Blown EPS bead in a vaulted roof?

[sean1933]

Hi all

 

I have been an avid reader of this website for the last 18 months and learnt a lot already. 

 

We have just had planning granted to build our 'dream' 5bed property and I'm now looking at the best ways to go about building.

One of the questions I have surrounds insulation, namely the roof. What with the deterioration factors and ever increasing PIR costs we have already decided on using a blown bead (grey EPS) full fill in a 200mm cavity, married to a jablite (or similar) EPS between and over B&B floor. The roof build up remains undecided and thats where I would like some input.

 

Our property is a large barn style property with a c.8m wide eave to eave roof - we waiting on SE advice but the rafters are therefore likely to be rather chunky (300mm at a guess) so I've been looking at alternative ways to insult, obviously with EPS being a hell of a lot cheaper than PIR. I am yet to run the numbers but I was wondering whether a warm roof with PIR over rafters and EPS fulfilled rafters would work. Does anyone have any experience of using EPS bead in the roof, or any comments - it would be greatly appreciated. Even if just to say dont bother!

 

thanks in advance.

Sean

[Jeremy Harris]

Welcome Sean.

 

My concern would be over the very low decrement delay of EPS.  You will probably find that this is so short as to allow a lot of heat into the in-roof rooms in bright summer sunshine, leading to them being perhaps a bit too warm.  I'm not sure whether blown bonded EPS beads have a BBA approval for use between rafters, either, might be worth checking.

 

In general EPS is about 2/3rds as effective (in terms of thermal resistance) as PIR, with much the same short decrement delay.  This means you need about 50% greater thickness than PIR to achieve the same U value.

 

If you want to reduce cost and increase the decrement delay then you could look at one of the higher heat capacity insulation materials.  Wood fibre, blown cellulose and even dense rock wool, all have a longer decrement delay that either EPS or PIR for a given U value, with blown cellulose being the longest, and wood fibre only slightly shorter.

[ProDave]

Our vaulted roof seems to be performing well with 100mm wood fibre on the outside acting as a sarking board, and 200mm of Frametherm 35 (earthwool) between the rafters.  I found this a lot cheaper than the blown in alternatives as it was a DIY job to install it.  Originally we were going to use blown in wood fibre beads.

 

Our span front to back is about 7 metres so not quite as large as yours.

[Big Jimbo]

I really dont mind if you all call me daft, but lets say you had say 150mm pir on top of the roof, which would prob satisfy B.R. outside the rafters, but you wanted to improve further ? The natural thought might be you stick some additional insulation between the rafters, say rockwool, beads, or whatever. On the underside you stick a vcl, some battens, plasterboard and skim. What makes this different to say a timber framed wall, where i believe that you should work on the basis of least permiable on the inside, to most permiable on the outside, so that any water vapour that gets into the wall will be able to dry to the outside. Is this not the same on a roof, either flat or tiled. am i making any sense. I am aware that quite often i don't.

[sean1933]

56 minutes ago, JSHarris said:

Welcome Sean.

 

My concern would be over the very low decrement delay of EPS.  You will probably find that this is so short as to allow a lot of heat into the in-roof rooms in bright summer sunshine, leading to them being perhaps a bit too warm.  I'm not sure whether blown bonded EPS beads have a BBA approval for use between rafters, either, might be worth checking.

 

In general EPS is about 2/3rds as effective (in terms of thermal resistance) as PIR, with much the same short decrement delay.  This means you need about 50% greater thickness than PIR to achieve the same U value.

 

If you want to reduce cost and increase the decrement delay then you could look at one of the higher heat capacity insulation materials.  Wood fibre, blown cellulose and even dense rock wool, all have a longer decrement delay that either EPS or PIR for a given U value, with blown cellulose being the longest, and wood fibre only slightly shorter.

 

Thanks for the welcome! 

You make a good point regarding BBA approval and that would probably explain why I'm struggling to get clarity.

 

I will take another look at all the options. I'm completely open to different options I just want to ensure the best bang for buck outcome (like I'm sure we all do) and take advantage of the deep rafter space. I have started with taking an above and between approach but from reading on here, despite it apparently being the best route it doesn't seem to be followed very often - many seem to go with full fill and insulated plasterboard. Do you perceive this to be to do with the extra external roof buildup/labour involved, or other reasons I'm missing!?

 

40 minutes ago, ProDave said:

Our vaulted roof seems to be performing well with 100mm wood fibre on the outside acting as a sarking board, and 200mm of Frametherm 35 (earthwool) between the rafters.  I found this a lot cheaper than the blown in alternatives as it was a DIY job to install it.  Originally we were going to use blown in wood fibre beads.

 

Our span front to back is about 7 metres so not quite as large as yours.

 

Thanks Dave - a similar size to ours then. Are you rafters full filled I assume? What is your build up, namely do you have insulated PB?

I'm approaching this build from a DIY everything possible (so most of it bar the brickwork/plastering) so a similar route to you. Only reason I was considering grey EPS beads is its going onto the walls and seems so cheap when there is a wide void compared to other materials.

 

[MikeGrahamT21]

According to U-wert Phenolic Set insulation (Kingspan Kooltherm) also has a very good heat storage capacity, not quite as good as wood fibre, which seems a bit odd for a foam product, but it is extremely dense compared with PIR, will get nearly a 10hr decrement delay, which isn't too bad, plus its the most insulating of materials, unless its an error on their calculator. I find buying from seconds and co is a good deal cheaper than new, the boards i received were barely marked, but they were effectively K5 EWI boards, so fibre lining on both sides, instead of foil, but that was fine for what i wanted them for.

