Insulation alternatives to PIR

[Alex]

Hi. Looking for some help from you guys who've been there/are going through it/etc.

 

With the ever increasing cost to PIR I'm looking for some alternative options to try and keep costs down but maintain performance for my build.

These are the current specs; 

 

Pitched roof: 0.15 w/m2k

• Slate
• 25x50 batton
• Breather membrane
• 50mm air gap
• 150mm PIR between rafters (200x50 @ 400 c/c)
• 25mm PIR under rafters
• 25x19mm batton
• 12.5 plasterboard and skim 

 

Flat roof: 0.14 w/m2k

• Single Ply
• 140 PIR crown bond
• VCL
• 22mm ply deck
• Firings
• Rafters (200x50 @ 400mm c/c) 
• 12.5 plaster board and skim 

 

Ground Floor: 0.11 w/m2k 

• Porcelain tiles
• 75mm screed
• 500 gauge separation layer
• 150mm PIR
• 1200 gauge dpm
• Slab

 

Basically since I originally budgeted the insulation prices have gone up several times. I've missed a trick in buying early and storing it until needed but the reality is prices are approx £3k over budget and I'm being told there's another price rise due in Jan! 

 

So, I'm looking for any advice / suggestions on alternative options! 

 

What I'm currently thinking is: 

Pitch Roof: squeeze the air gap down to 25mm  and go with 175mm of rockwool/equivalant or 175mm EPS with 50mm PIR under rafter 

• Rockwool and EPS both have thermal conductivity of around 0.035 to 0.040 W/m K which would still give me overall roof of around 0.14 to 0.15 w/m2k
• Is one better than the other? anyone suggest any problems with either of these? any other options? 

 

Flat roof: partial swap out for EPS, have option to increase to approx 200mm of insulation so was thinking 125mm EPS with 75mm PIR on top to maintain 0.14 w/m2k

 

Ground Floor: I have ability to squeeze insulation to around 210mm. basically I can push ceiling joists up slightly and loose a little space in rooms upstairs without any real impact as well as lifting thresholds slightly, again with little impact. Was thinking of 150mm EPS and 60mm PIR to again get to just over 0.11 w/m2k

 

I've read some of the other posts here and on other forums and tbh have information overload! So after some sanity checking of my thinking above as I feel I'm going around in circles at the moment. What have I missed / am I better to stick with PIR and look to save budget somewhere else?

Thanks. 

 

 

 

 

 

 

[PeterW]

What stage are you at...?

 

if the founds aren't in then ditch the PIR and go 250mm EPS for the floor

 

Flat roof leave as is - take the hit

 

Pitched roof go for 190mm Frametherm and 25mm PIR - and use a breather membrane that doesn't need a ventilated gap...

[Alex]

1 hour ago, PeterW said:

What stage are you at...?

 

if the founds aren't in then ditch the PIR and go 250mm EPS for the floor

 

Flat roof leave as is - take the hit

 

Pitched roof go for 190mm Frametherm and 25mm PIR - and use a breather membrane that doesn't need a ventilated gap...

 

I'm at wall plate now. Should have already ordered insulation as starting on the roof structure this week but dithering! 

Wishing I'd looked more closely at insulation for floor as could easily have made an extra 50-100mm of depth but am borrowing laser level tomorrow to remeasure finished floor heights and bottom of ceiling joists to see what I can play with. 

 

With frametherm are the batts or roll better? I've got 400mm centres so both will need to be cut to fit as 570mm wide. On basis gap between rafters is 353mm I'm going to be left with lots of thin (220mm) off cuts. How best to use them? lots of small horizontal sections working up the rafters? 

 

I thought breather membranes all needed a small gap or at least 25mm or can you get away with just 10mm? 

 

  

[PeterW]

Frametherm isn’t thay accurate..!! Cut each roll into 3 and  be done with it - it will expand to  fit the gap

[ProDave]

I used the frametherm rolls. Cut the rolls with a panel saw while they are still rolls. Dead easy and not very messy.  The small offcuts are perfect for floor insulation or odd gaps.

[Crofter]

Breather membranes should be OK with full fill insulation- that's what I've done anyway.

It's the old fashioned felts that need a 50mm gap, which also has to be vented top and bottom via soffit and tile/ridge vents.

 

Have you got anybody locally to you who does blown-in cellulose insulation? It should offer very good real-world performance, better than PIR because it tightly fills all the space (plus improved 'decrement delay' but I'll let someone else explain that one!). I don't know how it compares on costs, but as it's a totally different material to PIR it may not have bene hit by the same price hikes.

[Temp]

20 hours ago, Alex said:

I thought breather membranes all needed a small gap or at least 25mm or can you get away with just 10mm? 

 

Some makes say there must be a gap, others not. I know Kingspan used to do one that said no gap needed.

 

You should allow the membrane to drape between rafters so that water can run down under the battens. If the membrane is pressed against the batten then water (aided by dirt/dust) can pool above the batten causing the batten to rot.  The alternative is no drape and to use counter battens to raise the battens off the membrane.

[Crofter]

21 minutes ago, Temp said:

 

Some makes say there must be a gap, others not. I know Kingspan used to do one that said no gap needed.

 

You should allow the membrane to drape between rafters so that water can run down under the battens. If the membrane is pressed against the batten then water (aided by dirt/dust) can pool above the batten causing the batten to rot.  The alternative is no drape and to use counter battens to raise the battens off the membrane.

 

And of course if you have sarking, you need a non tenting membrane and also counterbattens.

[AliG]

Are you using the roof space? I changed the spec in my house to 400mm rock wool above the ceiling in areas where we didn't need a warm roof as this was vastly cheaper. You save on labour as well as the insulation.

 

200mm EPS under the screed will give you 0.15 U-value, you will likely get more benefit from the roof than the floor. Watch out for odd sizes which can be much more expensive, e.g. 90mm PIR can be more expensive than 100mm.

 

You could cut your screed thickness to 60/65mm. Screed is usually charged by volume, so more screed than necessary will increase your costs.