PIR above rafters only with dMEV

[allthatpebbledash]

After reading quite a few previously asked questions, would I be allowed to assume the easiest and least faffing way of insulating a roof is to go above rafters all over?

 

New pitched roof, covered in slate.

Gable end to end, simple form.

Probably hand cut timber rafters.

Intergrated PV panels.

Loft boarded for storage with hatch.

New ceiling joists below.

 

 

Was considering cold roof with mineral wool above new ceiling joists as cheaper but being advised it’ll open up potential to damp and mould.

 

Told to go warm roof instead. Either between and below, between and above, or all above. The latter seems most simplest to not go wrong.

 

Therefore, if I throw 140-150mm Recticel Eurothane or something above rafters, and as I have 2 ducts either end coming up from wet rooms on floor below for the dMEV units to extract via the roof, could I install an additional 2 units in the loft space on either side to tap into existing ductwork for any possible moisture to be extracted?

 

Need to achieve 0.15W or better.

 

[JohnMo]

Insulation above roof is in theory easy to do. But you need to detail well, think about how you seal the gaps between the roof insulation and join to the wall insulation. If you rafter is 150mm high you have a gap 150mm to fill at every rafter.

8 minutes ago, allthatpebbledash said:

dMEV units to extract via the roof

Why not through wall?

 

Hybrid roof insulation 

5 minutes ago, allthatpebbledash said:

between and below, between and above, or all above

Can be a recipe for disaster if you don't follow the rules and have a sound basis for construction.

[allthatpebbledash]

7 hours ago, JohnMo said:

If you rafter is 150mm high you have a gap 150mm to fill at every rafter.

Would that mean I’d be looking at a between and above solution then, like hybrid roof? Forgive my lack of understanding, I was under impression you could just plonk a ton of board on top of the rafters reducing the need to cut and fill?

 

7 hours ago, JohnMo said:

Why not through wall?

Aesthetic reasons, the soil stacks are also venting via same method so no visible pipes externally.

[Mr Punter]

If you have external insulation you will need to work out how to fix battens / counterbattens.  You will need a lot of very long fixings.  I think the helical type work well.  Expect to spend about £7/m2 in fixings for counterbattens.

[allthatpebbledash]

1 hour ago, Mr Punter said:

If you have external insulation you will need to work out how to fix battens / counterbattens.  You will need a lot of very long fixings.  I think the helical type work well.  Expect to spend about £7/m2 in fixings for counterbattens.

Theres no EWI. Anticipating a concrete block cavity wall with cavity fully filled 100-150-100mm. 
 

So is it possible to avoid the filling between rafters and just place boards above them?

[JohnMo]

5 hours ago, allthatpebbledash said:

Aesthetic reasons, the soil stacks are also venting via same method so no visible pipes externally.

Why MEV instead of dMEV if venting via roof. But that's another topic.

 

5 hours ago, allthatpebbledash said:

Would that mean I’d be looking at a between and above solution then, like hybrid roof?

Hybrid is fine as long as all the details are worked out and followed. Warm roof also fine as long as all details are worked out etc. Doesn't make either easy to do well

 

2 hours ago, allthatpebbledash said:

Anticipating a concrete block cavity wall with cavity fully filled 100-150-100mm. 
 

So is it possible to avoid the filling between rafters and just place boards above them?

You can do that, but you still need to fill between rafters at the wall to roof insulation gap and stop air movement and give continuous insulation layer.

[Mr Punter]

19 hours ago, allthatpebbledash said:

Theres no EWI. Anticipating a concrete block cavity wall with cavity fully filled 100-150-100mm. 
 

So is it possible to avoid the filling between rafters and just place boards above them?

 

No I didn't mention EWI.  You need to fit counterbattens on top of the rafters and fix through the externally fitted insulation boards, hence long helical fixings needed.  No need to fill between the rafters, although you could use a small amount subject to satisfactory condensation risk analysis.

[allthatpebbledash]

On 05/04/2026 at 22:21, JohnMo said:

Why MEV instead of dMEV if venting via roof. But that's another topic.

 

Tell me about this, if I’m venting through roof are you saying I should be looking at MEV? Like an MVHR system?

[JohnMo]

MEV is a single fan unit with all wet rooms feeding into a single unit and then to a single outlet through the roof.

 

Get an intelligent one that alters fan speed based on demand, and install humidity activated extract terminals. Then add humidity activated trickle vents, for a fully demand operated ventilation system.

[allthatpebbledash]

11 hours ago, JohnMo said:

MEV is a single fan unit with all wet rooms feeding into a single unit and then to a single outlet through the roof.

 

Get an intelligent one that alters fan speed based on demand, and install humidity activated extract terminals. Then add humidity activated trickle vents, for a fully demand operated ventilation system.

Oh. I have all moisture areas located either side of say a large rectangle. To have them extract through one roof outlet would mean I would need to run a duct across the attic space which isn’t possible due to raked ceilings and through runs.

 

So it’ll have to be 2 roof outlets. I suppose I can still run a MEV this way, but what advantage would that have over a dMEV stacked and vented vertically through roof in same way?