Insulation and wall construction improvements for a groovy 70s partial-brick ranch
![](https://secure.gravatar.com/avatar/7a2d62c5229a5c10db03c9b3edc446f8?s=52&d=https%3A%2F%2Fimages.greenbuildingadvisor.com%2Fapp%2Favatars%2F200x200%2Fs.png&r=g)
Hello,
We live in Boulder, CO (climate zone 5). Our house is a 1970 ranch (with basement) that is “dipped in brick” as someone put it – the lower half of the walls have brick siding, and plywood board and batten siding from there on up. We’re looking for suggestions on how to improve the insulation, and ultimately how to flash it.
Wall construction details: 2×4 studs on 16″ centers, with R-7.5 faced (to the inside) fiberglass batts cavity insulation. Out from there is 1/2″ Celotex Fiberboard (R-1.2?), although I expect to find 1/2″ plywood in the corners of the house. From there, there is a (surprisingly large) 1-1/4″ air gap between the Celotex and the brick in the lower half, but the upper half has the sheet siding directly against the Celotex. Painted drywall interior.
We are looking to (finally) augment the insulation in the walls, as well as replace windows during a siding replacement project.
Insulation considerations:
It would seem the best bang for our buck (money and time) would be to add rigid foam board insulation (over the existing Celotex). I have read here (https://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-minimum-thickness-rigid-foam-sheathing) that in Zone 5, the minimum foam board thickness is R-5 for a 2×4 wall. I believe I understand the article correctly in that the minimum foam board is a function of the assumed insulation in the walls and you could go thicker than the minimum. Furthermore, as long as you hit the minimum, vapor permeability becomes less of a concern since the wall assembly interior of the foam board will dry to the inside. That said, we have R-7.5 (I was surprised they did not install R-11) and the Celotex for a combined R value of our wall assembly somewhere around 8.7. According to the IECC:
https://energycode.pnl.gov/EnergyCodeReqs/?state=Colorado
the requirement for a wood frame wall is 20 or 13 + 5. A 2″ polyiso layer would add R-13 (label) or R-10 (derated for cold temperature) getting us close to the R-20 requirement.
Because of the brick on the lower part of the wall, we could add about 4.5″ of stuff (insulation, additional nailing substrate and/or furring strips) before the upper part of the wall would become proud of the bricks on the lower part of the wall – and consequently look weird. Another question to ask though would be whether maxing that out would really be worth it since the lower half of the wall isn’t going to get any additional insulation, only the top half.
Siding attachment concerns:
Not wanting to stray too far from the look of the neighborhood, we feel the brick should stay and we will add insulation to the top half of the wall when we replace the wood siding and windows. The board and batten look can be achieved with Hardipanel sheets and should go up fairly quickly:
1) Initially I thought a Zip-R wall from Huber would be a good idea (insulation, nailing substrate, and WRB in one), then fasten the siding to the Zip board. However, unlike Hardiplank lap siding, using the Hardipanel sheets appear to require furring strips to space the Hardipanel off of the Zip:
http://www.huberwood.com/assets/user/library/L-6-ZS-Lap_Siding_with_Hardie_Panel.pdf
(which seems to defeat the nailing substrate benefit somewhat). Seemingly counter to this is Hardie’s drawing showing the Hardipanel directly against the WRB:
http://www.jameshardie.com/d2w/installation/hardiepanel-single-family-hz5-us-en.pdf
2) If we have to go through the labor of installing furring strips, why not just save the money we would have spent on Zip-R and just install polyiso board with furring strips screwed to the framing through the polyiso, then nail the Hardipanel to the polyiso? Furthermore the foil-face on the polyiso can suffice as a WRB if the seams are taped.
See existing wall construction drawing attached below.
Once I get going in the right direction on the siding/insulation, I can probably figure out the flashing, although that may generate more questions.
Thanks in advance for any help!
Steve
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
![](https://www.greenbuildingadvisor.com/app/themes/greenbuildingadvisor/dist/img/modules/detail-library/thumb1.png)
![](https://www.greenbuildingadvisor.com/app/themes/greenbuildingadvisor/dist/img/modules/detail-library/thumb2.png)
Replies
Steve,
I think you forgot to ask a question.
Your plan to install exterior polyiso and furring strips sounds fine to me.
