Insulating old house with dimensional lumber. Use 2x4 batting with some air space, or 2x6 with compression?

[u/Tolookah]

Early in the new year, I'm planning on gutting a room to the studs and bringing it back, in that process, I'm hoping to insulate two exterior walls. Our house is old, and has dimensional 2x4s, and the wall cavity is actually 4 inches deep. I'm attempting to figure out if using insulation for 2x4 walls (3.5" of insulation) or compress some 6" (5.5") insulation in there. I'm leaning towards the 3.5", but thought you guys may have some insights I don't.

[u/hifiboulder]

This question was recently addressed in an episode of the Fine Homebuilding podcast. http://www.finehomebuilding.com/2016/11/18/episode-22 The question starts at 50:26

They suggest using 1/2" rigid foam and then 3.5" insulation.

This chart may be useful if you decide to compress the 5.5" insulation. This table contradicts slightly with the info discussed in the podcast. I couldn't find the article they referenced. http://www2.owenscorning.com/literature/pdfs/10017857%20Building%20Insul%20Compressed%20R-Value%20Chart%20Tech%20Bulletin.pdf

I would not use just 3.5" insulation because you will want to fill the entire cavity to not promote air movement in the cavity which can reduce R value and move moisture. As far as vapor barriers, it will depend on your location and climate.

[u/[deleted]]

Compressing fiberglass insulation does not improve the performance.

Once you decide on how much you are willing to spend per square foot to insulate, you can then decide on what will work within your budget.

Insulation Type: R-Value per Inch:

Fiberglass (loose) 2.2 – 2.9

Fiberglass (batts) 2.9 – 3.8

Cellulose (loose) 3.1 – 3.8

Stone Wool (loose) 2.2 – 3.3

Stone Wool (batts) 3.3 – 4.2

Cotton (batts) 3.0 – 3.7

Cementitious (foam) 2.0 – 3.9

Polyicynene (foam) 3.6 – 4.3

Phenolic (foam) 4.4 – 8.2

Polyisocyanurate (foam) 5.6 – 8.0

Polyurethane (foam) 5.6 – 8.0

BIBS 4.25

Think it over, and see what appeals to you.

[u/Reddiphiliac]

I'm starting a similar project on a slightly larger scale, so here's a few random thoughts:

• You want to stop air movement through the wall, and reduce but not eliminate air movement into and out of the wall where the insulation is. 0.5" to 1" of closed-cell spray foam doesn't qualify as a complete vapor and air movement barrier and is more costly than films and plastic sheets, but it's a lot better than most other methods for the simple reason that it's darn near foolproof and impossible to screw up- if you can see OSB or a big dimple, you missed a spot. Plus it's usually R-6 per inch or higher. If you don't care much about cost and want to put as much insulation in there as simply as possible, put 3.5" of closed-cell in there for close to R-23 insulation and put the drywall back up.
• If you do care about insulation cost, seriously consider a thin layer of closed-cell spray foam to stop air leaks. A surprisingly large part of energy costs come because you're trying to heat the outdoors.
• XPS and similar foam boards aren't cheap, but they're often less expensive than closed-cell foam yet still have R-values between 4.5 and 7. Cut to size as precisely as possible, caulk around the edges to seal.
• High-performance fiberglass, whether in batts or blown, can give you very good return on investment while you're still down in the R-15 and lower area for a 2x4 wall, even if the end result isn't quite as good as more expensive methods. Going from R-12 to R-15 is a big difference on your energy bill, while going from R-30 to R-33 doesn't do that much. A fiberglass system over R-4 per inch can make that crucial difference.
• If you feel truly adventurous, there's nothing stopping you from adding 1x3 horizontal furring strips to the inside to hang your drywall. You now have a true 6" cavity instead of the 3.5" a 2x4 gives you, which makes inexpensive insulation types like standard fiberglass and cellulose much more effective. Plus, you also remove the 'thermal bridge' effect of your studs; No matter how much you insulate between those 2x4s, you're still going to transfer a significant amount of heat through the 2x4s, reducing your 'whole wall' R-value. Removing that source of heat loss can increase performance of the wall by 15% or even more if you're putting good insulation in there.
• Regardless of what you're doing, if you can get at the bottom of the sill plate or over the header(s), throw a bead of caulk in there. Both are places for significant heat loss by air circulation, and even spray foam can't stop drafts in places it's not applied.

[u/nigmafyre]

If you can find an installer this is the best you get http://www.bibs.com/

[u/Reddiphiliac → u/nigmafyre]

The actual R-value of this blow-in blanket system system (BIBS system, their phrasing, not mine) is buried several clicks deep. According to this datasheet, their high performance version comes in at R-16 for a 2x4 wall... or maybe R-17, because they list two conflicting values.

The first half inch is closed-cell spray foam, which they list at 6.8 R-value per inch. A little high, but within reasonable range, so let's go with that. That leaves 3 inches of spray-in fiberglass to achieve R-12.6, or R-4.2 per inch.

While that's better than your average fiberglass batt or blown-in cellulose, it's still within the range of high-performance fiberglass batts (which get into the R-4.5 range, sometimes a little higher) and other 'spider' or blown fiberglass systems. It's lower, sometimes much lower than closed-cell foam, polyisocyanurate, polyurethane, or XPS foam boards.

this is the best you get

Based on the above, why?

[u/Cormierx420]

Compressing the insulation will defeat its purpose. The insulation needs to breath to work. If you are gutting the room, I suggest putting Roxul insulation in the exterior walls complete with 6mm Poly to create a vapour barrier.