Yet Another Timber Frame Question

[EquiumDuo]

Hi All,

I've scoured lots of threads and I don't think I am doing anything extraordinary. But I have some questions. This is going to be the wall of a regular 4x bedroom house, rectangle shaped. Nothing too exciting.

 

I've attached 2x photos, one is the wall make up:

12.5 mm Plasterboard

Service Void 25/34mm (tbc)

Vapour Control Layer (Dupont Airguard)

140mm Studs (filled with 140mm Rockwool Flexi)

19mm (tbc) OSB

Permeable Membrane (Tyvek Housewrap)

50mm Cavity Partial Fill (Rockwool High Perf Cavity Fill)

50mm Airgap

100mm Dense Block

External Render

 

Nothing groundbreaking and mostly based on advice I've read here and the rockwool catalogue (plus some youtube videos).

 

Please correct any mistakes, but this is what I'm looking at for the build. My main questions are:

 

1. A lot of suggestions for PIR boards (Celotex is mentioned often) to go in the cavity between the frame and the blockwork, but that doesnt make sense to me, PIR/foam boards seems air tight the way they should be layed out, whats the point opf a permeable membrane and rockwol etc to allow moisture out if you are going to put an insulated bin bag over the top? I'm literally imagining a wicking sports top, a hoodie and then a bin bag. You won't be very dry in that scenario. Is my analogy wrong or am I missing something?

 

2. With regards to the joists, I want to use engineered metal web joists (eg posi joist) between ground and first floor, how do these interact with the vapour control membrane and the permeable membrane? Looking at my second picture, is that how it all sits together? There would only be a small OSB/stud gap essentially between the end of the joist and the permeable mebrane, or is this just accepted as a comprimise?

 

3. For those of you choosing to go stick build yourself, how are you designing the walls/timber frame. Are you getting detailed plans from your architects? Are you buying a Haynes Manual or some other source? My google-fu is letting me down here.

 

Thanks for your time.

 

[Thedreamer]

1 hour ago, EquiumDuo said:

Hi All,

I've scoured lots of threads and I don't think I am doing anything extraordinary. But I have some questions. This is going to be the wall of a regular 4x bedroom house, rectangle shaped. Nothing too exciting.

 

I've attached 2x photos, one is the wall make up:

12.5 mm Plasterboard

Service Void 25/34mm (tbc)

Vapour Control Layer (Dupont Airguard)

140mm Studs (filled with 140mm Rockwool Flexi)

19mm (tbc) OSB

Permeable Membrane (Tyvek Housewrap)

50mm Cavity Partial Fill (Rockwool High Perf Cavity Fill)

50mm Airgap

100mm Dense Block

External Render

 

Nothing groundbreaking and mostly based on advice I've read here and the rockwool catalogue (plus some youtube videos).

 

Please correct any mistakes, but this is what I'm looking at for the build. My main questions are:

 

1. A lot of suggestions for PIR boards (Celotex is mentioned often) to go in the cavity between the frame and the blockwork, but that doesnt make sense to me, PIR/foam boards seems air tight the way they should be layed out, whats the point opf a permeable membrane and rockwol etc to allow moisture out if you are going to put an insulated bin bag over the top? I'm literally imagining a wicking sports top, a hoodie and then a bin bag. You won't be very dry in that scenario. Is my analogy wrong or am I missing something?

 

2. With regards to the joists, I want to use engineered metal web joists (eg posi joist) between ground and first floor, how do these interact with the vapour control membrane and the permeable membrane? Looking at my second picture, is that how it all sits together? There would only be a small OSB/stud gap essentially between the end of the joist and the permeable mebrane, or is this just accepted as a comprimise?

 

3. For those of you choosing to go stick build yourself, how are you designing the walls/timber frame. Are you getting detailed plans from your architects? Are you buying a Haynes Manual or some other source? My google-fu is letting me down here.

 

Thanks for your time.

 

 

How come you want to put glass wool in the cavity? Your set-up seems pretty traditional, therefore it would be unusual to have glass wool after the membrane. Usually the cavity for a timber frame is kept clear, unlike a block &  block build. 

 

Why not just go for a upper glass wool product such as Frametherm 32, I used a mixture of 32 and 35. 32 is much more superior. Frametherm is a lot easier and nicer to work with than PIR. I used both. If I could go back I would use even more glass wool.

 

We stick built our house, I have a blog here that might be of some interest.

 

https://forum.buildhub.org.uk/blogs/blog/28-druim-nan-darach/

 

The spec was done by our house designer and a structural engineer. I deconstructed both plans into a bill of quantities and ordered pretty much all the materials myself.

