Please comment on my pitched roof section - Am I missing something?

[BartW]

Hi,

I posted about it elsewhere on the forum, but it is still making me wonder.

 

Kingspan details "warm pitched roof" as:

- insulation above

- and inbetween

- like this:

 

 

 

I, however, have detailed it like this:

 

 

 

My questions:

- am I doing it wrong?

- should I have a layer of OSB on top of rafters before the insulation?

- will I struggle to anchor a 150mm PIR to the roof?

- should I consider means of preventing the roof sliding down under its own weight?

 

Advice highly appreciated. I got nowhere with it so far...

 

 

 

 

 

 

 

[nod]

I wouldn’t put insulation over the rafters Definitely wouldn’t use 12.5 plasterboard on 600 centers 

 

I would insulate between the rafters and use a 100 mil insured plasterboard under the rafters 

[BartW]

That roof is unventilated and there is no way it would be. So I somehow need to get it approved by Building Control. 
 

 

[JohnMo]

Flat roof looks fine, you would normally have a timber edge all round the edges of that roof.

 

Sloped roof. You should have a breather membrane above the insulation, you will need to batten and counter batten.  You will need a barge board 150mm thick at roof edge to stop the insulation sliding down.  First Battens are screwed through insulation into rafters.

 

[BartW]

1 hour ago, JohnMo said:

Flat roof looks fine, you would normally have a timber edge all round the edges of that roof.

 

Sloped roof. You should have a breather membrane above the insulation, you will need to batten and counter batten.  You will need a barge board 150mm thick at roof edge to stop the insulation sliding down.  First Battens are screwed through insulation into rafters.

 

 

 

Thanks for the comment.

 

Current remit from the TF supplier is that they fit OSB and breather on top.

On the basis of which I was leaning to just having cold unventilated roof (yes, got enlightened that it was possible and in line with all BBA / NHBC guidelines).

 

So like this:

 

 

 

 

Having said that, warm roof would be more consistent with the flat roof, less tricky junctions.

 

If I follow your advice, would I ask the TF supplier not to install a breather above OSB? Or should I just put PIR on top, and put another independent layer of breather on top, then battens and counter battens, slates?

 

 

 

 

[MikeSharp01]

We used long screws with sunk heads in plastic sleeve to hold our insulation on and then we used Helifix nails to hold the battens down. HELIFIX do the calcs for you and they are dead easy to fit with the HELIFIX tool in the SDS drill. 

[BartW]

10 minutes ago, MikeSharp01 said:

We used long screws with sunk heads in plastic sleeve to hold our insulation on and then we used Helifix nails to hold the battens down. HELIFIX do the calcs for you and they are dead easy to fit with the HELIFIX tool in the SDS drill. 

Trouble with this is that Helifix only go up to 200mm, and with 150mm pir and 25mm battens, then 18mm osb inderneath, I am not achieving the 38mm substrate penetration…

[MikeSharp01]

No ours is only 75mm so no problem. How heavy are your slates / tiles. When the architect drew your roof they must have had an idea of how to do it?

[BartW]

50 minutes ago, MikeSharp01 said:

No ours is only 75mm so no problem. How heavy are your slates / tiles. When the architect drew your roof they must have had an idea of how to do it?

I was my own architect and frankly have not given it enough thought. It stacked up on paper, but now we are nearly physical on site with it…

[Iceverge]

Hi @BartW

 

What exactly have you committed to with the TF company and can it be changed? 

 

Have you a minimum U value you need to meet? 

 

What materials have you paid for so far? 

[Linto]

Twistfix go upto 240mm, we're looking to use them to achieve a similar build up. 

[BartW]

15 minutes ago, Iceverge said:

Hi @BartW

 

What exactly have you committed to with the TF company and can it be changed? 

 

Have you a minimum U value you need to meet? 

 

What materials have you paid for so far? 

Hi,

Current commitment is roof with OSB and breather all around.

 

Target U is 0.15, which is same as for the the rest of the thermal envelope.

 

Warm above and in-between (as specified by Kingspan) seems an easier detail, which does away with thick stop rafter, and very long (and expensive) fixings. I just wonder if it would still function as designed if there is the "inherited" OSB layer and breather above it, between the rafter level PIR and the "above" layer.

 

Decisions, decisions...

[BartW]

24 minutes ago, Linto said:

Twistfix go upto 240mm, we're looking to use them to achieve a similar build up. 

Good find, thanks! And not bad a price too.

 

 

[Iceverge]

What is the detail on the left hand side of the roof?

 

Does it abut another roof or a wall?

 

How are you planning on the fall for a flat roof?

 

Firing strips or a fall in the rafters.

 

Why the posi joists, they're much dearer than cut timber, is it for load capacity or running services? 

 

Infact do you have a wider section  of the roof with requirements for head height etc?

 

 

[BartW]

19 minutes ago, Iceverge said:

What is the detail on the left hand side of the roof?

