Wall cross-sections, opinions appreciated

[btm3055]

(Apologies if this is in the wrong section.)

 

For context, I’ve zero experience of house building, but I am pretty experienced with wood (furniture etc).

 

I have a timber-frame wooden building on the shoreline in Northern Ireland. The weather can get a bit wild, and we’re quite exposed. However, it’s stood up for 100 years, and I’ve been told by locals this is in part due to the materials being pinched from the Belfast shipyards (and therefore treated for marine conditions).

 

The walls (internal and external) currently consist of 3x2 frame, clad with 12mm tongue and groove, and stuffed with sawdust and wood chippings. Obviously that’s not adequate.

 

I’m trying to arrive at a final plan for the walls before sending the plans off the building control. I’d really appreciate people’s opinions on the structure. Am I missing something, does something need to be chunkier, is there a newbie error in there, etc. (For example, I’m unsure if I need a moisture barrier between the frame and the plywood sheeting on the outside, given that there’s also a weather shield — I’m guessing it can’t actually hurt anything but my wallet.)

 

My priorities are structural rigidity, wind/weather-proofing, insulation, a utility channel for electrics etc., and to be realistically achievable.

 

Cross-section diagrams of external and internal walls attached. All opinions appreciated.

[saveasteading]

This seems over complex. 

4 layers of vapour barrier of different types  whereas one out and one in should do.

Plywood on the frame will make it very much stronger, laterally and vertically,  so that is a great idea.

 

I'm not a fan of foil insulation if there are other options. 6" between the studs as shown is a good start. Then some non- conducting liner over the studs.  

 

What external facing have you in mind?

[Iceverge]

Is this a demolish and rebuild scenario? 

 

There is too many moisture barriers. One "barrier" ( think plastic sheet) max in any buildup. Everything needs to be breathable ( think goretex jacket). Otherwise the wall will not be able to dry and stay damp and rot. 

 

No moisture barriers normally on internal walls typically. 

 

The ply Vs OSB has its protractors/detractors arguements. Good quality OSB for me every time. I like Smartyply. 

 

One factor I would be cautious of is using lightweight insulations like glasswool and foam considering you have no masonry externally leaf. It would be similar to a caravan in the summer regards overheating due to poor decrement delay/phase shift. 

 

This is the the time it takes for the initially daily blast of external summer heat to soak though the wall/roof. Two buildings will perform drastically differently in summer even if they have the same U values. The caravan and stone outhouse are the dramatic examples. 

 

The stone building remains comfortable but the caravan overheats in summer although in winter they take the same amount of energy to keep warm.

 

Have a play with Ubakus.com. it's great for this. 

 

 

 

 

 

[George]

Internal to external wants to go from less vapour permeable to more vapour permeable. 

 

Are the internal battens for a service void? 

 

You shouldn't need plywood on both sides (personally I think internal is better) and grade 3 OSB is a better choice.

 

Internal - plasterboard - 25x38mm battens / service void - vapour control layer - 25mm PIR insulation* - 9mm OSB** - 150x50mm*** timber timber structure with full fill PIR* - Breather membrane - 50x50mm battens (or 2No layers 25mm battens for vertical cladding) / ventilated cavity - cladding

 

*Insulation type and thickness to whatever is needed for BR in your location

**9mm OSB to be increased to whatever SE says

***150x50mm structure to whatever SE says

 

 

[btm3055]

Thanks for all the replies.

 

I have the winter to make these decisions, because the weather’s just too unpredictable now. But I do want a plan soon, if only to get a budget in place.

 

@saveasteading — I went for foil because the front part of the building is on stilts, so weight is an issue. I thought foil in the roof would save a bit there, and it seemed to make sense to continue the same insulation down the walls.

 

The external cladding is likely to be larch, and possibly beaten brass or copper for the bottom 30cm to keep the wood off the ground. We’re still debating that, and the price of wood means it’s likely to be recycled from the current cladding for a year or so (the whole building is tongue and groove inside and out).

 

@Iceverge — No, renovation. We’re living in it, and funding the build with what we save in rent/mortgage. So yes, it’s a bit of a jigsaw, and flattening it and starting from scratch would be easier and cheaper. But for a few reasons, that’s not happening.

 

I have spent an unexpected amount of time reading about the difference between OSB and ply. It seems to be more polarising than brexit. I guess I opted for ply because I’ve used it in the past, so I was going for something I was familiar with. Literally everyone else says OSB though.

 

The insulation we like is hemp (https://www.ecologicalbuildingsystems.com/product/thermo-hemp-combi-jute). Mainly because it doesn’t require any specialist equipment to install. The manufacturer recommended a moisture barrier on both sides of the frame to keep it in place, but I guess the racking would do the same job.

 

Thanks for the Ubakus tip, I will play with that.

 

@George — The foil insulation needs an air gap on both sides to be effective (minimum 38mm). I bumped it up to 50mm on the side closest to the plasterboard to give me a more generous service void (just in case).

 

If I dropped the internal ply/OSB, I could probably fix the foil straight to the frame instead of one of the vapour barriers — but maybe I should rethink the foil anyway.

 

[Iceverge]

1 hour ago, btm3055 said:

went for foil because the front part of the building is on stilts, so weight is an issue.

 

If you have structural issues foil won't save you.  I'd avoid it in any case, most of the performance numbers they claim comes from the unventilated airspaces created within a wall, much like double glazing. Any kind of air circulation would ruin the theoretical performance and it's a renovation so you'll have some air movement.  Mineral wool, cellulose/hemp is a better way to stop the air moving. Fire will love those air gaps too if it gets in there.

