Using multifoil under steico insulated roof

[lookseehear]

We’re in the final stages of planning our re-roof. The engineer has specified 150 mm rafters, supported by a steel ridge beam.

 

Our architect is encouraging us to use wood fibre insulation for its thermal mass and decrement delay (since the main living space will be upstairs), which I’m increasingly drawn to. The builder, however, would prefer PIR spray-foamed between the rafters, with multifoil stapled underneath, counter-battened to create a service void, and finished with plasterboard and skim.

 

We’re already raising the roof height slightly beyond what was approved in planning (approval was for a 200 mm increase).

If we fully fill the rafters with Steico Flex, plus add 60 mm of Steico Special Dry sarking board on top, we achieve roughly a 0.19 W/m²K U-value. I think adding multifoil underneath could bring us down to the required 0.15, while also serving as a vapour control layer—and it would keep the builder happy.

 

My understanding is that multifoil requires a low-emissivity cavity, which in practice means leaving an air gap between the multifoil and the plasterboard. Still, it feels unusual to see it specified as part of a roof build-up. Am I missing something here? Are there obvious alternatives I should be considering?

[SteamyTea]

44 minutes ago, lookseehear said:

The builder, however, would prefer PIR spray-foamed between the rafters, with multifoil stapled underneath,

This builder has not studied physics at university has he, or at school.  Or bothered to look up the insurance claims and sales problems associated with sprayed on foams (which can be good, but are now tarnished)

 

46 minutes ago, lookseehear said:

Our architect is encouraging us to use wood fibre insulation for its thermal mass and decrement delay (since the main living space will be upstairs)

Did he actually say 'thermal mass'?  They usually pride themselves in the use of the English language, they they often interpret it oddly.

Is the living area actually in the loft? or below it?

[lookseehear]

I should probably be clear that when I say spray foamed, I mean the PIR would be cut close to size and any gaps spray foamed so there are no issues with the PIR being cut accurately to the gaps between rafters. He doesn’t want to use spray foam as the insulation.

 

I don’t think the architect said thermal mass, I can’t remember the exact wording. Does it matter?

 

The living space is upstairs with vaulted ceilings, but the roof pitch is low, so floor to inside of the ridge is roughly 3m.

 

 

[bmj1]

I don't understand why your architect is making this complicated. 

 

The standard approach is PIR boards cut to fit with any gaps foamed and joints taped, then plasterboard. 

 

Keep it simple!

 

Multifoil is very controversial in the industry and so I would probably steer clear of it.

[saveasteading]

Multifoil does not perform as well as they suggest, esp mixed in some other construction.

Note that the suppliers target homeowners not the industry.

Is any used at all by house-building companies?

It will add to insulation but not a lot.

 

In your case it would be compressed which defeats the whole premise of it (gaps where the heat units bounce around and get stuck.).

 

I'd consider it for my garden shed or an attic that needed some protection because it would be loose and it's easy.

But I'd more likely use Bubble-wrap.

 

Have fun asking the builder to explain how insulation works and how multifoil works and then to prove it.

[SteamyTea]

1 hour ago, saveasteading said:

Have fun asking the builder to explain how insulation works and how multifoil works and then to prove it.

I did some experiments on this a while back, posted the results up somewhere on here.

9 hours ago, lookseehear said:

I should probably be clear that when I say spray foamed, I mean the PIR would be cut close to size and any gaps spray foamed so there are no issues with the PIR being cut accurately to the gaps between rafters

That's alright.

Quite a standard method.

One thing to look at is noise transmission, not specially if you are living in the attic.

[lookseehear]

10 hours ago, bmj1 said:

I don't understand why your architect is making this complicated. 

 

The standard approach is PIR boards cut to fit with any gaps foamed and joints taped, then plasterboard. 

 

Keep it simple!

 

Multifoil is very controversial in the industry and so I would probably steer clear of it.

 

Just to check you mean PIR under the rafters but still to use Steico within and above the rafters rather than PIR for the whole lot? The reason I didn't want to do this is it removes the service void, but I guess we could just add a service void below the PIR and put the plasterboard on battens.

[lookseehear]

 

This would get me to required U value with at least 30mm PIR under the rafters. Am I right in thinking that any cables within service void need to be 50mm back from the surfact, so would need thicker batten?

[mjc55]

15 minutes ago, lookseehear said:

 

This would get me to required U value with at least 30mm PIR under the rafters. Am I right in thinking that any cables within service void need to be 50mm back from the surfact, so would need thicker batten?

Or run in safe zones (within 150mm of corners or horizontally/vertically from switches etc.)

Edited Sunday at 10:31 by mjc55

[SteamyTea]

18 minutes ago, lookseehear said:

Am I right in thinking that any cables within service void need to be 50mm back from the surfact

Or with a protective layer (conduit or plate).

Best to stick to safe zones anyway.

 

Are you having recessed lighting in it?

