Feasibility: EWI with larsen truss + flexible wood fibre batts

Hi everybody!

I’ve been following CarbonCoop’s webinars and blog for a while now and am just approaching the detailed planning stage of my own home retrofit so thought it was time to join up and start asking a few feasibility questions.

I’m considering a bit fo a novel approach for external wall insulation for a single solid brick wall (terrace rear).

Using larsen trusses, with friction fit flexible wood fibre held into the wall by cross battens, with a breathable membrane beneath the battens.

It’s a variation on an approach I’ve seen used with EPS between the trusses, but I’m hoping to use wood fibre as a more sustainable material (and for me easier to work with).

At the moment I’m just evaluating whether there are any obvious problems with this approach before starting detailed plans. As far as I know, flexible wood fibre is suitable to be installed vertically like this in timber framed buildings, so

My main reasons for considering this are:

  1. It seems a better option for supporting the timber cladding at the thickness I’d like to achieve (200mm of insulation)
  2. It seems easier (in my head at least) to fix trusses to the walls with conventional length screws rather than use extremely long (250mm?) fixings for anchoring wood fibre boards to the wall
  3. Avoids the need for an adhesive layer/parge for bonding solid wood fibre boards to the (definitely uneven) brickwork.
  4. Overall weight should actually be much lower.

If this is the wrong place for asking such questions or if it’s the kind of question which actually needs an assessment of the building then let me know.

Thanks in advance!

Oh and here’s the project which gave me the idea for this, where they used EPS for both roof and walls with DIY larsen truss-esq C-shaped beams made with OSB strips. A very cool little trick.



Be cautious of any novel wall assemblies. Maybe novel in your area? In which case, make sure your contractor is comfortable with what you’re proposing.

Trusses are often used above 150 mm because, as you’ve said, fixing rigid insulation can become tricky beyond that. Rigid insulation systems, such as Gutex, can go up to 200mm. The insulation supplier/manufactures will specify everything, down to the very screw patterns and angles, to get warranty. Not sure what they do with very uneven walls.

Wall truss assemblies typically use blow in insulation because the exterior sheathing allows for consistent packing factors and densities. Vertical cavities are better than sloped and horizontal ones for this. You don’t need to worry so much about uneven masonry, as the trusses can be shimmed to be flat and plumb.

*edit, I just got to the bottom of the article you referenced, they did not go for a sheathing system, but a breather membrane on the outside, after several overlapping layers of insulation and bracing/furring strips. Looks like a very labour-intensive way to do it imo. They had a lot of money.

However, it is better practice to have the air control layer on the masonry side, as well as the sheathings water and vapour control layer.

The usual suspects; punched opening, vertices, edges, and services; are where all the detail and the skill is needed. I’m confident with the theory, but don’t ask me about doing this in practice, let alone doing it productively.

Windows in particular. “Outie” windows, with thermal bridging detailing around the casing and frame, along with flashing and sealing details, are complex.

Cellulose will likely be your most environmentally friendly option. Consider doing it alongside blowing in insulation elsewhere, such as your loft, because it’s the call-out fee and equipment that cost a lot, not necessarily the material.

You could go over 200mm without too much extra cost, if the transition to your roof system and soffits allows for the build up and convenient continuity of control layers.

Ultra-high levels of insulation can lead to hydrothermal issues on the inside of the exterior sheathing in cold, wet, climates. You’ll need a back ventilated drainage plane behind your cladding system, 10mm or so for exterior renders, 20+mm for timber cladding (check fire codes). Plywood is better than OSB with moisture, and some plywood is better than others. I think 10-20perm is recommended. Interior vapour barriers are highly suspect in these situations. Be sure you know what you’re doing at the very high end.

My personal view is that 150mm rigid insulation systems, that are proven, are fine for retrofits. Save the money for the windows and doors, or the mechanical systems such as underfloor heating and rigid low static duct systems.

1 Like

Thanks for your reply @Frank_Reif,

I should have probably mentioned that I’m planning other retrofitting along side this, roof etc, but wanted to keep the topic just about wall trusses for simplicity.

Looks like a very labour-intensive way to do it imo. They had a lot of money.

No doubt their budget was higher than mine, and also a much larger house too, but I think they did a lot of the works themselves so a simpler but labor-intensive approach made sense. It’s not too far from my house, and I recall seeing their black outer membrane for several months before they got their cladding on :sweat_smile:

I’m undecided about doing the EWI myself, which is partly why I’m considering this option as it is one which I’m fairly confident that I at least could do myself, mechanically, with perhaps guidance from a retrofit consultant on detailing.

