Advice on roof insulation please!

[Iceverge]

Yeah, no probs.

[The Bear]

Out of interest, would insulating the roof just be a mirror of the floor? So it’d be like doing a floor from underneath?

 

Also, what are the thoughts on using a layer of PIR beneath the rafters, once they’re filled with slab insulation? PIR isn’t breathable, right? 🤔 

[G and J]

I’m late to this thread so my apologies.  
 

On 29/06/2024 at 14:38, JohnMo said:

Could you just staple small hole chicken wire under you floor frame?

As someone who battles to control a wood mouse population (as well as their bigger cousins) I’d say the proposed flooring, if I’ve understood correctly, of membrane supporting slabs of insulation is perfect nesting for small rodents.  I’d urge you to consider fine mesh directly under the membrane and maybe do your best to exclude them from under the building too.  It is incredible how small a gap they can get in (guess how I know this).  Gravel can be effective at stopping rats burrowing in to get to a void, and in combination with some form of curtain closure round the perimeter might achieve an effective barrier.

[G and J]

48 minutes ago, The Bear said:

Out of interest, would insulating the roof just be a mirror of the floor? So it’d be like doing a floor from underneath?

 

Also, what are the thoughts on using a layer of PIR beneath the rafters, once they’re filled with slab insulation? PIR isn’t breathable, right? 🤔 

I’m talking to a timber frame company at the mo, and they insist that Celotex is breathable.  
 

One thought, is it too late to consider one of these metal skinned pir filled interlocking roof systems?  

[JohnMo]

3 hours ago, G and J said:

Celotex is breathable

Not if it has aluminium foil either side, not sure how it could be. May be different if it's one of the grades, that has tissue or whatever on it.

[saveasteading]

3 hours ago, G and J said:

timber frame company  insist that Celotex is breathable

Well they are wrong. Doesn't matter if faced or not, it is close cell and airtight. You still use a dpm though for the joints.

I wonder what else they don't know.

Edited July 11 by saveasteading

[Iceverge]

8 hours ago, G and J said:

talking to a timber frame company at the mo, and they insist that Celotex is breathable.  

 

They've been behind the bike sheds sniffing aerosols with the "thermal mass" preachers and all the other flat-earthers. 

 

 

 

 

[G and J]

[shuffles to his feet, looking intently at his own shoes while repeatedly clearing his throat, gathering courage until finally managing to look up and blurt out…]

 

I am a self builder and I believe in thermal mass.  
 

[sits down quickly, avoiding all eye contact.]

[joe90]

I have had rats dig their way through 150mm of compacted MOT so beware, a skirt into the ground as far as possible AND stout chicken wire under insulation.

[JohnMo]

13 hours ago, joe90 said:

I have had rats dig their way through 150mm of compacted MOT so beware, a skirt into the ground as far as possible AND stout chicken wire under insulation.

And add bait boxes, we had a rodent issue last winter, ended up with bait boxes, got through about 8kg of poison in 3 months. Still found a hole that wasn't there a year ago in the shed insulation.

[The Bear]

On 11/07/2024 at 09:28, G and J said:

I’m late to this thread so my apologies.  
 

As someone who battles to control a wood mouse population (as well as their bigger cousins) I’d say the proposed flooring, if I’ve understood correctly, of membrane supporting slabs of insulation is perfect nesting for small rodents.  I’d urge you to consider fine mesh directly under the membrane and maybe do your best to exclude them from under the building too.  It is incredible how small a gap they can get in (guess how I know this).  Gravel can be effective at stopping rats burrowing in to get to a void, and in combination with some form of curtain closure round the perimeter might achieve an effective barrier.

Thank you - I've bought some panel mesh to try out - 6mm squares in 1mm galvanised wire.  It's doable from beneath, apart from being a ballache and tearing my hands up..

 

You are right - I should also consider a curtain barrier. The list keeps on growing, but it has to be right!

[The Bear]

On 11/07/2024 at 09:31, G and J said:

I’m talking to a timber frame company at the mo, and they insist that Celotex is breathable.  
 

One thought, is it too late to consider one of these metal skinned pir filled interlocking roof systems?  

Even the government says PIR isn't breathable - so it must be true.  TLDR: "Second, impermeable materials made from petroleum products: phenolic foam, PIR, PUR and EPS. These offer broadly similar performance, and do not breathe, which is an important consideration for moisture control"

 

It's a bummer, given the difference in insulation value and cost compared to wood fibre etc., but I wouldn't believe a timber framing company with a vested interest. Even looking at the major suppliers for Celotex you'll find on Google, I'm not seeing anywhere in their blurb that it's breathable. You can draw your own conclusions..

 

This also makes me wonder why some people are using a layer of PIR beneath slab insulation. How does this work with moisture? I must be missing something, because I'd still love to use it for the cost and insulation benefits.

 

Yes, too late for a different roof system. Many, many nails have already gone in. 

