TF House about to start, and last minute pondering... Insulation...

[BartW]

Hi,

We are about to start with the Timber Frame. Shell is in production right now, and is expected to come fitted with 120mm PIR in-between the studs, and osb externally.

 

I have been forever planning on adding 50mm insulation (PIR) internally. But speaking to my neighbour, who is insulating externally, it suddenly made more sense to me:

- quicker as no need to negotiate internal studs or floors

- more continuous, thus fewer possible cold bridges

 

 

Most importantly, in view of my recent dilemmas about void ventilation:

- I could extend insulation right down "to the ground"

- hide the void ventilation telescopes in sections of removed insulation

- install cement board across the lower part of the wall, and decorate / render / brick slip it

 

 

We are cladding open rainscreen. 

 

Our timber frame supplier will finish the building by wrapping the whole frame with OSB to the face with VCL. Am I right in thinking I then go:

- 50mm PIR

- vertical battens

- cross battens

- vertical rain screen cladding

 

Any downsides to doing it like this?

 

 

 

Many thanks!

 

Bart

[Thorfun]

7 minutes ago, BartW said:

Any downsides to doing it like this?

it makes the building bigger so planning might have an issue, assuming they ever come and check I guess.

[Thorfun]

also, there's a lot of discussion about decrement delay of insulation and it's effect. PIR has poor decrement delay and also poor sound attenuation when compared to mineral wool. what size timbers is your TF being made of? I presume they're bigger than 120mm so you could fit mineral wool between the studs and then insulate internally/externally with extra PIR to get better U-values but also get the benefit of sound attenuation and decrement delay of the mineral wool.

 

just a thought and sorry to throw the spanner in the works when you're so far down the line!

[BartW]

Just now, Thorfun said:

also, there's a lot of discussion about decrement delay of insulation and it's effect. PIR has poor decrement delay and also poor sound attenuation when compared to mineral wool. what size timbers is your TF being made of? I presume they're bigger than 120mm so you could fit mineral wool between the studs and then insulate internally/externally with extra PIR to get better U-values but also get the benefit of sound attenuation and decrement delay of the mineral wool.

 

just a thought and sorry to throw the spanner in the works when you're so far down the line!

 

Hi,

Yes decrement delay has been on my mind some time ago, and ways of mitigating its effects, but economical and other reasons took charge sadly...

 

Studs are 140mm, and PIR between is 120mm. I could technically got more PIR externally, and full fill with rock wool in the studs, but it's sadly too late now, as I have now committed to factory fit PIR, and I would imagine it is nearly in!

 

Is the rest of my thoughts process logical and more importantly correct?

 

 

 

[Thorfun]

not sure I'm qualified to answer your questions about your thought process. EWI is possible but you need to be sure that you have a condensation analysis done (as you should for IWI though!). There are quite a few on here that have done it so maybe one of those will be along to assist. might be worth doing a search or trawling the insulation subforum

[Nickfromwales]

Just put 30mm of PIR internally on top of the studs, foamed to the factory-fitted PIR to prevent air gaps between the two layers, and use acoustic "sound barrier" plasterboard internally on all the external walls. That will stop the cold bridging from the studs, eg the connection from the outside layer of OSB to the room interior, and take you where you need to be.

You'll need to live 2 more lifetimes to reap the benefit of adding EWI to this detail, so time to worry about something else now  

[BartW]

Thanks guys,

 

@Nickfromwales I am was meaning to say that I am considering doing external PIR instead of Internal. Is this not a wise move?

 

 

[Nickfromwales]

13 minutes ago, BartW said:

Thanks guys,

 

@Nickfromwales I am was meaning to say that I am considering doing external PIR instead of Internal. Is this not a wise move?

 

 

Insulation is wise, wherever it goes. If EWI is laborious, adding the 30mm as I suggest is very simple and does not create any more detailing / work internally. Yes, you'll lose a fraction of internal room volume, but I would be confident this would be a cheaper and simpler route. 

[BartW]

14 minutes ago, Nickfromwales said:

Insulation is wise, wherever it goes. If EWI is laborious, adding the 30mm as I suggest is very simple and does not create any more detailing / work internally. Yes, you'll lose a fraction of internal room volume, but I would be confident this would be a cheaper and simpler route. 

