What thickness of PIR to span 300mm between rafters.

[epsilonGreedy]

I was planning to use flexible rafter roll to create ventilation channels to promote air circulation from the eaves to over the top of the loft insulation rolls in my cold attic space. However the Manthrope fitting instructions warn that the loft insulation should not be pushed tightly into the gap between the wall plate and rafter roll. I assume there is risk that the loft insulation will push up against the roof breather membrane and obstruct any water dribbles flowing down the gradient of the drape between rafters.

 

My roof has a 30 degree pitch and in on section of roof the slender rafters result in a height above plate of just 150mm. This increases the risk of an upwards bulge in the membrane.

 

So I am going to change the design and fit rigid PIR sheet between the rafters instead of rafter roll. The rafters are at 300mm and 400mm centers.

 

What thickness of PIR sheet should I buy that will be strong enough not to buckle in the event that loft insulation is stuffed with excess force to push it down to the eaves?

 

In practice I will try to fit some loft insulation roll to create a snug fit down to the cavity closure before the PIR panels, membrane and roofing battens are fitted.

 

I anticipate most inter rafter sections of PIR will be 265mm x 800mm slotted into a groove formed by slim wood battens top and bottom.

[epsilonGreedy]

I hope this extra drawing illustrates where the PIR will be fitted. I am referring to the part "spacer maintains 25mm clear airway".

 

[dpmiller]

be careful putting anything in too tight. SWMBO rammed the insulation a bit to tight against the rafter roll in places and it squeaks annoyingly in the wind...

[epsilonGreedy]

7 minutes ago, dpmiller said:

be careful putting anything in too tight. SWMBO rammed the insulation a bit to tight against the rafter roll in places and it squeaks annoyingly in the wind...

 

Ok thanks for the warning, Plan-B is to ditch the rafter roll and use cut sections of PIR as a spacer between the rafters to achieve the same ventilation airgap.

 

That diagram above illustrates the limited height for loft insulation roll at the wall plate with a 30 degree pitch.

 

 

[Mr Punter]

I think the PIR is a really good solution.  You could fix strips of 25 x 38mm batten to the sides of the rafters to maintain your airflow gap.  You could use further short pieces under to hold the PIR firmly in place.  40mm PIR is easy to cut but quite rigid.

[joe90]

I would fit the thickest PIR you can as the insulation total thickness there is below optimal. Also the thicker it is the less it will bow.

[epsilonGreedy]

7 minutes ago, Mr Punter said:

I think the PIR is a really good solution.  You could fix strips of 25 x 38mm batten to the sides of the rafters to maintain your airflow gap.  You could use further short pieces under to hold the PIR firmly in place.  40mm PIR is easy to cut but quite rigid.

 

 

Thanks, in the absence of any advice I would have guessed 40mm or maybe 30mm was strong enough for a 10" span though I have never handled or cut the stuff before. Plenty of threads here on the subject of cutting PIR I recall.

 

I take your point about holding it in position as I don't wont the PIR sections sliding out of place during a gale because it would be a pain to reach the wall plate from inside the loft with 350mm of insulation down and a 30 degree pitch. It is surprising how far back the PIR (or rafter roll) has to come to remain above 350mm of loft insulation given a 30 degree pitch.

 

I was going to use a slimmer batten cross section to create the slot to hold the PIR, was thinking one size down, maybe 18mm x 38mm?

 

Ok I feel happier with Plan-B because I could imagine the loft space getting damp near the wall plate if water pooled on the membrane and leaked through slowly. I think it would make sense to fit some loft insulation roll before closing up the eaves and fitting these PIR spacer panels.

 

[epsilonGreedy]

26 minutes ago, joe90 said:

I would fit the thickest PIR you can as the insulation total thickness there is below optimal. Also the thicker it is the less it will bow.

 

 

Ok interesting you were a roofing pro I recall.

 

Just one matter concerns me if going even thicker than 40mm. At 40mm in the smaller roof I will only have 150 - 25 (airgap) - 40(PIR)  = 90mm above the wall plate. I think I will need a continuous  section of insulation roll stretching from the cavity up under the PIR at the wallplate and further into the attic space. If just bundle a section of insulation on top of the cavity and upto the wall plate but not through into the attic it might just flop out over time and drop into the boxed eave.

 

Is this a real concern?

Edited September 28, 2021 by epsilonGreedy

[joe90]

14 minutes ago, epsilonGreedy said:

Ok interesting you were a roofing pro I recall.

I would not call myself a pro but done a lot of roofing/insulation jobs in the past 

 

14 minutes ago, epsilonGreedy said:

Just one matter concerns me if going even thicker than 40mm. At 40mm in the smaller roof I will only have 150 - 25 - 40 = 90mm above the wall plate.

Insulation roll will compress a lot. Is the roof on? If not, roll the insulation over the wall plate to where it needs to go, push the pir down on it till achieving the gap above required and fit you battens to stop it rising, your first diagram shows three layers, first between joists (100mm?) then two layers of 150mm?, roll this second layer over and down to the cavity, fit PIR and battens to maintain gap then roll top layer up to the PIR ?

Edited September 28, 2021 by joe90

[ETC]

What about some cardboard?

[Iceverge]

What stage of the build are you at? 

 

If it is before roofing membrane I would just fit the insulation roll ensuring continuity of insulation walls to roof and then fit the rafter roll and then the membrane. It's difficult to ensure theres no gap between the wall and the roof batts otherwise. Have you discounted blown in insulation? 

 

 

[ETC]

26 minutes ago, Iceverge said:

Have you discounted blown in insulation? 

If you do you’ll definitely need cardboard to stop it touching the underlay. Plus don’t forget to prime all metal.

[Iceverge]

Are you referring to the boric acid additive in cellulose Vs metal?

 

Any references on this for my perusal?

[ETC]

Referring to spray foam insulation, underlays and zinc coated fixings. Check the BBA certificates out.

[Iceverge]

Ah, I was talking about blown cellulose, mineral wool and woodfiber. 

[ETC]

Aha….apologies…..I’m still a fan of keeping an air gap along the eaves using over fascia or soffit vents with rafter roll.

[Iceverge]

1 hour ago, ETC said:

Aha….apologies…..I’m still a fan of keeping an air gap along the eaves using over fascia or soffit vents with rafter roll.

Yup. It's required with a ventilated cold roof AFAIK