Adding Exterior Insulation over Stucco
CorporateCowboy
| Posted in Energy Efficiency and Durability on
I have a client who is looking at remodel and wants to improve the insulation and sealing. This is a rental property and wishes to minimize cost. The exterior is stucco (45 Years old) and I want to know if there is any way to insulate on top of the stucco and then provide a new exterior. Will be taking the interior down to studs.
Location is Western Canada, Souther Alberta.
Thanks for the thoughts
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Replies
Is there a vented air gap between the stucco & sheathing?
Good question! If the stucco were to be sealed somehow to create a rain shield, then an air vent could be created. But how do you seal the stucco?
Christopher,
Dana Dorsett's question is a good one. But assuming there is no ventilation gap between the stucco and the sheathing, it would certainly be possible to install a layer of rigid foam on the exterior side of the stucco.
Of course, it would be necessary to follow the rules explained in this article: Calculating the Minimum Thickness of Rigid Foam Sheathing.
It would also be essential to come up with a plan to flash the windows. Ideally, the existing windows would be removed so that the rough openings could be properly flashed.
-- Martin Holladay
I did this job with my New Mexico house two years ago. The crew fastened the foam right through the existing stucco and into the studs beneath. I re-did the windows at the same time, but if you don't, you can simply bevel or inset the new cladding at the penetrations to create pseudo-innie windows and doors.
Christopher,
Nate's advice -- "you can simply bevel or inset the new cladding at the penetrations to create pseudo-innie windows and doors" -- leaves out a few steps. You need to detail the window openings carefully with flashing so that water can't gain entry into your wall assembly. The problem with leaving the existing windows in place is that it is tricky to insert flashing under the windows (as is usually done during new construction).
Making these flashing decisions takes judgment. Different details will be necessary depending on your climate, your roof overhangs, and your exposure to wind.
-- Martin Holladay
If there's a skinny air gap you may need to fill it with non-expanding injection foam (yuck!). If there's a 1" or deeper gap a slow-rise half-pound pour of polyurethane may be in order. But any significant air gap would be a thermal bypass for any exterior insulation, so it has to be made as air tight as reasonably possible.
But if there's none you can go ahead and put rigid foam or Larson Trusses on the exterior of the stucco and hit just about any arbitrary performance point, with a wood or fiberboard exterior sheathing and a proper rainscreen assembly siding approach on the exterior.
Thanks for the all of the contributions. There will be new windows installed and therefore proper flashing will be installed. There may be some deterioration of the substructure behind the stucco, so this might prohibit this application.
Definitely if you're replacing the windows, then bring them to the flame of the new WRB/wall, or else add a deep sloped sill (at least). Martin is absolutely right that the flashing is critical.
Dana, in your response you mentioned the Larson Truss system. After review of the system, which originated from Mr. Larson just three hours from me, I wonder how the system works regarding gaps. In the article from Green Building Advisor (https://www.greenbuildingadvisor.com/blogs/dept/musings/all-about-larsen-trusses) it describes installing right over the siding. What about the gap in this case? There must be a gap associated with the uneven siding?
Christopher,
1. Not all siding is bumpy. Some siding types (T-111 siding, vertical shiplap board siding, Hardie panel siding) are smooth and co-planar.
2. Larsen trusses are usually filled with blown-in cellulose or blown-in fiberglass. These insulation materials fill all spaces, even when the siding is bumpy.
If someone wanted to install fiberglass batts between the Larsen trusses, the batts can also conform to the uneven shape of the siding.
-- Martin Holladay
Excellent, I like this solution for this and maybe another project. Thanks very much for your comments and those of the others. This forum and the magazine are great learning and mind provoking sources.
Hi Chris,
I’m interested in your results. Did you end up completing the project?
Thanks,
Paul
I have a similar question for a retrofit stucco house with fiberboard sheathing. Rather than demo the stucco and fiberboard and installing Zip panels to the studs, could you leave the stucco and fasten the Zip through it? Stucco is somewhat air and vapor impermeable to begin with. Would there be any concern with the minor airgap between the Zip and the uneven texture of the stucco?