 

Recommendations on manuals, books etc Home Builder's Bible covers a lot and gives a broad overview of self building.

 

[EquiumDuo]

Thanks for the input, I was looking at a very well insulated house. And following some guidance on youtube videos. Lots of the american guys in the northern regions use these rockwool bats on the outside of the tyvek housewrap. The rockwool is breathable so the idea being it is permeable to moisture but resists the temperature gradient and adds more sound insulation. Also puts a fire barrier between the outside and the inside fo the building. Which is a nice to have (not really needed for me though).

 

Also according to this website https://www.changeplan.co.uk/u_value_calculator.php

The difference between with and without the outer mineral wool is 0.19 (with) and 0.25 (without). Is that significant?

 

Thank you for the blog link, currently getting throuigh ProDave's blog atm.

 

I also do have and am chugging through the home builders bible, really good book but it is quite dry at times 

Any thoughts on how to route the mebranes around the joists?

 

 

 

 

[Hobbiniho]

Up here in shetland the "standard" timber frame would be similar to your build up except without the rockwool in the cavity, keep the cavity clear. However i would put a layer of 50mm PIR on the inside of the studs this can double up as the airtight later and you can put a 25mm layer in the ingoes in to the windows

[the_r_sole]

Get the mineral wool out of the cavity and put some rigid insulation on the inside of the kit panel.

Check the joist manufacturer details for the fixings, most of the ones I've seen are on hangers rather than built into the wall like you've shown... engineers will give you panel drawings, although it's not rocket science!

[Hobbiniho]

Also what you have drawn in the second drawing showing the air tight layer is often called a " tony tray"

[ProDave]

I would seriously think of making the frame thicker to get more insulation in, as I did with my frame made of 195mm timber.

 

Your floor joist detail.  As has been mentioned, wrapping the air tight membrane around the joist ends is known on here as a "Tony Tray"  You also need to enclose the ends of the joists with a ring beam all the way round the same thickness as the joists, mine were Glulam beams 300mm by 50mm.

[EquiumDuo]

Thanks guys, I wont be attempting this completely blind, the old man's a builder so will mostly be following his lead. But I am keen to make sure we've got the best solutions for our budget, so all your feedback is appreciated.

 

The other source fo rockwool in the cavity was rockwool's own datasheet.

 

Also I assume Tony Tray is good? I can't tell haha!

[Simplysimon]

i wouldn't be doing the joists like that, horrible cold bridge. put them in hangers on the inside face. as to a vcl if you use something akin to smartply propassive, it is effectively puncture proof, more expensive than poly sheeting but i would say considerably better.

[ProDave]

2 minutes ago, Simplysimon said:

i wouldn't be doing the joists like that, horrible cold bridge. put them in hangers on the inside face. as to a vcl if you use something akin to smartply propassive, it is effectively puncture proof, more expensive than poly sheeting but i would say considerably better.

Standard detail on most timber frames. It leaves the joist ends helping to support the upper storey timber frame and makes the joists very rigid.

 

Yes, it gives you a cavity that needs filling with insulation but that is straightforward if a little tedious, and a good place to use up insulation offcuts.

[JamesP]

I have a similar build up to your own drawing except used 3 layers of Rockwool Flexi in my wall build up, 140, 70 and 50. Very good fit, easy to cut and not to toxic. . I had a 40mm layer of t&g  Pavatex on the outside. No blockwork, just cladding.

The first floor joists sit on the walls, no joist hangers with a similar detail to your original drawing. Very solid, no squeaks or movement. I felt it provided a very stable platform to add the first floor walls.

Have fun.

[EquiumDuo]

Once again thank you all for the comments, I'm thinking hard about what to change.

[SteamyTea]

On 22/10/2020 at 14:12, EquiumDuo said:

Lots of the american guys in the northern regions use these rockwool bats on the outside of the tyvek housewrap.

Different climate.

Very cold and dry winters, very hot, and sometimes humid summers. Not much spring or autumn.

 

[ADLIan]

Might not be a common detail but no reason not to insulate in cavity provided 50mm min cavity kept before brick outer.

 

[SimonD]

On 22/10/2020 at 14:12, EquiumDuo said:

Lots of the american guys in the northern regions use these rockwool bats on the outside of the tyvek housewrap.

 

8 hours ago, SteamyTea said:

Different climate.

Very cold and dry winters, very hot, and sometimes humid summers. Not much spring or autumn.

 

 

Adding insulation to the outside of the timber frame reduces cold bridging, compared to, for example, additional layers on the inside, which for some reason seems popular over here in the UK.

[SteamyTea]

32 minutes ago, SimonD said:

Adding insulation to the outside of the timber frame reduces cold bridging, compared to, for example, additional layers on the inside, which for some reason seems popular over here in the UK.