 

PIR between studs is fine if you like suffering, don't care about summer overheating, of fire risk, or materials wastage!

 

 

Do you have any pictures of the building as it stands, inside and out?

Im guessing that you don't have a tremendous budget so it's important to save as much cash as possible.

 

I think you don't need to remove the tounge and groove boards outside if they are in good condition, just lap, staple and tape a good breather membrane over the top, then some 38mm or 22mm battens for ventilation and some more cladding.

 

Internally you could take down the T&G and put hemp batts between the 6x2 studs. Then cut 45deg tracks in the studs and push in some T L or C profiles to provide cross bracing screwing them into the studs. Then cover and really diligently tape with a good vapour membrane, fix 47mm cross battens for a service cavity insulated with 50mm hemp batts and then reuse the tounge and groove cladding.

 

It could be done piece by piece, fairly cheaply and would be windtight, airtight, have little thermal bridging, good phase shift, breathable, good sound protection, low embodied carbon, good fire performance, low materials wastage.

 

 

 

 

 

 

 

 

 

 

 

[George]

The only time I've seen foil insulation suggested seriously is for retrofit where nothing else was feasible. The idea is that radiant heat gets reflected back into the room to be absorbed. Which is great. but with a well insulated wall, that same radiant heat is absorbed by the wall, so still remains within the same thermal envelope. So what have you gained by reflecting it back in?

 

Foil can make make some improvement in badly insulated walls. But on a new build that is not where you should be.

 

If weight is an issue, improve the foundations. 

Edited October 25, 2022 by George

[Temp]

How about treating the existing structure as just ventilated cladding?

 

Something like..

 

Remove existing insulation to allow ventilation of that void.

 

Line with vapour permeable membrane and OSB.

 

Build internal structure from say 2*5 with 125mm insulation between...or 100mm and a 25mm service void.

 

Plasterboard.

 

Is there a DPC already under the existing structure?

Edited October 26, 2022 by Temp

[btm3055]

@Iceverge — thanks, I’m going to have a second look at the foil. It’s starting to sound like a waste of money.

 

I’ve attached a before and after image of an internal wall. Left: 3x2 frame, verticals ~80cm on center. Right: 4x2 treated CLS, 40cm on center, left the original frame in place where possible and added battens to the 3x2 to get it roughly level.

 

This wall’s urgent because I need to run electrics along a utility channel in here. (The current wiring is very old and probably dangerous, so the sooner the better.) However, because it’s internal, I can get to the other side no bother, so as long as I don’t need to change things like the membrane, I can do most of the work from the other side. I may just put up some sheeting, add battens for the utility space and call it done.

 

(Incidentally, you can see the end wall that’s an emergency repair to an external wall. That moved about 10cm in each direction when you leant on it. So that frame got hastily patched up this summer. However, it needs to be rebuilt again as it’s just a frame the outer T&G and some membrane to stop drafts.)

 

The budget is more about time than money. I can afford to spend £1000–£2000 per month. I’m expecting it to take five years, and cost in the region of £100k. As I work full time I can only work on it evenings and weekends, so at the moment I run out of time before I run out of money.

 

However, once I have to pay people (like electricians) I will eat through the budget pretty fast.

 

The T&G on the outside is dirt. It was replaced in the 70s and they’ve used untreated softwood painted with some kind of gloss paint. Somewhere around 40% is rotten, so it all has to go before it gets into the frame. However, I can probably save ~60% of it and make do for now until the price of wood comes down. (Oddly, the side of the building that faces the sea isn’t so bad, it’s the sheltered side that seems to be worse.

 

@George — thanks. Yeah, the foil is probably off the table by now. No one seems to rate it.

 

@Temp — Nope, no DPC at all. Just a wooden frame fixed to a concrete slab at the back, and a wooden frame on stilts at the front.

 

I’m planning to build a short 30cm concrete wall and rebuild the external frame on top of that. But I need to work out the width of the wall first. The damp course will get slotted in there somewhere once I’ve worked out the details.

Edited October 26, 2022 by btm3055

[Iceverge]

@btm3055

 I've had a look back at your previous posts. 

 

It's daunting but you'll need to have a look at this objectively. 

 

At the moment your house has rotten cladding, insufficient structure. No DPC. No insulation, needs rewiring, needs plumbing.  Needs windows, needs decorating. There's nothing left! 

 

Unless you value your time you will be committed to make poor choices. I did. 

 

The work you have done is tidy but can you say objectively how long it's taken you? 

 

 

 

 

 

Edited October 26, 2022 by Iceverge

[btm3055]

That internal wall took me about 5 hours, going slowly and drinking lots of tea. I imagine I’ll get faster when I’m not triple-checking everything.

 

I’m aware it’s a big task, but objectively:

 

The last place I lived was smaller, with one fewer bedroom, was made of flint and had zero insulation, was rodent infested, had rising damp, and black mould. Plus it cost me £1500/month to rent with zero legal guarantees. And that was probably the best place I ever rented — there aren’t that many options.

 

This place is basic, but liveable. We have mains electricity (although I’m having that updated now), we have running water (albeit only one tap), we have a toilet and a septic tank, most of the work that needs doing is doable by me, with the tools I already own (excluding the electrics). Plus it’s free and we can live in it while I work.

 

The key (I think) is to keep the plan nice and simple. Get it signed off by building control so I’m confident I’m not doing something really stupid, and then go section by section until it’s all good.