 

Does it abut another roof or a wall?

 

How are you planning on the fall for a flat roof?

 

Firing strips or a fall in the rafters.

 

Why the posi joists, they're much dearer than cut timber, is it for load capacity or running services? 

 

Infact do you have a wider section  of the roof with requirements for head height etc?

 

 

 

It's just a loft room, so typical trim detail at the edge of flat GRP roof.

Falls are being achieved with firings, and pointing towards the pitched roof. 

We have pozi joists contracted as part of the build, but there will be services there indeed.

 

Wider section of the building here:

 

[Iceverge]

Ok and by services do you mean wires/pipes or MVHR ducting?

[BartW]

4 minutes ago, Iceverge said:

Ok and by services do you mean wires/pipes or MVHR ducting?

 

Both electrical and MVHR ducts, just no plumbing.

[Iceverge]

Right I've thrown an hour at this based on what I would do starting from day 1 if it was my own project.

 

It won't exactly fit yours given the further details you've provided. I'm not a fan of PIR in general, especially not in the roof. 

 

 

I would have framed it all in 220x45mm timbers, as it's readily available and cheaper than posi's. I would have lived with the 1.5deg slope internally rather than use firing strips. 

 

Then sheathed it with 11mm OSB on the slope and 18mm on the flat for extra strength.

Put a good breather membrane over it, taped at all joints for windtightness, 

Battened along the rafters and then counterbattened throughout to get 50mm ventilation on the slope and 90mm on the flat section. Detailed at the junction with an insect mesh and drip as appropriate. 

Then 18mm OSB+EPDM/GRP for the flat section and slates for the pitched. 

 

Internally a continuous A/T membrane counterbattened with 45mm battens for an insulated cavity and then 15mm plasterboard. 

 

Blown cellulose between the rafters too I forgot to say. 

 

 

 

Advantages,

 

1. All off the shelf, reasonably priced materials, no specially made joists etc. 

2. All nail gun compatible, no screws ( faster and cheaper) 

3. continuous wind tightness and  airtightness layer.  

4. Good decrement delay, good heat protection.  

5. None of the PIR issues. 

6. Minimum wastage of insulation. 

7. Minimal thermal bridging. 

8. Moisture tolerant during construction, Nowhere for moisture to get trapped inside the structure causing decay. 

9. Very good sound proofing. 

 

 

 

If I get a chance later I'll have a ago with the updated details you've provided, but maybe there's some inspiration there. 

[Mr Punter]

That is a fairly simple, workable and cost effective suggestion from @Iceverge.   I guess it is mineral wool in the service cavity.  I know that BC can get sticky with cold flat roof designs and would probably want a condensation risk analysis for this one.  There is a lot of reliance on the inner "airtight" membrane stopping water vapour migrating into the build-up and there is not solid surface to attach it to.

[Iceverge]

2 hours ago, Mr Punter said:

 I guess it is mineral wool in the service cavity

 

Yup, woodfiber/sheepswool or hemp may add to the decrement delay and be nicer to work with but with 220mm cellulose between the rafters, it's not critical. 

 

 

2 hours ago, Mr Punter said:

I know that BC can get sticky with cold flat roof designs and would probably want a condensation risk analysis for this one.  There is a lot of reliance on the inner "airtight" membrane stopping water vapour migrating into the build-up and there is not solid surface to attach it to.

 

If I could have personal control over the build quality myself, and could ensure the airtight membrane would be perfectly continuous, I would be happy to omit the ventilation from the flat/pitched roof junction and allow continuous ventilation from eave to eave. 

 

The 90mm void as drawn with ventilation at both ends should be fairly idiot proof.  We had an airtight membrane below roof trusses and achieved 0.31 ACH. That part was pretty straightforward really. Much easier than the window taping for example.  

 

We battened and counter battened to leave an 88mm void for the 75mm MVHR ducting, me being totally paranoid about breaching the airtight layer.  Awkwardly this also involved pre cutting the battens along the route of the ducting. Also with no nail gun it was probably 1200 screws! Typical self builder over thinking everything and putting zero value on my own time!! 

 

 

 

 

With the benefit of hindsight, next time I would happily run the MVHR ducting into the attic through the membrane and back inside at the plenums provided I was the one taping the holes. (Ducting all buried under the 400mm of blown cellulose of course). Then battened underneath with 22*50 or 22*57mm strapping for plasterboard.

 

Also I would upgrade to at least 15mm plasterboard if not 12.5 x 2 or 15mm Sound block as the house is almost completely silent otherwise. 

 

Here is a pic of how we ran cables into the attic. 2 x outside lights, 1 x 5g router, 2 x TV antenna/dish. 

 

 

A scrap piece of ply ( hence the irregular shape) screwed from below and then taped from above. Each cable got it's own hole and then were sealed with a flexible airtight sealant from both sides. 

 

 

 

 

 

Edited May 22, 2022 by Iceverge