Insulating exterior walls – best practices vs ROI vs budget
I feel that I am fairly well versed and have read articles here and know what is an ideal situation would be but I want to ask about my specific requirements.
I have a nearly 100 year old home which I will be renovating extensively soon in Saskatchewan 7A climate. The exterior walls are not insulated but the typical lath and plaster over wood 1×8 (or similar). The exterior is fully stucco and will not be removed. No matter the benefits of exterior insulation it is simply is not in the budget for a decade+ or ever.
We have four basic choices:
1. Do no insulating in the exterior walls (Free)
2. Blow in cellulose + patch holes ($5,000-$10,000)
3. Demo walls and install rockwool + drywall ($25,000-$35,000)
4. Demo walls and install spray foam +drywall ($40,000+)
I understand most of the pros and cons, but I’d be interested in hearing your recommendations. Option 1 seems ridiculous given the other upgrades. Option 2 appeals to the wallet, but has insulation draw backs. Options 3 & 4 clearly have best insulation potential but are a big budget items that combined with the remainder of the renovations make it more challenging.
When comparing cost and efficiency how should I weigh this?
For reference the cathedral roof (third story/attic) will almost certainly get 4″ of closed cell spray foam regardless of the choices above.
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Replies
Option 2 seems the obvious choice. You can get better results by coupling it with blower directed air sealing, where you run a fan and then use a smoke stick to find air leaks and seal them. A house of that vintage was probably built quite leaky.
How thick are the walls? The only reason I would consider demoing the walls would be to make the walls thicker. Then you can really pile on the insulation.
The studs are 2x4, double sided with 1x plank. (Assumed to be true prior to double full demolition.
At most we could add 1.5" on some walls to create a 2x6 cavity, but it would be inconsistent based on the layout, etc.
Unless you're demoing the walls for other reasons, I'd go 2.
1 or 2.
1 if your current heat load is small or medium and you have cheap fuel.
The current old boiler is north of $300 per month in natural gas costs as it heats the whole neighbourhood.
Gas rates today are as such ($CAD):
Delivery Charge: $0.1113 per m3
Commodity Rate: $0.1264 per m3 ($3.20/GJ)
How much gas per month? Not $, m3.
That's cheap gas! $6.73/MMBtu if I did the conversion right. It'll be difficult for insulation to touch that.
I would estimate that January, the highest month, would be about 700-900 M3 out of the boiler.
North of $300 quickly becomes $175 -$225 for the coldest month :). I think if financial savings are the goal, you might not find them here. Look for other value!
You have to be careful with old exterior stucco. Lot of time water control layers are missing and flashing is not done properly. This is not an issue with an uninsulated structure but can be a big problem with insulation. There has been a number of posts here about issues with insulated walls and badly detailed stucco.
If the details are sketchy, your best bet is to insulated while keeping an air gap behind the sheathing similar to buildings without sheathing:
https://www.greenbuildingadvisor.com/article/managing-water-and-insulating-walls-without-sheathing
This does mean demo the interior walls which feels like a big job but lot of times simpler especially if you have to do wiring upgrades as well.
This is my main concern. While we are doing an electrical upgrade, I can do that myself without demoing the walls. So the cost full demolition and drywall is budgeted as a 20% increase to the entire project which is an eyewatering amount. But intellectually I know this is "best".
Based on the link you provided that insulation type is quite involved and limits the insulation anyway. Breaking down the mineral wool to like 2" or whatever seems painful. Obviously I know water/moisture is an issue, but with new windows properly flashed and living in a semi-arid climate I wonder if the moisture issues can be reduced to make the dense-pack an acceptable compromise.
Couple of ways of looking at it.
You can dense pack the walls, if something goes wrong you would have to demo anyways so if it holds up, saves you demoing. I would not insulate under any windows for now just in case.