The R20 requirement is only if the insulation is thermally bridged by the framing, R13 + R5 c.i. is only R18, but slightly outperforms a 16" o.c. 2x6/R20 wall.
On a U-factor basis the U-factor only needs to be under U0.060 , which is R16.7 "whole-wall", that factors in the thermal bridging of the framing, and the R-values of the wallboard, sheathing, siding, interior & exterior air films, etc.
https://up.codes/viewer/utah/irc-2015/chapter/11/re-energy-efficiency#N1102.1.4
For the brick clad section, blowing low density cellulose or low density fiberglass into the remaining 1.5" of cavity depth left by the econo-batts is do-able drilling from the inside. You won't be able to get it fully up to code-min there, but the extra air films in the masory cavity and the continuous R1.5-R2 of the Celotex gets you pretty close.
Adding a continuous R10-R13 or even a skinny inch of foil faced polyiso over the pre-existing Celotex and under he new siding would get you there with significant margin, assuming you filled the wall cavities. A continuous R13 over the Celotex would get you under U0.60 with just the econobatts, but it's still worth filling the stud bays completely.
Don't try to dense-pack behind a single layer of Celotex fiberboard, or there's a risk if blowing it out, and it will belly-out significantly. The easiest might be 1lb density fiberglass (Spider, Optima, or L77, etc), which flows into narrow cavity spaces more readily than cellulose.
Check with Hardie, but I believe it can be mounted on 1x4 furring through-screwed to the studs 16" o.c. with pancake head masonry screws.
Martin: Thank you! Yes, it was late at night when I finally posted (slap forehead). My questions really were:
1) Is my understanding of the minimum foam board thickness correct given the econobatts in the stud cavities, and therefore 2" is a reasonable target for the polyiso thickness?
2) Is there any reason to press forward with Zip system or other additional sheathing despite the points I brought up?
3) Is it foolish to go beyond any given thickness given the fact that we're only adding insulation to the top half of the wall?
4) Are there any moisture-migration issues (or any other unexpected detriment) that would result from only adding insulation to the top part of the wall?
It sounds from your answer that #1 is a yes, and #s 2 through 4 are all nos.
Dana: Thank you! I understand the clarification of R20 vs the R13 + R5 now, thank you. I need to understand your further suggestion of blowing in insulation to the lower part of the wall. Is the suggestion to drill through the drywall on the inside, blow some insulation, so that it settles on the inside of the batt facing?
I guess some further info from me would be helpful. The small amount of exterior wall insulation that I've uncovered (see attached photo) seems to have been installed with the lower batt lapped by the upper batt with an overlap of about 3" in the middle of the wall. The facer surface is set about 1-1.5" into the cavity as measured from the inside. All that said, drilling into each wall cavity, blowing insulation, and patching drywall sounds like a large, labor-intensive project and I'd like to be able to quantify the benefit.
Again, thank you, gentlemen.
Steve
Even with the lapped batts you can still blow fiberglass or cellulose from the interior. Even if the batts weren't lapped with cellulose it would take two 1.25-1.5" in diameter holes (drilled with a hole saw for easy repair) per stud bay. The typical technique for low-density cellulose is a hole 1-1.5' or so from the bottom of the stud bay and another a foot or so below the top. The nozzle gets inserted in the bottom hole and the material is directed downward, then as the blower begins to stall it's re-directed upward. When copious amounts of material starts coming out the top hole they move the nozzle to the top hole, then blow downward, then upward until the blower begins to stall. Depending on the blower and the blower settings this results in 2-2.5 lbs per cubic foot average density in the wall. In your case it will compress the batts a bit, but won't pop the fasteners on the Celotex or cause excessive bowing. With the overlap at mid-level the blower may begin to stall before much cellulose is coming out the top hole, but that's OK- they'll just move to the top hole when that happens- it'll still be a full cavity fill.
With 1lb density fiberglass the process is similar, but they can usually do it with one hole somewhere in the middle. If you point out the batt- lapping problem they can do it with two holes just to be sure.
Talk to some contractors- they'll tell you what the problems & solutions are, some may even try to talk you into dense packing it, but unless you're replacing the Celotex with OSB or plywood dense packing would be a mistake, since it risks compromising the Celotex, which is probably your structural sheathing. Even low density blown fiber tightens up the air leakage of the assembly quite a bit (more so with cellulose than fiberglass.)