 

Ok, so just to clarify, I have always planned on 50mm internally. That + 120mm in-between the studs would achieve 0.149 u Value.

All I was thinking of was to move the internal insulation to sit on the outer surface for the reasons mentioned in my first post.

Are you suggesting, this is not as straightforward as I picture it?

 

 

 

 

[bob the builder 2]

Hi All

 

I am in the same situation but have the the choice to go on one of the following options. 

 

My main concern is lack of thermal mass (and swings in internal temp) and sound to a lesser extent 

 

Option 1 - Mineral wool between the studs  + Extra Layer of PIR

 

• 140mm mineral wool between the studs
• 80mm rigid insulation (PIR) between the studs and VCL and battens

 

or

 

Option 2 - PIR between the studs + Extra Layer of PIR

 

• 140mm Rigid insulation the studs
• 50mm rigid insulation (PIR) between the studs and VCL and battens

 

Any thoughts appreciated 

 

bob

[dpmiller]

while PIR between the studs is theoretically better, the difficulty getting a thermally-good fit vs just shoving in rockwool makes the rockwool my preferred choice.

[BartW]

18 minutes ago, dpmiller said:

while PIR between the studs is theoretically better, the difficulty getting a thermally-good fit vs just shoving in rockwool makes the rockwool my preferred choice.

 

Yes, unless PIR being done in factory.

[Thorfun]

13 minutes ago, BartW said:

 

Yes, unless PIR being done in factory.

call me cynical (and I am) once the PIR is factory fitted and covered by a VCL membrane, also factory fitted, how do you know that they've done a good job in the factory? you're just assuming that the factory workers give a **** and do a bang up job. but I wonder what the reality is?

 

45 minutes ago, bob the builder 2 said:

Option 1 - Mineral wool between the studs  + Extra Layer of PIR

 

• 140mm mineral wool between the studs
• 80mm rigid insulation (PIR) between the studs and VCL and battens

this is what we did. it wasn't hard work but it is time consuming and not completely pleasant! the roof was a real pain though. but I know that the mineral wool is well fitted and that every join of PIR is foamed.

 

if I had the choice again I would seriously consider a twin stud/I-joist wall filled with blown cellulose for performance and speed of fit.

[BartW]

55 minutes ago, Thorfun said:

call me cynical (and I am) once the PIR is factory fitted and covered by a VCL membrane, also factory fitted, how do you know that they've done a good job in the factory? you're just assuming that the factory workers give a **** and do a bang up job. but I wonder what the reality is?

With that approach you would be doing the whole job yourself, to include everything else you won’t be able to see. The chance of them not giving a toss is as big as me doing it wrong (even if I were to commit to doing it in my lack of spare time). 
 

Back on topic, is there anything wrong with applying secondary PIR externally, rather than internally?

 

thanks

Bart

[Thorfun]

49 minutes ago, BartW said:

With that approach you would be doing the whole job yourself, to include everything else you won’t be able to see. The chance of them not giving a toss is as big as me doing it wrong (even if I were to commit to doing it in my lack of spare time). 
 

Back on topic, is there anything wrong with applying secondary PIR externally, rather than internally?

 

thanks

Bart

ahhh....I wish I shared your faith in the world and the building industry. I have hired good trades but have still had to keep an eye to ensure that things are done properly and as designed rather than just 'how they've always done it'. 

[Iceverge]

1 hour ago, BartW said:

Back on topic, is there anything wrong with applying secondary PIR externally, rather than internally?

 

 

Have you considered an airtight/vapour barrier anywhere in the buildup?

 

If you add external PIR there's a risk of condensate forming on the back of the sheets and rotting the sheathing (OSB I guess). It'll probably be fine if you've got a good airtightness/vapour control layer with no holes inboard somewhere. 

 

Airtightness will be much more important from an energy use and comfort point than insulation, what target do you have? 

 

 

[BartW]

2 hours ago, Thorfun said:

ahhh....I wish I shared your faith in the world and the building industry. I have hired good trades but have still had to keep an eye to ensure that things are done properly and as designed rather than just 'how they've always done it'. 

 

Yes, I can see where you are coming from, but certain things you have no control over. For me, this being one of them.