It is all to do with the condensation risk.  The risks are different for different climate regimes.  I think this caused a few problems when the UK built TF in the 1980s.

Since then a lot has been learnt (my place is a late 80's TF and not a problem here as far as I know).

[ADLIan]

Very little insulation in a 1980s timber frame house so condensation risk very different from current standards. From memory issue in 1980s was bad building practice and poor detailing leading to moisture penetration and subsequent rot, may also have been issues with fire performance. Also remember reports of media overhyping the issue of only a few actual problems - didn’t help Barrats though. Current timber frame construction is a world away from back then.

[SteamyTea]

26 minutes ago, ADLIan said:

Very little insulation in a 1980s timber frame house so condensation risk very different from current standards. From memory issue in 1980s was bad building practice and poor detailing leading to moisture penetration and subsequent rot, may also have been issues with fire performance. Also remember reports of media overhyping the issue of only a few actual problems - didn’t help Barrats though. Current timber frame construction is a world away from back then.

Yes, they were only a little disaster.

My house has 200mm of mineral wool insulation and good airtightness.  It was a pre-production model constructed for testing.  The plastering is crap though.

[EquiumDuo]

This is the exact thinking that drove me to the design i chose.

 

I think we can all agree that mineralwool/rockwool is moisture permeable, Looking at my wall layout, there is the hard line dupont airguard on the inside, and all breathable insulation outwards. I don't personally see the logic of adding insulation to the inside of that dupont airguard. In my mind that airguard is the boundary layer between what is considered inside and outside of my home. Insulation internally is driven primarily by acoustic performance after that.

 

Not that I am not considering all your suggestions very carefully. I've modelled a lot of your ideas and its really interesting. I really hate the idea of the dense block because honestly I dont even see a point to it anymore, BUT thats very uncommon in my part of wales, so not sure how the powers that be will think about it (its even mentioned in the detailed planning as a condition of the external render look. "Textured render on dense block"

[Gone West]

On 22/10/2020 at 19:26, EquiumDuo said:

Thanks guys, I wont be attempting this completely blind, the old man's a builder so will mostly be following his lead. But I am keen to make sure we've got the best solutions for our budget, so all your feedback is appreciated.

 

The other source fo rockwool in the cavity was rockwool's own datasheet.

 

Also I assume Tony Tray is good? I can't tell haha!

That is a similar type of construction as my house. I have 350mm I-beams as my main frame filled with Icynene and OSB3 racking on the outside. I then have the membrane and 50mm RW6 Rockwool batts followed by a 50mm cavity and cedar cladding as the rainscreen.

[BartW]

Hi,

Quickly borrowing the thread to ask how packing oversized rock wool into the TF frame would stack up as a practice. I mean 150mm rock wool into a 140mm Timber Frame. I have been doing all sorts of calculations, and somehow a 150mm Knauf Dritherm 37 works out cheaper than Dritherm 32 125mm, whilst providing similar uValue performance.

 

I appreciate packing the whole thing would be a bit more of a faff, but insulation cost difference would be around a £1000 for just the external walls.

 

Any opinions please?

Bart

[PeterW]

150mm rockwool in a 140mm frame would fit as it is in compressed bales when it is sent to site but you would find your Frametherm (Not DriTherm as that is for masonry cavities) 37

would probably be closer to 0.385 due to the compression of the product reducing the air space. Either way it will be suitable and full fill means it is easier to install and not worry about it moving in a partial fill. 

[BartW]

Thanks Peter,

 

 

good point. 
 

I have checked again and 140mm Frametherm 35 roll would be available with the same level of savings, whilst providing better R value. Happy days

[Thedreamer]

19 minutes ago, BartW said:

Thanks Peter,

 

 

good point. 
 

I have checked again and 140mm Frametherm 35 roll would be available with the same level of savings, whilst providing better R value. Happy days

 

I used frametherm 35 140mm in my walls good stuff. 

 

Might worth considering frametherm 32. I used it in my floor and roof space, it looked like a thick rug.

[BartW]

24 minutes ago, Thedreamer said:

 

I used frametherm 35 140mm in my walls good stuff. 

 

Might worth considering frametherm 32. I used it in my floor and roof space, it looked like a thick rug.

Thanks. Trouble is the 32 seems to command double premium compared. 
 

I need 210m2 for external walls, and been working out price difference between TF manufacturer supplied insulation vs PIR (GA4120) vs variables of rockwool, and between PIR and dri35 there was roughly £1500 difference. Let alone, floors, internal partitions, roof, intermediate floors, inner face of external, etc.