Adding WRB to Stucco-Finish Assembly
This is my first post – I’ve learned so much from this group. I’m a civil engineer by trade in Victoria BC (zone 4 marine) who knows just enough building science to be dangerous.
I’m in the process of slowly renovating my 1941 house with a focus on increased building performance. The current wall assembly is
– 80 years of paint
– 1 inch of lathe and plaster
– 2×4 (actual) with no insulation
– 1 inch board sheathing
– glass bottle stucco in medium good condition
-single paned windows
I believe this works out to R5 or so
My current plan is:
– replace all windows with double paned casements and picture windows
– blow havelock sheeps wool into the walls
– 2” of rockwool insulation over the existing stucco
– install a wrb (likely siga majvest)
-3/4 inch strapping for rainscreen
– fiber cement siding
I’d like to keep the stucco on if possible – it’s in decent shape and hard to remove, and I don’t want to landfill it. I’m aiming for overall sustainability and low carbon intensity.
My current plan is to place the rockwool directly over the stucco, build window bucks, and then put the wrb over the rockwool. This aligns the new drainage plane to the outside where I want the outie windows. Are there issues folks see with this approach? My worry with putting the wrb against the stucco is both the hard rock and glass chunks could damage it, and it’ll be a pain to flash the windows well.
Thanks in advance, and I look forward to collaborating and sharing some of my knowledge with folks on here. My day to day is more on large civil projects so if you’re building a big development or have site infrastructure issues thats what I’m here for.
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Just a few thoughts on your plan as I see them.
"I believe this works out to R5 or so " - I doubt it really gets close to R5, stucco has a high thermal conductivity, as does plaster, and the board sheathing has so many gaps that air / energy can pass directly through that I'd consider it to be R0 for planning purposes. Any insulation that it does provide is just 'nice to have'. While wood does have R value of ~ 1.4/inch, I don't think the wall is robust enough in its current rendition to designate it any specific number.
The Havelok will cost you 10X over comparable insulation for the same R value, and it is virtually untested in its long-term performance. It's up to you if that's worth it or not, but I'd go with a safer option for the money. There's nothing wrong with cellulose, fiberglass, mineral wool, or SPF if done correctly.
Below are my preferences only -
I'd ditch the stucco. It's had a good run, but if your goal is a high performance house, and if you're replacing windows anyways, it's time to let it go. It's not going to be seen ever again, and It'll make so many more of the details easier to implement and get right. The WRB can be installed smooth and air tight against the board sheathing, which is probably in fine shape. You'll be able to get the rockwool attached, and the furring strips, without predrilling every hole through the stucco, and you'll be able to properly flash without the 3D matrix of granules at every location.
Thanks for your insights. You're right on the R-value - I'm viewing it as uninsulated.
I decided on the havelock for a couple of reasons. I've seen some cellulose wall failures here on retrofits and it concerned me enough. The classic energy retrofit here is to drill a bunch of holes on the outside, fill the walls with dense pack cellulose, and then paint the stucco. It seems to work fine for some houses, and cause substantial damage in others. My understanding of this is the years of paint on the lathe and plaster prevents inward drying, and then you have cold sheathing on the outside edge due to the insulation, and the stucco paint prevents drying to the outside. Please correct me if I'm wrong here.
If I was completely opening up the wall I'd be going with mineral wool, but I'd prefer something I can blow in. There's always an emotional aspect to building, and I'd be lying if it wasn't my partner's love of knitting and and comfort with wool, as well as good deal from the local distributor that tipped me that way.
One project that has the WRB over the exterior mineral wool is documented in https://www.jlconline.com/projects/energy-efficient/high-performance-insulated-wall-retrofits_o, I'm following some of the details I found there.
The stucco on/off is the bigger challenge and thank you again for your thoughts there. Your approach similar to my initial approach, and there are benefits for sure and I might ultimately do that. What I'm balancing is the level of effort and waste as a tradeoff to keeping the stucco on and working around it, or removing it and landfilling/recycling it. Either way I'll update as I move on.
" My understanding of this is the years of paint on the lathe and plaster prevents inward drying, and then you have cold sheathing on the outside edge due to the insulation, and the stucco paint prevents drying to the outside. Please correct me if I'm wrong here"
I think you have the right understanding. I was thinking (to myself) that if you were taking off the stucco, and then applying a WRB / rockwool / furring strips / fiber cement, that it would be vapor open to the outside, and the inside wouldn't have to be touched. I don't think I vocalized that well.
I have seen the WRB over the mineral wool, but I think that would apply better if the air barrier already existed, such as taped plywood / osb seams. Again here it assumes the removal of the stucco down to the bare 1" board sheathing, where the WRB was detailed as an air barrier there as well.
"The stucco on/off is the bigger challenge and thank you again for your thoughts there. "
I have no doubts it a tremendous amount of work! It's just that from this side of the keyboard I can make suggestions without having skin in the game, but focusing on outcomes :). Keep us updated either way, it sounds like a fun project!
So far it’s been quite fun. The property market is just insane here - our assessed value has gone up 40% since we bought in 2019 and what it means is that our only option is to renovate or leave the region. And I love building so that’s not a burden.
The supply chain issues have been more of a problem. I’m slowly stockpiling until I have everything I need. Fortunately there’s enough local or semi local manufacturers I’ve been able to get most of my materials lined up.
I’m just finialzing the drawings and then I’ll share some sections for comments.
I'm just up the road from you near French Beach park.
Leaving the stucco makes sense to me for the reasons you gave. The challenges are that it will be harder to locate the studs to attach the rain-screen battens, and because it won't be entirely straight, harder to seal the resulting cavity, and get the battens co-planar. Against that is the stucco provides a great barrier to stop pests.
Sleggs metal shop will make up perforated metal flashing in any custom profile. That's what I'd use at the bottom to cover both the rock-wool and battens.
I'd put the WRB outside the rock-wool too. I'm not sure there is any appreciable advantage to using the Majvest here. I'd probably use Tyvek Commercial instead.
Malcolm, do you have any tips, insight or links to projects or details where the WRB is outboard of rockwool? I'm having difficulty finding anything.
I suspect the more areas you block out the better. If you are adding 2" bucks to the windows, I would consider running a 2" block along the base of the walls, perhaps at the top too, and deeper mounting blocks for any penetrations like electrical boxes or duct terminations. You may even want to add vertical blocking at the corners.
Once that's done, infill with the rockwool and you will have a new flat plane. Install the new windows, and add head-flashing with end dams. To put up the WRB I would staple it heavily at the blocking, and with as few cap screws as possible just to keep it in place until you add the rain-screen furring. Add flashing tape to penetrations and openings and you are ready for the cladding.
Thanks for the response. My parents are building a new place in Parksville, I’m up in your area pretty often. I really like the mid island and wouldn’t be surprised if I end up there eventually.
Good tip on Slegg - I got most of my materials at convoy supply because I was having issues getting a slegg cash account but I’ll check in with them again. Also good input on the Tyvek. I was leaning towards the majvest due to some reviews and availability but will take a look at tyvek commercial.
Yeah, during the booms Sleggs becomes very difficult to deal with. They are pumping truckloads of materials into the bottomless pit known as Langford, and have lost interest in small accounts.
Thinking about it again, as the WRB is outside the rock-wool, you could set the insulation on a solid 2" block and just use the stock 3/4" perforated J trim under the furring that they always have in their warehouse.
That’s it 100%. I was impressed with how well convoy treated me as a small cash account. And that’s a good point on the perf trim. I was planning on using coravent but that would work well.