Cut & cobble polyiso with fiberglass batts – good or bad idea?
I am a homeowner in the construction management industry and a plumber by trade.
I recently purchased a home in Worcester County Massachusetts (Climate Zone 5A). The “Original” Part of the home (which will be the focus for the purpose of this question) is a bungalow/cape style built in 1890 with a full unfinished basement and attic. I am currently re-locating the bathroom in the old part of the house by doing a complete gut of that area. Here is some info on the existing construction:
-2×4 exterior walls. Typical construction from outside to in (See attached photos for reference): Vinyl siding>1/2″ rigid insulation (seams do not appear sealed)>(2?) layers of cedar shingles>1x8x3 shiplap sheathing>2×4 stud wall with blown-in cellulose (I had to remove the cellulose within the extent of the bathroom because the lathe had to come off).
Final finish on the interior side of the wall will be 1/2″ durorock with tile to 42″ & 1/2″ drywall to the ceiling
The previous homeowner added an addition to the rear gable end of the home as seen in the attached exterior photo. There is a small portion of (old) exterior wall that creates a covered entryway/porch. The rest of the “Old” gable end of the house can be considered an interior wall with the exception of the attic potion (I’ll save that for another project/post…) The small section of the exterior gable wall was insulated with R-13 FFB by the previous owner. as seen in the attached photo.
There is various to-be-expected signs of moisture infiltration over 130 years, which appears to have been reasonably mitigated relatively recently, as you can see in the photos.
Our goal is to do a slow, phased gut remodel of the old portion of the home, including tightening the entire envelope. The previous owner was a roofing/siding contractor so I’m hoping that added layer of 1/2″ exterior insulation will help in my efforts.
As I mentioned, the first phase is re-locating the current “galley” bathroom. The new bathroom/laundry room will be 7’x11′ with 15 Linear feet (120SF) of exterior wall. I am replacing the existing window seen in the attached photo with a 48″ wide x 24″ tall vinyl awning window with u=.26.
The new tub will be located so the “long” wall will be the exterior wall, and the two “short” walls will be interior walls. This posed my original problem. The tub location is necessary for the layout, however we want to install a 20″x 12″ Hydroban Pre-formed Niche (By Laticrete) in the long exterior wall. The niche is a full 3-1/2″ deep so I did not want to lose the full R-rating at that small location.
My original solution was to pad the exterior wall only at the extent of the tub with 2×3’s, cut & cobble a piece of 2-1/2″ polyiso behind the niche, insulate the rest of the 2×4 wall with R-15 FB & 6-mil Poly, and call it a day.
However, after reading up on this website and realizing how many gaps there were in the shiplap/holes from the blown-in cellulose contractor, I think it would be wise for me to further tighten the envelope, beyond the 1/2″ rigid on the exterior, which probably isn’t even sealed (I will know more when we tear siding off to re-frame the window that arrives in 2 weeks.) I think I have devised a method that will effectively tighten the entire wall assembly, provide ~R-21.5 rating while only adding 1″ of wall construction, and can be installed incrementally from the interior as the phased renovation progresses over time. I would like to avoid disturbing as little as possible of the exterior portion of the envelope, because I fear I would be opening a can of worms.
I would achieve this by padding the entire wall by 1″. Preferably by nailing 2×4’s directly to the existing/new framing (for the niche,) like you would to plumb an existing interior wall or sister studs. This seems easier but will add a lot more lumber to the assembly i.e less insulation. Alternatively, I could rip 1″ off of 2×6’s and replace all the exterior studs with the new 2×5’s, then run 1″ polyiso over the T&B plates. to make it even tighter. After writing it out, the second idea seems much better, but also seems like more work for my cousin and I.
To achieve the new rating, I would cut & cobble 1″ Polyiso directly to the exterior sheathing in each stud bay with Door&window great stuff, then simply add R-15 FB in each bay. I would omit 6-mil Poly or any other type of vapor retarder/barrier on the interior to allow the wall to dry in.
I will also apply one of these methods to the entire “short” exterior wall, as I will be re-purposing a 3-1/2″ deep built-in ironing board that will be 1/2 interior, 1/2 exterior (not sure if you can see my mark in the attached photo), so I would like 1″ Polyiso behind that as well. But the entire wall needs to be padded so it is flat for tile.
I am fortunate enough to have other family members in the trades and therefore have been inundated with various new owens corning FB (a new 7-pack of 15″x93″ R-15 FFB, two lengths of 15″x93″ FB, and a new 11-pack of 15″x47″ R-13 FB) I will likely rip all the facing off the R-15 and use all that up, hopefully I don’t run short but worst case I can just use some of the R-13 for the remaining short wall. But I wanted to at least ask the question/get some feedback here before proceeding/buying rigid insulation and great stuff.
I apologize if this post is longer than most, but I wanted to give as much detail as possible as I realize how important that is on this site, especially for initial questions. Thank you in advance for any input! I look forward to hearing from the community! I have a couple more insulating projects coming up including insulating my entire rim joist, joist bays, and basement walls that I may end up posting about as well.
TL/DR; I have a pretty good knowledge of building envelopes, and a good idea for how to proceed given my situation, and based on info from GBA; but I would greatly appreciate any advice on how to proceed given my situation/constraints from this very helpful community! Thank you in advance!
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