 

As far as a ceiling is concerned, decrement delay definitely needs to be considered, it is very important in terms of comfort in the summer, but also balancing temperature in the winter.

[ProDave]

1 hour ago, sean1933 said:

 

Thanks Dave - a similar size to ours then. Are you rafters full filled I assume? What is your build up, namely do you have insulated PB?

I'm approaching this build from a DIY everything possible (so most of it bar the brickwork/plastering) so a similar route to you. Only reason I was considering grey EPS beads is its going onto the walls and seems so cheap when there is a wide void compared to other materials.

 

Inside to out:

 

skimmed plasterboard

Battened service void

Air tightness membrane (Protect Bariair)

11mm OSB (for racking strength)

195mm rafters full filled with frametherm 35

100mm Pavatex wood fibre board over rafters

Protect VP400 breathable membrane

Counter battens

Tile battens

concrete tiles.

 

The builders built the frame putting the Wood fibre and breathable membrane on the outside.  I then fitted everything else from the inside, i.e frametherm between the rafters, then the OSB then the air tight membrane, then the battens and plasterdoard.

[Russell griffiths]

If your span is 8m then you need to look at what product you will use as rafters

you certainly won’t be using solid timber, so it will be an engineered product. 

If you don’t use a blown in product you need to look at how you will fit the insulation, you really don’t want to try and fit pir between I joists as that will be a ball ache of a job to do well. 

You will need a blown product, or something squishy to fill the awkward shape of an i joist. 

 

My i joists turned up today, and apart from the cost I can’t fault the product. 

[Jeremy Harris]

Good point about I joists, @Russell griffiths.  We have 400mm modified I joist rafters, and realistically blown insulation was the only sensible way to insulate the roof.  We have 400mm of cellulose, with rooms in the roof, and have found that the U value and decrement delay is such that there is no rise in upstairs room temperature at all on very hot days in summer (and one face of our roof is just a bit West of South).  An unexpected benefit of the fairly heavy cellulose is that it's an extremely good acoustic insulator, so we can't hear rain on the roof, or any sound from outside.  It wasn't something I'd thought about before, but it's very noticeable when living in the house just how quiet it is inside, spookily so at times.

[ProDave]

42 minutes ago, Russell griffiths said:

If your span is 8m then you need to look at what product you will use as rafters

you certainly won’t be using solid timber, so it will be an engineered product. 

 

My roof span is 7M and uses 195mm timber rafters. Remember these span from the ridge beam to the eaves so are less than 7M long.  

[Jeremy Harris]

9 minutes ago, ProDave said:

My roof span is 7M and uses 195mm timber rafters. Remember these span from the ridge beam to the eaves so are less than 7M long.  

 

 

Our rafter span is about 5.5m, but because they are ridge hung they need to be quite deep.  The basic rafter is a 302mm deep I beam, with an additional 100mm deep spaced stud nailplated to the underside to give 402mm overall.

[Russell griffiths]

8 minutes ago, ProDave said:

My roof span is 7M and uses 195mm timber rafters. Remember these span from the ridge beam to the eaves so are less than 7M long.  

 

You must have a purlin under there, surely 

 

just re read it, do you mean span, as in wall to wall, or do you mean 7m from wall plate to ridge

 

i struggled to find any timber longer than 6m unless it’s a telegraph pole. 

[ProDave]

30 minutes ago, Russell griffiths said:

 

You must have a purlin under there, surely 

 

just re read it, do you mean span, as in wall to wall, or do you mean 7m from wall plate to ridge

 

i struggled to find any timber longer than 6m unless it’s a telegraph pole. 

7M from front wall plate to rear wall plate.  45 degree roof pitch. You can do the maths to get the length of each rafter from the ridge beam to the wall plate.

 

They are quite close spaced, 400mm I believe.

[PeterW]

23 minutes ago, ProDave said:

7M from front wall plate to rear wall plate.  45 degree roof pitch. You can do the maths to get the length of each rafter from the ridge beam to the wall plate.

 

They are quite close spaced, 400mm I believe.

 

4.95m if anyone doesn’t want to do the maths .... ? Would guess it’s C24 and either 195x47 or 220x47 

 

 

[sean1933]

I just wanted to come back and say thanks for all the helpful advice - I moved house inbetween posting so was somewhat sidetracked, apologies for the rude silence!

 

We're yet to make a final decision as I'm waiting on SE to confirm depth of rafters - expecting them to be pretty big at something like 250mm or 300mm so assuming thats right we'll probably go with something like 280mm FrameTherm(40) between + 50mm celotex underneath OR 230mm FrameTherm(40) between + 60mm celotex underneath which by my calc should be good for 0.10-0.11. Reason for the Frametherm40 rather than 35 is that it seems to be much much cheaper (140mm is c.£4m2 vs £9m2) and does the job assuming rafters are as deep as I assume.

 

Subject to what SE says about I'm thinking:

wet plastered plasterboard

Battened service void

VCL/Air tightness membrane

Celotex 

rafters full filled with frametherm 40 (save for enough space for membrane drape) 

breathable membrane

Tile battens

slates

 

Any further comments always welcome!