 

[saveasteading]

4 minutes ago, The Bear said:

 I must be missing something

Yes. Please expand on your concern. Slab on pir on dpm on stone....no moisture concerns.

[The Bear]

On 11/07/2024 at 19:27, joe90 said:

I have had rats dig their way through 150mm of compacted MOT so beware, a skirt into the ground as far as possible AND stout chicken wire under insulation.

 

Christ, now I'm wondering how long it would take a determined rat to gnaw through 1mm of metal wire. He'll have his missus nagging him, umpteen mouths to keep warm.. he's gonna chew til he's through!

[The Bear]

Just now, saveasteading said:

Yes. Please expand on your concern. Slab on pir on dpm on stone....no moisture concerns.

 

I was thinking of the Ubakus models of slab insulation between roof rafters, then a complete layer of PIR beneath. I'll see if I can find a pic..

[saveasteading]

2 minutes ago, The Bear said:

slab

Sorry, I thought you meant floor slab. Ignore me.

[The Bear]

51 minutes ago, saveasteading said:

Sorry, I thought you meant floor slab. Ignore me.

 

No worries! This is what I was thinking of..

 

 

Not sure how to put a vapour barrier in here yet (or why Ubakus doesn't flag this), but this is pretty much how my roof would look if I were to use thermo jute and PIR/PUR

[Iceverge]

Ubakus is a simple tool, very useful for calculating U values but limited in judging moisture issues. It assumes, for instance, that the PIR layer will be totally continuous with no gaps. 

 

This is unrealistic unless you foam and tape all joints.

 

A solid understanding of the core principles of safe layered construction and good workmanship are more important than a calculator. 

 

1. Your primary aim is to prevent any moisture laden internal air migrating to any part of the wall/roof that the temperature can fall below the dew point and condense. This is done by excellent airtighess.

 

Take any material that is airtight, like a membrane, OSB or even plasterboard and make a perfectly continuous layer somewhere in the wall/roof. It can be internal or external but it must not have any gaps or cracks anywhere. This will stop the flow of vapour laden internal air outwards to beyond the dew point. 

 

2. Next ensure that any small amount of moisture that does get in there (through inevitable slips in assembly or construction moisture) can dry readily.

 

This is where your materials of low vapour permanently* and sometimes external ventilation are important. The lightly availability of "drying" at the dew point determines your required materials and ventilation. 

 

TBC

 

*Vapour Permanbility is a much better term than "breathable." It has SI units attached and can be accurately measured.  "Breathable" gets conflated with "ventilation", it gets used by salesmen to sell everything from anoraks to lime plaster with little intelligent insight. In my opinion the it belongs in the bin along with "thermal mass" and other such terms that are thrown around with little other than guesswork and superstition. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[Iceverge]

Anyway how does this help you..... @The Bear

 

Your as drawn picture would work fine. 

 

Principal 1: Keep the damp internal air out of the wall through airtighess. You are doing this as I'm sure you will make a pucker job of taping and foaming all the PIR 

 

Principal 2: Make sure the amount of drying inside the wall is sufficient for the expected amount of moisture. You have fluffy insulation between the rafters and a ventilated space above it so you're ok here. AKA plenty of "drying" at the dew point. 

 

This is particularly important with lots of insulation inboard (PIR in your case) as your rafters will be frequently below the dew point without the room warmth to keep them toasty.

 

It's one of the reasons I'm not a fan of internal insulations and ventilated external cavities. Cold timber is always more vulnerable to condensation and also the ventilation required can "wind wash" the fluffy insulation degrading it's performance.  However it is largely a safe construction. 

 

 

 

[The Bear]

@Iceverge Thank you for your comments. Like most other people on this site, I've gone down the rabbithole and become slightly overwhelmed. Warnings to KISS are noted. My saving grace may well be half-decent workmanship, fingers crossed.  I do have a couple of queries tho..

 

Airtightness is all very well, and doable in the floor and ceiling. But Tuin also strongly recommends adding air vents at opposite sides, one high and one low. I can see the sense in this, keeping a small but regular change of air. But won't that negate me spending plenty of time and cash on taping the floor and ceiling? My understanding is lacking here. Or is it subtler? I want to keep moist air away from places it could condense and become trapped (within layers of the floor or ceiling), but inside the building overall, I want moisture inside to balance moisture outside?

 

Cold roof seemed the best option for this cabin, what with height restrictions for permitted development. And as I've made progress on the roof, I see it also has "built-in" vents front and back.. I hammered those purlins down *hard* too - I'll post a pic shortly.

 

To be honest, right now I'm still wavering on how to do this. I'm just so tempted to use PIR! But I still have some time to decide..

 

For the floor, the eco-build schedule does make sense - clear instructions, I can save some cash by using alternates to Solitex Plus and Proclima DA, and if I follow those instructions well, I can be confident jobs a good un.  Even if I'm spending more and getting a lower U value than I'd have with PIR.