I also feel that a timber cladding is probably something which could be replaced more easily as it wears with age, than a rendering system… but that’s just a feeling…

However, it is better practice to have the air control layer on the masonry side, as well as the sheathings water and vapour control layer.

I haven’t seen any EWI systems specify an external air control layer, unless that is because in most hard-fit systems the parge/adhesive performs that function?

The usual suspects; punched opening, vertices, edges, and services; are where all the detail and the skill is needed. I’m confident with the theory, but don’t ask me about doing this in practice, let alone doing it productively.

Yep, absolutely thinking about all of those and planning to involve a consultant on the details once a general whole house plan is formed. But knowing whether an approach is even a possibility at this stage lets me know whether it’s something I could consider taking on myself with support, or something to just find an experienced retrofit contractor in Sheffield to do “off the shelf”.

Cellulose will likely be your most environmentally friendly option. Consider doing it alongside blowing in insulation elsewhere, such as your loft, because it’s the call-out fee and equipment that cost a lot, not necessarily the material.

There isn’t really any other application for a blown insulation:

  • Roof & room-in-the-roof
    • Is going to need rebuilding this-decade anyway by my reckoning, at which point I hope to just do roof and the EWI together
    • Blown cellulose could possibly work with my plans for a warm roof, but flexible wood fibre can be fitted DIY from inside after the exterior works are done… so…
  • Suspended timber ground floors
    • Eh, possibly, but IDK if it would be “easier” than my plans for flexible fibre, as I assume both would need similar detailing for air and vapour control…
  • Front stone wall IWI
    • I’m planning wood or cork board for this at 100mm, and will be plastered anyway unlike the EWI which I’d prefer to clad with timber

Interior vapour barriers are highly suspect in these situations.

Could you explain that please? Do you meant that an internal vapour barrier would/could trap moisture in the fabric? I’m not planning to use internal vapour barriers against any of the exterior walls (but will be using vapour-closed boards in the bathroom to keep vapour in and vent it out)

My personal view is that 150mm rigid insulation systems, that are proven, are fine for retrofits. Save the money for the windows and doors, or the mechanical systems such as underfloor heating and rigid low static duct systems.

Could timber cladding be used at 150mm ish depths? Wouldn’t battens need fixing to the brick wall, leaving some thermal bridging? (I realise it’d be small screw heads and they could be covered but still seems counter-intuitive…

It may be a little like the tail wagging the dog, but I’m fairly taken with the style of timber cladding Bilton Design used, having seen it in person and also having looked at the cost of rendering…

Honestly those are about the only elements I don’t plan to replace this decade. Both are from early 2010s and have passable U-values, almost as good as the solid walls :sweat_smile:

And by my calculations any energy savings won’t come close to paying off the material costs of low-U alternatives within my lifetime (and I’m only 32!) And unless I’m mistaken, they shouldn’t result in condensation unless the internal temperature and humidity rises in future with the insulation… (I hope I’m right)

So until they require replacing materially, I’d like to try to retain them. After all, there’s a fair chunk of embodied energy in them too.

I’m hoping that even the 3 windows on the rear wall could be retained, but I accept that may well end up costing more to engineer supported boxes at the size of the existing window reveal than to replace them, at which point they’ll just go to wherever uPVC windows go when they die…

A big advantage of having the air & vapour barrier membranes on the outside of the existing wall is that once work is complete it is extremely unlikely to get damaged by someone doing DIY in the future.
Imagine a future owner hanging pictures or even shelves and the potential for damaging internal membranes and consequent long term damage.

The easiest method is spray on Blowerproof, which covers all flaws and penetrations:

1 Like

You’re a braver person than I am! I’d love to be on site for QC from time to time, to help make quick decisions, or even just have lunch with the crew and to muck in labouring and clearing up for a few days to get to know everyone. You’ll learn much faster that way, and if they like you, then they’re more likely to care about your building!

Generally, however, people who do it for a living do it so quickly that it depends on how much others value your time, and whether it’s better to work elsewhere and pay pros to do it instead. The warranty is important if you need to move, or god forbid, there’s a fire and the insurance people snoop around.

DIY culture in this country is a bit mad. TV propaganda for 40 years will do that to us.

I like timber cladding too. My choice would be natural cladding, the silver faded look is beautiful. I don’t care if it doesn’t last as long.

Modern plasters are durable, and more fire/water-resistant. I wouldn’t be surprised if over its lifecycle it’d be cheaper, if anyone else knows I’d curious to find out, especially with the recent supply chain issues. It’s cement, so high GHG emissions is a con.

It’s more about the aesthetic look you want. Does it fit in with the surrounding built environment? The Bilton Design retrofit looked a bit out of place to me. A beautiful building will be looked after better, and others in the neighbourhood will be more likely to copy, and therefore it is more sustainable. Subtlety and modesty is underrated.

