only 200mm so I couldn't get more than that in there.

The insulation the designer had sitting at ceiling height, underneath false pitch is PIR boards presently in his written spec. But if there was insulation at ceiling level, either PIR or Loft type, presumably you wouldn't bother with anything between rafters, as that just creates an insulated void which could trap vapour and limit air circ, which it looks like in the detail section I attached?

One way or another insulation at both ceiling level (besides flat roof joists) and in between false pitch rafters seems wrong. One position, done properly, for the insulation would make more sense to me.

I have soffits. They are small at 100mm. No ventilation specified but that is easy enough to add. The plain tiles I suppose add an element of ventilation themselves as well.

 

About 200mm, conveniently.

 

OK. So thoughts are something like knauf loft roll 44? Only thing is that recommends between joists and overtop as well. Down the other two false pitch sides, the insulation will be broken by the flat roof joists every 400mm... Without any ability to go over the top due to the warm deck height. Presumably 200mm only between joists will push it noticeably below performance of the warm roof 120mm insualtion board over the rest of roof or is it close enough?

The other slight practicality is the flat roof and false pitch tile roof would need to go on before ceiling plaster. Then the area beneath the false pitch is pretty inaccessible so roll would have to be laid from beneath, sounds awful. Unless noggins / battens used thoughout to retain it all prior to ceiling going in, probably less awful.

Also, in the current detail, there is an air gap at the top of the insulation shown at ceiling level between the bottom of the warm roof overlap. Presumably that is the designer worrying about cold roof ventilation above the ceiling level insulation. But if the false pitch rafters are left uninsulated, with a breather membrane and plain tiles wouldn't, that be enough for condensation to escape from this small cold roof area??

It appears me if you leave an air gap open at the top of the ceiling level insulation, there is a direct path for warm air from the ceiling void below the warm roof to head straight outside through the false pitch. It would ventilate the area under the warm deck effectively. Particularly if it is agreed the false pitch insulation is not recommended in conjunction.

Above the rear elevation at least it is partioned a bit by the flat roof joists but down the other two sides would be a direct path to outside along the ceiling void following the flat roof joists corridors straight out, unbroken by any insulation or even portioning, if no insulation is introduced between false pitch rafters. That can't be good.

On the other hand, if no air gap above ceiling level insulation, is condensation going to be a problem in that small 200mm overlap of warm flat roof and ceiling level insulation with no air gap?

Would PIR board be better if filling the ceiling void fully so, 200mm?

I've tied myself in knots now. Insulate at ceiling level or between rafters. Both as the designer seems to have hedged bets in his drawing. Seems to be pros and cons of both. The one thing I'm still clinging to is it doesn't seem right to do both ceiling and the rafters.......

 

Although Pilsbury is the false pitched/flat roof meister, I'll add my 2p's worth.

Fill it all with insualtion. Celotex directly under the deck and tiles, a then quilt. No air gaps, no ventilation.

 

Thanks. MMmmmmmm. More head scratching. I can kind of see where you are coming from. Ok, revision, the only thing I am still clinging onto at all is it can't be the best approach to insulate at ceiling level and between the false pitch rafters and then leave an air gap creating a void between the two, as surely that is a condensation risk being fed from all the warm roof area of the ceiling everywhere else.

Doesn't seem I am closer to deciding! And my beliefs on what can and can't possibly be right seem to be flexible......

 

Unless it's changed the current standard for roof insulation is 100 between and 160 over the joists. With joists all the way up, it would be more than 260 overall.
U value would only be .22 with no joists whatsoever with 200mm, so 200 between wouldn't really be enough.

 

Ps your designers should have calculated u values and thermal bridging limiting u values, also done condensation risk analysis for any non standard details they are proposing.
It's not for you to go changing it afterwards as they might not take responsibility for it being ok.
I'd recommend going back to them and asking for more details.

 

Makes more sense why the designer has the 165mm PIR board at ceiling height then. But that ventilation gap worries me.

I have tried going back but they weren't offering much by way of explanation or sounding that convincing. I can go back again but I doubt he has prepared those. Is a false pitch that non standard? Can I get them prepared myself somewhere? None of the U value calculators on the board manufacturers are capable of mixing roofs / insulation set ups. And certainly wouldn't allow considering options between an air gap or no air gap above that ceiling insulation, below the overhang of the warm deck. I have no idea where I would get a condensation risk analysis...... Would all of that give total certainty?

Thanks for the input..

 

The whole principle is that if you remove cold surfaces, and remove air voids, and remove potential for air to get into the structure for good measure, then you remove the condensation risk.

The problem with vented flat roofs (your problem) is getting through-ventilation. Your lantern well, and the pitched section will prevent this without ridge and abutment ventilation - which can be unsightly and awkward (plus costs and potential for defects) and which can all be avoided by designing them out.

 

The detail in the drawing is lacking!

The flat roof is a warm roof construction. Pitched tile roofs are generally cold roofs, esp on houses as insulation is generally at ceiling level.

The last tiled skirt job I did, an orangery, I built with a warm tiled roof. That was insulation in between and over rafters. I did counterbattens to form ventilation for the tiles.

The detailing I got from the Tyvek/Dupont site (membrane supported over insulation fig 34).

I did it that way as it seemed to be a more robust details, the insulation of the whole roof is a warm construction method, with insulation running across the outside all the way. The eve detail is awkward to avoid a big lump at the gutter line, but I managed it work it out fine and the project was for a listed building.

http://www.dupont.co.uk/products-an...ticles/technical-library.html#technicalguides

its the technical guides for roofs download -useful range of options

 

Yes. ok.

Regarding ventilation, isn't the only bit if this roof that could, possibly be needing ventilation, the cold roof area below the false pitch. Which maybe isn't cold if filled totally with insulation...

Isn't the rest of the roof conventional warm roof? So no ventilation required beneath the deck, assuming a VCL installed above the deck (bonded to it) would be installed and then insulation on top. That is tried and tested? It just seems the junction between the false pitch and area under it is messy and so much less clear.... Or am I missing a bigger picture....

 

Thanks. I'm with you on those 3 points. I'm certain on those. The detailing in that document you linked to does look good but my head hurts now thinking about it.

There is some text to go with that roof section I attached but clearly it is nowhere near the level of detail of that Tyvec guide and no more sections or diagrams.

His spec does specify the deck, VCL and Insulation of the warm roof and roof covering layers.

It does say to use plain tiles on battens of 38x25mm. It does say to us Tyvec Supro breather membrane.

But it only includes text for the use 165mm PIR board over ceiling. It does not even say use insulation between or over or under rafters even though diagram shows it. So it is not very clear what type of roof around the perimeter that has been created.....

 

Why construct a warm flat roof and a cold piched roof? Why not have them both as warm roofs?

Moving on from that, why have an area of air below the warm roof insualtion when it can be filled with insulation? This will allow the reduction of thickness of Celotex under the deck/tiles by using quilt to compensate for less Celotex, or will create a much better insualted roof by using quilt to supplement Celotex.

That's what I was alluding to in my first post, in the construction of a completely insulated roof structure.

Then there are no issues in venting, part venting, designing an interface between the two or even the potential for air leakage by having any sort of void - vented or not.

It just seems so simple.