If so then consider the airtightness and thermal ventilation for them. Even low power LEDs get quite hot.

[Iceverge]

Multifoil=Snakeoil.

 

Decrement delay is somewhat important but if you achieve any sort of reasonable U value it very quickly pales into insignificance beside things like unwanted sunlight through the windows. 

 

PIR between rafters cannot be done correctly at a competitive price. Gapotape and Robin Clevett "insulation joinery" maybe but it's be cheaper to build your house from gold bars. Foams shrink differentially to timbers too. It's a recipe for loose insulation. 

 

The single thing that will make the biggest difference here is Airtightness and you haven't mentioned how you're going to deal with this. 

 

What stage is construction at the moment? Any plans and pics would help for further suggestions. 

[Iceverge]

5 minutes ago, SteamyTea said:

thermal ventilation

 

What units are your using for this new parameter? 

 

Edited Sunday at 10:42 by Iceverge

[SteamyTea]

9 minutes ago, Iceverge said:

What units are your using for this new parameter? 

I am not, but as you mention it, U-Values will cover most of it.

W.m^-2.K^-1 is a good way of describing a lot of things as it includes energy, time, area and temperature.

[lookseehear]

21 minutes ago, SteamyTea said:

Are you having recessed lighting in it?

If so then consider the airtightness and thermal ventilation for them. Even low power LEDs get quite hot.

Haven't decided but am leaning towards LED strips recessed along the top of the walls which will illuminate the vaulted ceiling upwards - we're creating a battened insulated service void against the walls, so there should be a neat spot to do this. Service void in the vaulted ceiling was more for flexibility but maybe we don't need it.

 

20 minutes ago, Iceverge said:

The single thing that will make the biggest difference here is Airtightness and you haven't mentioned how you're going to deal with this. 

 

I was going to use multifoil taped to parged walls as VCL/airtightness layer on the inside of rafters but if I do PIR underneath I would assume that a continuous layer with joints taped and taped back to parged wall would be sufficient. Steico flex is (from my understanding) easy to fit between rafters in a snug way (held in place with battens), and over-boarding with steico special dry sarking board. I understand a breather membrane isn't required over steico special dry, but Ecomerchant said it's probably a good idea in case any of the tongue and groove joints are damaged.

 

25 minutes ago, Iceverge said:

What stage is construction at the moment?

 

Concrete pads are in to take roof steels which are being measured for next week and hopefully installed in September. These will be on top of existing roof, so that roof timbers can go on top and the existing roof can be removed in a phased way.

[JamesP]

2 hours ago, lookseehear said:

 

This would get me to required U value with at least 30mm PIR under the rafters. Am I right in thinking that any cables within service void need to be 50mm back from the surfact, so would need thicker batten?

Similar warm roof build up to mine, though have an additional 120mm insulation and 11mm OSB( for racking) instead of your 30mm PIR.

I would avoid Multifoil, I saw some after installation in a new property and it had sagged and no mass.

I also would not use PIR between the rafters, never seen a decent fit.

Steico, though costly are great products.

[bmj1]

Is the reason for choosing steico because you need it to be vapour permeable ? If so - then PIR underneath will defeat this element.

 

If the overall buildup does not need to be vapour permeable, then it's fine to have a mixture of impermeable (like PIR) and permeable, as long as the vapour permeable is on the outer layer (doing it the other way around risks interstitial condensation).

[lookseehear]

55 minutes ago, bmj1 said:

Is the reason for choosing steico because you need it to be vapour permeable ? If so - then PIR underneath will defeat this element.

 

If the overall buildup does not need to be vapour permeable, then it's fine to have a mixture of impermeable (like PIR) and permeable, as long as the vapour permeable is on the outer layer (doing it the other way around risks interstitial condensation).

 

I don't plan on making the roof vapour permeable from the inside, and am planning a VCL under the rafters to achieve this. The Steico is for decrement delay reasons as well as 'better for the planet' reasons. It's hard to know how a space is going to react in the heat of summer, but it would suck to go through all of this then feel we need some kind of cooling solution when we could have fixed it with an insulation that performs better vs heat.

1 hour ago, JamesP said:

and 11mm OSB( for racking) instead of your 30mm PIR.

Steico's standard roof buildup doesn't seem to include a structural board for racking, but I imagine it's probably preferable? How do you find your roof performs on hot days?

[Beau]

Just for balance we have PIR boards between the rafters with multifoil below and have been delighted with how easy the place is to heat. 

[Iceverge]

3 hours ago, lookseehear said:

was going to use multifoil taped to parged walls as VCL/airtightness layer on the inside of rafters but if I do PIR underneath I would assume that a continuous layer with joints taped and taped back to parged wall would be sufficient.

 

Yes that would be fine. 

 

The internal layout matters somewhat on the location of your airtight layer however 

 

If you have a lot of funky junctions or timbers or  internal walls that can't be erected post roof build I'd be in favour of moving the Airtightness layer to above the rafters.