I like when there’s a rendered ~300mm skirting to manage rain splashback onto the cladding. Bilton have it on the front side, but not on the pavement side. I quite like some options that extend up to the windows, or the full height of the first floor.

Consider flooding risk. It’s nice to have the insulation and cladding system close to the ground made of cement and rock wool, so you don’t need to worry about dirty floodwater damaging the building (we’re on a redeveloped water mill, and my poor neighbours are going to be flooded every few years due to climate change - they are going to have to tank up to the windows including a second DPC above the flood line, removable internal panels for drying etc, yikes - thanks Cala homes clearly in the floodplain).


Recycled systems may also not be such a bad idea. The bad thing about plastic is that it lasts forever, the good thing is that it lasts forever. Until the waste stream gets cleared up, I’m open to using these materials.

Having a proper water management system on the base of the cladding is cheaper than retrofitting massive overhangs on outrigger roof assemblies to keep the water away that way. Usually you can’t mess with the roof scape to create large overhangs in retrofits anyway, either for conservation or because of third party roof along the terrace, or because it’ll just block your windows.

Another thing I’d like to see more of, for a friend’s retrofit in particular, would simply to raise the whole roof and add a raised heel to the trusses. It’s how they get a thick continuous insulation detail round under the eves. But alas, corralling our current bunch of leaseholders is beyond my powers of persuasion.

If you had an unconditioned ventilated roof, you can blow 400 mm in there and be done with it - just ensure the soffits don’t get blocked. I agree with the rest.

Poly membranes are vapour barriers, so the wall assembly can’t dry inwards and outwards, which is needed in AC climates because the internal walls will be cooler, and therefore act as a condensing surface due to solar driven vapour from a reservoir cladding. Interstitial failures are the worst because they take a long time to be spotted. In ultra-high performance walls >200mm, you need to really worry about the old cliché “it needs to breathe”. The bonus is that you immediately convince all the Karens across the land :slightly_smiling_face: (like my mum).

I doubt you’ll be using AC in the bathroom, could be a fad in the future? Keeping the water in the room followed swiftly venting it through the MHRV sounds like a great plan. In some climate zones they even have to back ventilate wall mounted mirrors! They must have a back ventilated tanking system otherwise? IDK.

Sure. With horizontal cladding, you have a set of vertical furring strips and epoxy coated steel screws holding it to the wall. It acts as a very decent positive attachment for the insulation system, and acts as a nailing board for the cladding. The cladding provides the horizontal shear strength. However, I think you sometimes require extra screws for the vertical shear depending on the weight of the cladding and depth and load bearing capacity of the rigid insulation etc.

Cladding options vary, obviously. Heartwood out is standard because the growth ring like to straighten out as they age, bowing the boards slightly. It also has nicer growth ring patterns. The lapped variety, with exposed stainless steel nails would be my choice.

You can use dark open joints as a feature, then a UV and bug resistant weather/wind washing membrane is added. Vertical cladding needs a second set of horizontal furring strips to fix to.

Do not underestimate bug birds and rodents getting into a nice cosy insulation system. Detailing by the ground is super important. At least we don’t have termites.

You don’t necessarily need to replace them. But spend money upgrading them. Cassette thermal blinds are better than glazing anyway. Better still, also add an internal/integral automated Venetian blind - which is sealed from the dustier inside to eliminate cleaning.

I ended up permanently sealing most of our crap windows once our ventilation was installed. I never needed to open them anyway, and they’re the fold up variety, so they can’t even be cleaned from the inside!

Curiously, I didn’t insulate the disused vents, because one side was on the garden where we liked keeping an ear on the kids.

Eventually I think I’ll go for the fitted Venetian blinds system because they provide a much nicer light by reflecting the direct sunlight onto the ceiling.

Furthermore, I definitely want the automated ones, because you then get perfect active solar gain control. At the moment the controllers are not great. I find the Loxone home automation systems ridiculous. Soon they’ll be more stand-alone units that have the features I want without being connected to skynet.

Right, it’s way too late. gn.

1 Like

Ok yeh that makes sense, and looks like a cool product. I’ve seen paint-on air tightness gunks for detailing but that’s the first time I’ve seen a whole wall painted with something.

It would be harder to seal any new/modified penetrating fixtures like pipes or wires, as it wouldn’t be feasible to take the insulation off to re-seal at the exterior barrier line. But also, eh, probably still a damn sight better to seal internally and have a tiny risk of air seeping through brickwork into the exit hole… And anyway, decent pipework shouldn’t need replacing in the lifetime of the cladding, and I plan to use consuit and sealant for the external wiring for expandability so the external barrier might never be disturbed anyway.