 

For the roof, I have no problem with attempting a mirror of the floor, using a membrane either wrapped around the joists as per the floor, or individually fixed like this

 

but fixed lower to keep my airgap. Then either jute and a membrane; or jute, membrane, PIR, t&g ceiling (however that would work).

 

I also know I should, at some point, insulate the walls. But I need to get back to work at some point, so that might have to wait. I've also not thought about voids to run electrics through, hoping a solution would present itself while I'm working on something else..

 

On the whole, I am finding this fun, most of the time. Plenty to learn and the principles and benefits of good insulation are kind of fascinating. Very helpful to have people such as yourself on this site to bounce ideas off, or just say "stop overthinking and crack on!"

[The Bear]

Bottom pic shows my “built-in” vents. Although there’s a little variance between purlin levels, they were never going to be flush. 
 

Top pic shows similar at rear wall

[Iceverge]

2 hours ago, The Bear said:

Thank you for your comments. Like most other people on this site, I've gone down the rabbithole and become slightly overwhelmed.

 

Very easy to do. It's a poorly understood topic, dominated by hearsay and rules of thumb. Advice comes typically from litigation wary professionals (who must assume terrible workmanship) or sales targeted companies. 

 

2 hours ago, The Bear said:

 

 

2 hours ago, The Bear said:

But Tuin also strongly recommends adding air vents at opposite sides, one high and one low. I can see the sense in this, keeping a small but regular change of air

 

Airtighess serves two purposes. Firstly to allow airflow within a building to be controlled for the health and comfort of the occupants. 

 

Secondly and (equally as importantly) to ensure that no vapour laden internal air can get where it shouldn't. I think this was the driving factor in getting to the 0.6ACH target for passivhaus. Essentially the health of the building itself. 

 

In your case if you do want "hole in the wall" ventilation I would still certainly ensure that there is a perfect airtighess layer. 

 

2 hours ago, The Bear said:

I want to keep moist air away from places it could condense and become trapped (within layers of the floor or ceiling), but inside the building overall, I want moisture inside to balance moisture outside?

 

You want to prevent the internal air, which will have more g/M3 of water vapour than the external air, from getting inside your walls.  Airtighess AND appropriate planned ventilation is the key. 

 

 

 

 

 

[Iceverge]

Practically speaking 2 staggered layers of PIR top of the roof and then a membrane would be easy to install but although you're solving one problem you're creating 2. The increased height and the difficulty in making a continuous airtight layer with the walls. 

 

What is your preferred roof covering and how much thickness could you achieve? 

 

How much below the ceiling are you prepared to loose? 

 

Personally I would be tempted to do something like this. 

 

1. Metal profiled roof sheeting. 

2. 20*75mm battens to take the sheeting. 

3. Breather membrane, all joints taped. 

4. T&G sarking as shown. 

5. Full fill mineral wool between the joists.

6. Vapour control membrane, diligently taped to the airtighess layer on walls. 

7. 45*50mm battens at 90deg to the rafters with mineral wool infill as service cavity. 

8. Plasterboard and skim. 

 

Given you can guarantee a good job on the workmanship then there'll be almost zero moisture make it's way into the structure and what does can dry to the outside. 

 

 

 

 

 

 

[G and J]

Methinks fingerless gloves, a thick wool beanie, a Sarah Lund sweater, thermal leggings and moon boots (all matching or at least coordinating, natch) for the eight days a year it’s really, really cold would be a cheaper and more environmentally friendly solution.  
 

And on the bright side, the computers will only overheat for three months of each normal year. Obviously not this year mind, so that’s next year’s problem….  

[The Bear]

20 hours ago, Iceverge said:

What is your preferred roof covering and how much thickness could you achieve? 

 

If you mean the very top layer, I went for EPDM, to go straight onto the roof boards. 

 

20 hours ago, Iceverge said:

How much below the ceiling are you prepared to loose? 

 

I'm prepared to push it, within reason, given that a) it's for my wife (women are short), and b) as the ceiling slopes towards the back, no big deal if she can't stand up straight against the wall. Note to self - measure wife..

 

20 hours ago, Iceverge said:

Personally I would be tempted to do something like this.

 

I'm not sure how or whether that would work with the roof boards I've just finished nailing in..

 

I'm also unsure about making the walls airtight, and here's why: 

 

1) By "airtight", do you also mean "moisture tight"?  Reading your post above, they're not quite the same things, am I right?

2) It's a log cabin, which I am expecting to absorb moisture and move, despite my best efforts of..

3) Hitting all the end grain with Osmo protector wax, and

4) Buying Osmo paint for the exterior

5) Wife not bothered about paint or finish on the inside

6) But I know what can happen when only one side of a timber is painted..

7) So would it be sensible to put something - even a clear matt finish - on the inside?

8 ) Or would insulating the walls, with airtightness and vapour control layers, take care of this for me?

 

Sorry, you can see I'm not quite out of the rabbithole yet!