 

2 hours ago, Iceverge said:

 

Have you considered an airtight/vapour barrier anywhere in the buildup?

 

If you add external PIR there's a risk of condensate forming on the back of the sheets and rotting the sheathing (OSB I guess). It'll probably be fine if you've got a good airtightness/vapour control layer with no holes inboard somewhere. 

 

Airtightness will be much more important from an energy use and comfort point than insulation, what target do you have? 

 

 

 

Yes, there would be the usual VCL internally, and breather membrane externally. Of course, that would have been the assumption on the standard timber frame wall buildup. Would I need to shuffle things up if I go external PIR?

 

 

[Iceverge]

I should probably ask a more specific question. 

 

Can you describe your total wall build up from outside to inside specifying will be done when the timber frame factory has done and what remains to do yours self. 

[BartW]

6 hours ago, Iceverge said:

I should probably ask a more specific question. 

 

Can you describe your total wall build up from outside to inside specifying will be done when the timber frame factory has done and what remains to do yours self. 

 

So, the anticipated build-up from inside out was going to be (does not detail the membranes):

 

 

 

[Nickfromwales]

21 hours ago, BartW said:

is there anything wrong with applying secondary PIR externally, rather than internally?

 

 

9 hours ago, BartW said:

 

So, the anticipated build-up from inside out was going to be (does not detail the membranes):

 

 

 

185mm gross is a healthy amount of PIR, so I would stick with this tbh. Now I see the 60mm internally the concern of cold bridging from the frame is dealt with. Don't apply to the outside, as others have said, as it will unnecessarily complicate things ( with little reward imo ).

 

Looks good as-is.

[BartW]

Thanks Nick

 

off to worrying about exposed floor void ventilation now again 😆

[JohnMo]

Looking at your wall build with respect your service cavity.  You have plasterboard, OSB, batten for service cavity.

 

This should be plasterboard, batten for service cavity, OSB.  

 

If you floor has a ventilation within the void it's there for good reason.  If your insulation is above the ventilation why are you worried?

[Thorfun]

3 minutes ago, JohnMo said:

This should be plasterboard, batten for service cavity, OSB.  

why do you specifically state this? I see no issues with putting OSB on top of the service cavity with plasterboard on top of that. I've spoken to a number of builder friends and they all say that they put plasterboard on top of OSB as it gives the wall a rock solid feel. it is something I am very seriously considering.

 

On 15/05/2022 at 22:07, BartW said:

So, the anticipated build-up from inside out was going to be (does not detail the membranes):

can I ask if you have a specific reason for using 2" x 2" battens for your timber cladding? we used 2" x 2" for ours due to the fact that our external blinds needed the space between the building and the cladding to function but it cost us a LOT of money especially as we, like you, are going for vertical cladding so needed to batten and counter batten. if you don't specifically need that size battens it might be worth using 2" x 1" roofing battens as that should save a ton of money? 

 

just a thought.

[JohnMo]

5 minutes ago, Thorfun said:

why do you specifically state this? I see no issues with putting OSB on top of the service cavity with plasterboard on top of that. I've spoken to a number of builder friends and they all say that they put plasterboard on top of OSB as it gives the wall a rock solid feel. it is something I am very seriously considering.

First the OSB layer has done 90% of your airtightness especially if you use airtight OSB on the joints to worry about.  Or if using a membrane a great flat surface to affix to.

 

Your service cavity will have your plumbing and electric stuff within it,  why would you want then to make loads of holes for light switches etc in the OSB to allow it to be installed?

 

Seems like a double work effort to me for less gain

[Nickfromwales]

5 minutes ago, JohnMo said:

First the OSB layer has done 90% of your airtightness especially if you use airtight OSB on the joints to worry about.  Or if using a membrane a great flat surface to affix to.

 

Your service cavity will have your plumbing and electric stuff within it,  why would you want then to make loads of holes for light switches etc in the OSB to allow it to be installed?

 

Seems like a double work effort to me for less gain

Membrane can go atop the 60mm PIR and be fixed by the battens. After that 1st fix electrics / plumbing. The OSB would be affixed AFTER 1st fix, so all metal back boxes would be in already and the OSB cut out to suit.

Sounds like a good plan IMO, and AT detailing with a membrane behind the battens is shit-loads easier to manage and preserve IMHO.