Wow that’s a huge amount to think about, thanks I’ll digest it tomorrow!

Consider flooding risk.

One perk of everything being on hills in Sheffield is most are safe from flooding :+1:

(other than a bit of runoff pooling in torrential rains and the unlucky houses in the valleys who suffered in 2007)

Some materials tolerate water better than others. You also need to consider that if they absorb water and then dry, anything that was in the water will remain, both contaminating the material and blocking pores otherwise filled with air.

I had considered phenolic board a few years ago but the back of my house gets a few inches of flooding in heavy rain, so in case of future extra severe weather, perhaps combined with blocked drains, I decided that the lowest metre above DPC should be EPS. In the end I was refused permission to insulate with EWI due to my proximity to a conservation area.

Had a think about this point this morning. Vapour control is notoriously difficult and highly circumstantial, with respect to climate and assemblies, when you go to very light levels of insulation, and I should have been more cautious here. The nice thing with less thermal resistance is that it provides you with more resiliency due to more energy being available for drying. So this may not be the most sound advice after all. It all depends on how valuable the energy is in the future, I suspect in urban environments it won’t be quite so expensive due to heat networks and the potential of TES would make it very cheap. High performance envelopes will likely be useful for the ease of mechanical systems to quietly and calmly provide comfort.

Generally speaking, vapour open flow through assemblies, which can dry in both directions, are the best. If you have to control for vapour pressure, a vapour throttle/retarder/smart membrane is better than a barrier. Lastly, if we have constant high RH internally, then the cumulative diffusion outwards could warrant a barrier. Again, far too complex to cover in a post like this.

Ideally, in new builds, in our climate, the proportion of external/internal resistance around the interstitial air control/water control layer should be about 50/50. In retrofits, you may not be able to get this ratio, so you may get some very minor condensation due to vapour in the warmer humid months, but it’ll just be drained off the water control/air barrier, or redistributed and stored in the insulation before drying back inside/outside when it’s a bit drier. Natural fibres are great at this - trees, after all, do need to transport water about the place.

Due to Covid, there’s been a lot of interest in higher relative humidities for air quality and viral transmission. If 50% RH all day long becomes standard, then an internal barrier/semi-impermeable/smart vapour control layer might be the best choice after all.

Hospitals, galleries, OAP homes and swimming pools in our climate will have done very detailed simulation, so I’d look and see what they’re doing with similar masonry assemblies. Their liabilities a huge, so litigation will force good design eventually.

A service cavity can separate the internal walls, and be used to add thermal and sound insulation from an internal control layer.

I like service cavities in rooms likely to change function over time anyway. Especially for lighting; I like uplighting to avoid compromising the ceiling air control barrier, and because it generally creates a nicer light.

I suspect that the spotlight down lighters will eventually go out of fashion. Floor standing and table-top lamps, switched from a fixed switching panel by the door, are the way to go. But you need to be able to easily relocate the dedicated lighting plugs if you move the furniture and lamp. Modern LEDs have wireless switching, but you still need the sockets nearby as not to have long, unsightly, cables all over the place.

If I recall correctly, the Passivhaus standard requires 40-60% RH 80-90% of the time (depending on whether or not the house has active cooling), and consequently does need a vapour control layer (and mechanical ventilation).

50-60%RH all the time would be hard for new builds! For enerphit I think they’re more lenient than that. Mold starts at about 80%, so you do not have a large buffer before things go wrong. The thermal resistance a truss assembly can go up to, say 400mm of cellulose, would just compound your problems.

OSB is fairly impermeable and can be used as the air control layer and to throttle vapour drive outwards. In which case you’ll have to go for highly vapour open sheathing, like the paperless fibreglass gypsum board or the bitumen impregnated fibreboard. 60% RH all the time, it would not surprise me if a barrier is safer tbh. With those sorts of niche assemblies and conditions, you’d have to be mad not to do a full hydrothermal analysis.

I’ve seen the Americans use a ducted heating system with a whole house dehumidifier that can be scheduled to help avoid problematic months. I used to hate the idea of using air as the working fluid in heating, but I’m warming to the idea with low temperature heat pumps because of the other benefits of homogenizing the air.

For what it’s worth, I spotted that Steico actually now propose a truss solution like I was imagining on their website:

“Renovation type: Multi layer external insulation”

And similarly for internal-walls they propose using their own propritary laminate-verneer-lumber and metal angle brackets for studding:

“Renovation type: Multi layer internal insulation”

So I’m not as crazy as I thought at least :sweat_smile:

1 Like

There is that thin line between genius & …

Been there and I have the T-shirt

These animations are great.