Thoughts on insulating a balloon-frame brick house
Hey guys I’m new here and have been reading just about every article there is but I’ve run into some gray area, where it seems it’s too open ended to actually have a straight forward answer.
So im planning on insulating a balloon frame brick house built in 68. Im in region 5a, pennsylvania. Heres what im working with: 3.5inch brick, .25in air gap, a couple mil fiberboard wrap(?), and then another layer of thin cardboard looking stuff, I think the name ends with a sote. Homasote? Then balloon framing, 2x4studs every 16inches on center. As is, there is no noticeable drafting in house. Winter heating is significantly cheaper than summer cooling in terms of energy consumption (3x energy difference comparing apples to apples). No known water infiltration.
my plan is to use a rigid foam in the wall and finish the fill with a slightly compressed fiberglass for additional r value. I was planning on using xps, or possibly eps, at no more than 1inch, but was thinking of going 3/4 if internal thickness of rigid doesn’t matter. Internally, would the thickness of the rigid mean I wouldn’t need kraft paper or a vapor retarder? Should I even bother with the rigid, and just go with batts or blow in fill, and caulk seams?
The thickness of the rigid is what I’m stuck on. I’m unsure If the rules change for internal rigid/batt insulation. If I need to go thicker foam I’m probably going to end up doing 2inch foam with cellulose or fiberglass blow in. Thoughts? I’m planning to stay with the original 2×4 cavity, I’m not planning on worrying about the thermal bridging issues from studs, but if someone’s willing to enlighten me I’ll bite.
Also if anyone has preference over another foam, iso, eps or xps I’d appreciate your thoughts for internal insulation and air stopping. (I am aware od the blowing agents and green values) For my wall space and climate I was thinking iso would be too thick to be the main insulator.
So any thoughts or modifications is greatly appreciated. If I forgot to mention something or neglected a forum rule let Me know I’ll try to oblige.
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Replies
Joseph,
You are planning to use the "cut-and-cobble" approach to wall insulation. When using this approach for a 2x4 wall in Climate Zone 5, the rigid foam needs to have a minimum R-value equal to 27% of the total R-value of the wall.
You could use either 3/4 inch of XPS (27% of the R-value of the whole wall) or 1 inch of EPS (30% of the total R-value of the wall) and you would be safe.
If you insulate using the cut-and-cobble approach, there is no need for an interior vapor retarder or vapor barrier.
For more information on this issue, see these three articles:
Cut-and-Cobble Insulation
Combining Exterior Rigid Foam With Fluffy Insulation
Calculating the Minimum Thickness of Rigid Foam Sheathing
For more information to help you choose which type of rigid foam to use, read this article: Choosing Rigid Foam.
The type of wall system you are describing is at high risk for inward solar vapor drive. It's essential for you to include a layer of rigid foam or spray foam on the exterior side of your stud cavity (as you are planning to do), or you could end up destroying your walls (especially if you made the mistake of installing interior polyethylene). For more on this topic, see When Sunshine Drives Moisture Into Walls.
1" EPS and compressed R11-R13 would be fine, and would deliver ~R13-R14 of center-cavity performance, which is fine.
But installing rock wool or HD fiberglass R15s and MemBrain on the interior under the gypsum might be easier & cheaper.
With vapor permeable materials on the exterior such as the thin asphalted felt & cardboard described there isn't much of a wintertime condensation issue, but the MemBrain would keep frost from filling up the 0.25" gap from vapor diffusion. If it hasn't been frost-damaged already, even that might not be necessary, but it doesn't hurt.
Summertime moisture drive from the brick is generally manageable in zone 5A unless you air condition to some ridiculously low temperature (68F, anyone?). Outdoor dew points in most of zone 5A rarely break 75F for long enough to matter, and with latex paint (with or without MemBrain) the cavities can still dry toward the interior. The further south you go the riskier that is (so with global warming, when you eventually become zone 3A maybe it'll be different).
Or, you can stick with plan-A. An inch of EPS is sufficient for wintertime dew point control, and somewhat better than 3/4" XPS- the foam would be 28.5% of the total R-value instead of 27.5%, and stable over time, whereas the foam-fraction would shrink with time with XPS. For lower vapor permeance you could use foil-faced or plastic faced Type-I EPS, which is usually pretty cheap. With compressed R13s the slightly lower R Type-I EPS would leave you at ~27.8% foam-R, which is still pretty good, and still higher than the ratio you get with XPS. (And comparable to R5 sheathing over an R13 insulated studwall.)
If the 0.25 cavity doesn't have weep holes every couple of feet near the bottom course of brick and similar vents near the top, add them, no matter what approach you take on the insulation. That allows convection to purge moisture from behind the brick, keeping the exterior stud edges drier.
I'd consider how you will do air sealing and some other insulation options. For example, putting the rigid foam on the interior side of the studs and then taping it.
Thanks for confirming my thoughts, and for the thoughtful input. I'm still tossing ideas around. So let's say I plan to do a dense pack cellulose. Would it be ill advised to blow into cavity without rigid, or other barrier ((what would be a safe and effective"other" barrier?) (Just for a worst possible scenario in hard to reach places). If so, say I used a smart barrier like membrain on interior, would that negate the fact? I feel either way a 1 inch rfoam layer is my best bet (and im starting to warm up to dense cellulose).
One more thing. I have a wall that may be hard to access for now, and am planning on doing a temporary bandaid by getting into it from above. So say I shove my rigid down the stud cavity from above, and get a pretty tight fit, how would you seal, if you would at all, the bottom seam? Maybe there's an expansion tape that could be recommended? The only thing I can think of is a sloppy amount of caulk. (This is where id be wondering if just dense cellulose would be okay)
I appreciate the input and knowledge.
Joseph,
I don't think that you'll find it easy to slide 8 feet of rigid foam board down each stud bay from above, and position the rigid foam board properly near the exterior side of the stud bay -- and your idea of air-sealing the perimeter of each piece of foam board is even more of a fantasy. You need to remove the interior plaster or drywall to do a decent job.
As long as your exterior bricks are somewhat protected by roof overhangs, and not regularly exposed to wind-blown rain, I think that you can insulate these stud bays with dense-packed cellulose.
Yeah. That's what I was afraid of. So say I was able to slide the foam in and seat it properly but not seal. That would not offer any benefit at all, even for some protection for the cellulose?
And one more thing, (I think I'm running out of questions), now I know products aren't do all be all and this may be out of your spectrum or just nonsense in general. But I'm considering covering my brick with a decent (if I find one) waterproofer. Supposedly (I haven't looked into it yet, and plan to test on samples) they'll let water out, but not in, within reason. If such a product existed would that be enough peace of mind would you think?
I do have tree coverage in every direction, so I don't do too bad with weather.
And in the end I think I have enough same level access to properly secure the rigid. I was just hoping to wait until I had full access.
I really appreciate your answers.
Joseph,
Q. "Say I was able to slide the foam in and seat it properly but not seal. That would not offer any benefit at all, even for some protection for the cellulose?"
A. If you could do that, it would offer some benefit.
Q. "I'm considering covering my brick with a decent (if I find one) waterproofer. Supposedly (I haven't looked into it yet, and plan to test on samples) they'll let water out, but not in, within reason. If such a product existed would that be enough peace of mind would you think?"
A. Most brick experts advise against the use of exterior sealers or waterproofing agents, because they can do more harm than good.