You can demo the walls, build the air gap and insulate. In zone 7 you are looking at about 1.5" of rigid. Assuming real 2x4 studs with 1/2" air gap, you need an 2x2 strapping over the studs to get you back to a 3.5" cavity so you can use regular batts. This would be a pretty significant energy efficiency upgrade. This is not as bad as it sounds, I've done it with older houses with brick veneer without a wrb and it doesn't take all that much. Might take you about a day or two to do 100' linear feet of wall (about one floor). One bonus is you can shim the 2x2 to get your old crooked walls perfectly plumb. This also gives you a chance to install blocking at each floor and at the end of the joists bays which makes a big difference in air leaks and improves fire safety.
The real problem is when you start out with a 100-year-old building there is almost never a ROI. It is a labor of love or a failure to plan.
“We have four basic choices:
1. Do no insulating in the exterior walls (Free)
2. Blow in cellulose + patch holes ($5,000-$10,000)
3. Demo walls and install rockwool + drywall ($25,000-$35,000)
4. Demo walls and install spray foam +drywall ($40,000+)”
Option #1 is not free you pay for this option every month in the form of higher heating bills.
Option #2 is probably not possible as it seems unlikely the walls will contain a viable water-resistant barrier and acquit flashing to keep the insulation dry. You also need to subtract the fuel costs that could be saved.
If anyone does a written budget for a deep energy retrofit the dream dies and the project is over.
Walta
I'm sure there's at least felt on the walls under the stucco so that's something. I know it's not a modern product. It's a very dry climate so I feel like the risk of moisture is lower than other places. Is it that risky that I need to increase the entire renovation budget by 20%?
rsporter,
I empathize: 143-year-old house, climate zone 6, with very few meaningful upgrades done over the years, but there is inconsistent cellulose in some walls / stud cavities. My dreams of a full gut-remodel are likely to remain dreams unless I pull that lucky Powerball ticket. Where I have landed is this:
- When I get a little extra money, I try to knock out one meaningful infrastructure improvement. A few years ago I moved our power from a masthead to underground and upgraded the service. This year I installed a new electrical panel, and relocated the existing wiring to it.
- If a room is "good enough for now," I'll do some painting and air-sealing. In my case: living room has a little insulation, but needs new windows, refinished floor, updated electrical - I caulked and painted it.
- If I open a wall for minor improvements, I'll insulate and air seal as I can. I know this is piecemeal, but it's not nothing.
- If a room needs major improvements and the budget is okay, I will gut the room, insulate (I've been cut-and-cobbling), rewire to the new panel, replace windows. I haven't yet had the budget to replace all windows in a wall, which means I haven't resided or added exterior insulation, so I pull the old window, install new one, patch in siding for now.
- If I ever get to the point where a whole wall has new windows, I'll reside (exterior insulation + rain screen) and just re-trim the new windows.
Not sure any of that's helpful - I'm doing the work myself, which is as slow as you'd imagine.
If it were me, I'd go for Option 2 to save money - it will probably increase comfort, and while $5k-$10k isn't nothing, if you're able to make more meaningful upgrades down the line, it won't be the end of the world to re-do some of that work.
If I had the money for Option 4, I'd want to be doing a full gut-remodel to improve everything at once, which also means a lot more money than just spray foam + new interior walls (but see above: despite my best efforts, haven't found that winning ticket yet).
Paul,
Sounds like an eminently sensible approach.
I'm just a little unclear on "renovating extensively" but you will be doing foam on the attic ceiling.
You have some knowledge that the spray foam cost is actually worth it in your project?
The large company that did my spray foam cathedral and wall batts I asked several times that I would be interested in the easier to install and thinner R21 in my 2x6 walls vs the R19 they were planning. I said I would take whatever material cost they wanted to charge. The bidder guy finally called me back and said it has nothing to do with not being able to get it or install or anything like that, they just didn't think it was worth the cost difference to me. ROI for them at a R3 hit for a hundred or two wasn't reasonable to them. Who knows.
The cathedral ceiling third floorspace is 2x4 construction. It seem to me that putting in only R-12 batt insulation is foolhardy at best.
In a 3.5" cavity we can get R21. With furring out the walls 1 inch, we can get R28. In climate zone 7 that seems significant to me.