Fiberglass insulation
jtune
| Posted in Energy Efficiency and Durability on
I read your article on insulation an older home that does not have sheathing behind the ceder lap siding. The house is in southern Missouri so I was planning on 0.75 inch foiled rigid foam board (foil facing the exterior siding with a 0.75 inch air space. My question is should the fiberglass batting have kraft paper or just the fiberglass batting. I was planning on 0.5 inch sheet rock for the interior walls.
Thank you for your time.
Jim
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Replies
I'm sure Martin will comment, but I read the article differently. It indicates you can use builder's felt or house wrap in combination with spray foam. Or you can install air-sealed rigid foam in combination with fiberglass, cellulose, etc. In other words, you can't use a air permeable backing material in combination with an air permeable insulation.
Jim,
As Steve pointed out, my article (Insulating Walls in an Old House With No Sheathing) suggested that it's OK to use air-sealed rigid foam in a case like yours. The 3/4" foam is a little thin, though, so I suggest that you re-read the section of the article under the heading, "Does foam thickness matter?"
If you thicken up your rigid foam to meet the guidelines in my article titled Calculating the Minimum Thickness of Rigid Foam Sheathing, then it's best to use fiberglass batts that don't have a kraft facing.
If you go with thin rigid foam, you should heed the advice of the article and choose EPS, not the foil-faced product you are considering, so that the moisture can dry outward. In that case, kraft-faced batts make sense.
In no case do I advise the use of foil-faced rigid foam that is only 3/4" thick, however.
Kraft facers are fine, no matter what you have on the exterior, though the facers can sometimes make it difficult to fully inspect the quality of the batt job.
Most of Missouri is in US climate zone 4A, and southern MO is the warm edge of 4A. In US climate zone 4A it would be fine to use foil faced foam board on a 2x4 wall as long as it's R-value was at least R2.5. At 3/4" foil faced polyiso would be nearly twice that at about R4.5, whereas 3/4" Type-II EPS would come in at under R3.2, and Type-I EPS under R3, which isn't a lot of margin. (EPS with foil on one side only is usually Type-I). While the wintertime performance of EPS would be somewhat higher, Type-II would still average less than R3.5 in your location/climate, and Type-I would be averaging maybe just a hair above or below R3, again, not a lot of margin.
Polyisocyanurate would underperform it's labeled value when it's cold out, but it would still average around R4 @ 3/4" during the colder weeks. So, if you're stuck at 3/4" for fit reasons go with polyiso, and don't sweat the low vapor permeance of the foil facer- it has plenty of dew-point margin for R13-R15 batts, and the foil facer would limit the risk of moisture accumulation in an air conditioned house during the sticky heights of the cooling season.
If you can go an inch or more, Type-II EPS (faced or unfaced) would be fine or Type-I, if it has a facer to control summertime moisture drives. At 1" Type-II EPS has about the same vapor permeance as standard interior latex paint, but at 3/4" it does not. So at 1" the air conditioning would roughly keep up, drying the cavities via vapor diffusion. Type-I EPS is still too vapor permeable for summertime moisture drives at 1", but would average ~R4 during the winter season at that thickness, giving it adequate dew point margin.
Dana,
Good point about the climate zone -- in zone 4, just 3/4" of polyiso works fine. My mistake.
Everyone thank you for your advice and direction.
JIm
Thank all of you for your input. I am following Steve and Martin's direction and will use 1 inch EPS with R13 unfaced batting which will put my total R value at 17. Adding 1/4 inch more to the wall depth is not an issue.
Jim
Sounds like a good plan! With the foil facer facing a 3/4" gap it adds about R0.5- R1 to the performance.
If it's easier to install or easier to find with kraft facers, kraft faced R13s are fine- it won't/can't create a moisture trap.
With the higher R-value of the cedar compared to plywood or OSB sheathing, 1" foam, and the foil facer w/ 0.75"air gap, you'll be pretty much at current IRC 2015 code minimum performance with that stack up (R13 + R5 continuous insulation is IRC code minimum for climate zone 3 through climate zone 5.)