 

3 hours ago, lookseehear said:

Steico flex is (from my understanding) easy to fit between rafters in a snug way (held in place with battens),

 

It should just friction fit I think. It's nice stuff but much dearer than the equivalent mineral wool. 

 

I'm a big fan of including insulation above the rafters. Keeps the timber thermally stable and you'll have less chance of rot etc. 

 

3 hours ago, lookseehear said:

These will be on top of existing roof, so that roof timbers can go on top and the existing roof can be removed in a phased way.

 

Ok cool. Is the design of the cut roof set in stone? 

 

Can I get a cross section of the proposed structure?

 

2 hours ago, bmj1 said:

then it's fine to have a mixture of impermeable (like PIR) and permeable, as long as the vapour permeable is on the outer layer (doing it the other way around risks interstitial condensation).

 

Works both ways actually, structures can dry in as well as out. 

 

It's bad airtightness that is the culprit for interstitial condensation. A single hole the size of a 50p will let more moisture into a roof than the entirety of diffusion over a 100m².

[JamesP]

3 hours ago, lookseehear said:

How do you find your roof performs on hot days?

We have a metal seamed roof so on +25c days we overheat in the summer months in the 1st floor bedrooms, I think mainly because we have zero shade and the building gets 16 hours of direct sun during the peak summer months. I plumbed for AC.

1 hour ago, Iceverge said:

It should just friction fit I think. It's nice stuff but much dearer than the equivalent mineral wool. 

 

I'm a big fan of including insulation above the rafters. Keeps the timber thermally stable and you'll have less chance of rot etc. 

 

We have T&G fibre board over both roof and walls, would not build any other way now.

I put the VCL inbound between the insulation and OSB, lot of attention to detail and plenty. of airtight tape.

[Iceverge]

2 hours ago, JamesP said:

I put the VCL inbound between the insulation and OSB, lot of attention to detail and plenty. of airtight tape.

 

Why not just tape the OSB? 

[Iceverge]

2 hours ago, JamesP said:

I plumbed for AC.

Wise. 

 

No amount of shading or decrements delay will counteract 30+ prolonged external temps.

[Gus Potter]

23 hours ago, lookseehear said:

Am I missing something here?

START HERE!

 

You might be as your SE may be relying on the sarking boards to stiffen the roof.

 

Just ask your SE if the sarking you propose is compatible with the building stability design intent. 

 

If your Architect has not picked up on this then you have options, but ask the question now and direct these to the SE,  rather than leaving it to later. If a loft conversion say this could blow the gaff in terms of clearance height when you go up the stair and so on. 

 

If your SE has relied on the boards on top of the rafters to stiffen the roof then it's probable that your Steico boards are not providing enough stiffeness, and then you need to rearrange the roof make up. 

 

 

 

 

 

 

 

[ADLIan]

Perhaps a bit controversial but multifoils do work. The problem goes back 15-20 years when Actis was the main manufacturer and using their own spurious test method were claiming an exaggerated thermal performance. They were taken to task by the ASA and mainstream insulation manufacturers - I believe individuals may even have started court action.

Multifoil manufacturers now have proper test methods (BS EN 16012?) in place and some products are 3rd party certified. The actual thermal performance is less than half of what Actis (and others) claimed in the past. This performance is based on reflective foil faces so need an unventilated airspace each side for optimum performannce, plus the core (wadding and other foils?) has a thermal resistance, though relatively low as only perhaps 20-40mm thick. With taped joints some products can create a good AVCL.

[JamesP]

32 minutes ago, Iceverge said:

 

Why not just tape the OSB? 

I did tape all OSB joints as well. The VCL was fitted to the timber frame using double sided tape. This was to prevent the warm air migrating to the insulation.

All works well, have been using about 1200 - 1400kWh per year for heating in a 275 sqm home. Mild winters though.

 

31 minutes ago, Iceverge said:

Wise. 

 

No amount of shading or decrements delay will counteract 30+ prolonged external temps.

We can cope during the day but night was a challenge. Would consider Solar glass and external blinds next time. The AC is great especially  if you have PV.

2 minutes ago, Gus Potter said:

START HERE!

 

You might be as your SE may be relying on the sarking boards to stiffen the roof.

 

Just ask your SE if the sarking you propose is compatible with the building stability design intent. 

 

If your Architect has not picked up on this then you have options, but ask the question now and direct these to the SE,  rather than leaving it to later. If a loft conversion say this could blow the gaff in terms of clearance height when you go up the stair and so on. 

 

If your SE has relied on the boards on top of the rafters to stiffen the roof then it's probable that your Steico boards are not providing enough stiffeness, and then you need to rearrange the roof make up. 

Perfect timing from Gus, On top of the 200mm rafters I counter battened 70mm C16 to then fix the Fibre board and add another layer of insulation.

Every external wall and roof had 11mm OSB fixed on the inside before batten and plasterboard. That was our racking layer.