It doesn't take 2" of exterior foam for dew point control on R7.5 batts, but to hit code performance without filling the stud bays it does. With the bays filled and keeping the Celotex you can get there with half-inch polyiso, but that would lead to high moisture content in the Celotex. The Celotex can take it, but it's not really great. With the bays filled with fluff and half-inch celotex you can hit code-min with 3/4" polyiso, but more is clearly better.
It's not at all foolish to go with more insulation than the minimum just because the bottom half won't have quite as high an R-value. The labor cost of installing 2" isn't significantly more than installing 3/4". If using 2" reclaimed roofing polyiso the material cost is about the same as virgin stock 3/4" foil faced goods too. With 2" of roofing polyiso over the sheathing the performance will be about twice that of a 2x4/R13 wall, and more than 1.25x higher than a R13 + R5 c.i. wall.
Steve,
Dana is giving you good advice. Only you can decide whether the improved performance associated with the cellulose approach is worth the trouble of drywall patching. It's your house, so you get to decide.
Q. "Is my understanding of the minimum foam board thickness correct given the econobatts in the stud cavities, and therefore 2 inches is a reasonable target for the polyiso thickness?"
A. As Dana explained, 2 inches is reasonable because it gets you closer to minimum code requirements than thinner foam -- but the full 2 inches isn't needed for condensation control.
Q. "Is there any reason to press forward with Zip system or other additional sheathing despite the points I brought up?"
A. Either Zip-R sheathing or rigid foam plus furring strips will work. Consider the costs and make your decision.
Q. "Is it foolish to go beyond any given thickness given the fact that we're only adding insulation to the top half of the wall?"
A. As Dana said, adding more R-value to one section of your walls isn't foolish, even if you can't do the whole wall.
Q. "Are there any moisture-migration issues (or any other unexpected detriment) that would result from only adding insulation to the top part of the wall?"
A. No.
Dana, Martin,
Thank you very much for your responses!
Steve
Hello again,
I am looking to install a patio door in conjunction with the re-siding. I have encountered many flashing details with rigid foam installation and many describe what to do with window head/sill areas, but I have yet to find one that is specific to a patio door - so thus far, I've assumed you can approach it the same way: innie or outie installation. Am I correct in thinking this? I would tend to think an outie will simplify my drainage plane. It seems I should be able to create a buck out of 3/4" plywood in order to set the patio door far enough out to allow room for the 2" polyiso (still screwing the door frame into the house framing) then tape the door to the polyiso and use the window head/sill details. Am I nuts for trying to do an outie patio door and this should only be done for windows? It is possible that I could cut back the subfloor and install a new piece the width of the rough opening that would extend into the house further to provide a stronger cantilever.
Another thing I am trying to find is a flexible flashing product for the sill corners that will stick to polyiso. The polyiso seam sealing tapes I've come across do not appear to be (very) flexible (they are for sealing seams), and the flexible flashing products (zip stretch tape, dupont flexwrap) appear to be great for sticking to housewrap or zip, but nowhere do they mention they will stick to polyiso. What flexible sill flashing product is recommended for sticking to foil-faced polyiso?
Thanks in advance,
Steve
Update: rather than finding a tape I like and trying to find in the tape's literature that it sticks to polyiso (which has been frustrating), I have tried looking at the polyiso manufacturer's literature and seeing what they recommend.
Rmax thermasheath is what is carried at the local HD, but as for their R-SEAL recommended flashing/construction tapes:
https://www.rmax.com/accessories-products/
I am not seeing these for sale online like I am their polyiso board. I'll see what I can find out by calling Rmax.
Johns Manville's polyiso board installation guide:
https://www.jm.com/content/dam/jm/global/en/building-insulation/Files/BI%20Toolbox/BID-0150-Residential-AP-Foil-Above-Grade-Exterior-Walls-9.10.13.pdf
says to use Flashing tape such as 3M 8067, Grace Vycor Pro
So between the grace vycor pro and the Rmax R-SEAL 6000, we have a couple butyl flashing tapes recommended by the polyiso manufacturers, but do they stretch into the corners?
The search continues. As always, thank you for any comments.
Steve