Rigid foam on inside of stud cavity?
Looking to maximum the insulation levels as high as I can and not use spray foam due to price.
Looking to see if anyone has installed rigid foam on the inside of their stud cavity with the combination of fiberglass. 2 inch rigid foam and then R-13 fiberglass insulation to fill the entire stud cavity. Would seal the joints between studs and foam with a caulking. Would give me a r value of r-23. walls are 2×6. can’t do outside installation. I haven’t found a whole lot on this topic but what little i did find was people talking about possibly moisture but didn’t hear of any actually mold issues over time. Will be taking place in Iowa.
any feedback appriceated.
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Replies
Kurtis,
Yes, you can install interior rigid foam.
The main disadvantage of this approach (compared to exterior rigid foam) is that it is much trickier to provide adequate insulation at partition intersections and rim joists. But as long as you are willing to open up your walls and ceilings, these areas can be addressed from the interior.
It's a new construction. Had planned on spraying it
Till I started running numbers and added other things to the construction. Would I need to put a barrier up or would the closed cell rigid foam act as the barrier?
Kurtis,
I'm not sure what you mean by a "barrier." There are many kinds of barriers.
Every wall needs a water-resistive barrier (WRB). For more information on this topic, see All About Water-Resistive Barriers.
There are no code requirements for interior vapor barriers, although interior vapor retarders are required in some climates. For more information on this topic, see Vapor Retarders and Vapor Barriers and Do I Need a Vapor Retarder?
If you install interior rigid foam, you don't need an additional vapor retarder in any climate. All varieties of rigid foam provide enough vapor resistance to keep your walls safe.
In new construction the cut'n'cobble + fiberglass approach will net you a whole-wall R (after accounting for the thermal bridging of the framing) of about R14-R15, not even R1 better than R23 rock wool would deliver.
At the same wall thickness you could build a 2x4 / R15 wall with 2" of exterior EPS, which comes in at about R18-R19 whole-wall. This would take at most ~30% more foam board (or maybe 0% more, since there would be a lower scrap rate on the foam), but would be dramatically less work, and would be a ~30% improvement in total performance. It would also be lower risk/higher resilience, since all of the structural wood would now be partially inside the thermal envelope, running much warmer (=drier) on average.
If you want slightly better performance than that, using 1" polyiso against the structural sheathing, with 1" of EPS on the exterior of the polyiso, you'd be at R20-21-ish performance. (The EPS is necessary to keep the polyiso warm enough to not fall off it's cold-temp derating cliff. EPS performance climbs at the cold temperature extremes, whereas polyiso's sweet spot is north of 40F average temp through the foam, and performs miserably when its under 30F through the foam.)
If I could put it on the outside I would do that but we are past that point with new siding already on
With the siding already on the thermal performance is pretty much baked in the cake, unless you can accommodate interior-side rigid foam, which has a lot more detailing issues But you can add foam strips on the stud edges for a thermal break and deeper fiber layer, then use smart interior-side vapor retarders and still do pretty well.
Half of Iowa is in climate zone 6, the other half is zone 5. In the zone 5 half you need only 25% of the total center-cavity R to be air-impermeable foam to not need interior side vapor retarders. In zone 6 it needs to be at least 35%. A 2x6 wall in zone 5 with 1" of ccSPF(R6) has 4.5" and a compressed R10-R23 batt or blown cellulose wold not need an interior side vapor retarder. In zone 6 it would need an interior side vapor retarder.
From a bang/buck point of view, given where you are in the process, the best bet is to caulk the framing to the sheathing with acoustic sealant caulk, use housewrap tape reinforced by painting over with duct mastic to seal any lateral seams in the sheathing, then blow cellulose (damp sprayed or dense packed) in the cavities, or new-school fiberglass at 1.8lbs minimum density, and use a smart vapor retarder such as Certainteed MemBrain or Intello Plus on the interior side, and spend the foam budget on performance upgrades elsewhere. If you want to add an inch of polyiso in strips on the interior side of the stud edges and fill the now 6.5" cavities with cellulose or fiberglass it would boost the whole-wall performance from ~R14-ish to over R20 with a minimum amount of foam, but that may complicate the window & door detailing.
In zone 5 if it's vinyl siding, or if there is a vented rainscreen gap between the siding and sheathing you can skip the interior side vapor retarder, but not zone 6.
With or without vented siding, the air-tightness on the interior side is more important than vapor retardency. Without vented siding keeping the interior side at least somewhat vapor open can be critical to making it moisture-resilient, which is why variable permeance smart vapor retarders are preferable to poly sheeting or foil-faced foam.
It would be in south central Iowa. It has lp smart siding on the outside. I was just trying to get the highest r value possible in the wall with this hybrid way of using foam Board to make the stud cavity air tight as possible. People I have talked to have no knowledge of anyone trying this in our area and worry that there could be condensation issues where the Fiberglass meets up with the foam board. The house will have geo thermal but I worry the wind in the winter will make my heating bills high if I don't address this insulation with it being on a hill
If you're south of Des Moines, you're in zone 5. That means that an inch of closed cell foam would be sufficient for dew point control at the foam/fiber boundary without interior vapor retarders. But price/performance wise it isn't superior to using a smart vapor retarder and all high density fiber for cavity fill.
You don't need spray foam or foam board to make the assembly air tight, in fact using foam board between the studs is less reliable than other air sealing methods. Making the SHEATHING the primary air barrier is usually the cheapest and most robust way to go. Caulk between the top plates of the studs, caulk every stud & plate to the sheathing, and caulk the bottom plate to the subfloor, etc. With a powered caulk gun and a case of acoustic sealant caulk it goes a lot quicker than you might think.
With 16" o.c. framing the framing fraction is about 25% of the wall area, after adding up all of the top/bottom plates headers, jack studs etc. Nothing that you put in the cavity between the studs can overcome that thermal bridging, which is why it's a poor place to invest the high R/inch foam budget. For 2x6 framing you can hit R23-R24 center-cavity using high density rock wool or high density fiberglass, and it's a lot cheaper than cutting up a bunch of foam board to no good end. But the end result is still less than R15 (on average) after thermal bridging, in fact it may not even beat R14. If using 24" o.c. single top-plate and advanced framing corners you can bring that up to R15, but even with some amazing R-infinity/inch cavity fill, you won't hit R20 whole-wall performance, since that R1.2/inch thermal bridge of framing timber is still conducting the same amount of heat. Foam only "pays" when it's is thermally breaking the framing, and delivering it's full R-value to the whole-wall performance. When it's between the framing, it's performance is being robbed by the thermal bridging, which is why using the cheap stuff for cavity fill makes more sense.
With an all-fiber cavity fill you'll still want to use a smart vapor retarder in zone 5. MemBrain is really CHEAP compared to closed cell spray foam.