R13 insulation in 2x6 walls

[Zogman]

Time to insulate my new garage construction. It's 2x6 construction. I had planned on using R13 insulation but I notice R13 is only 3 inches thick. Is it OK to have a 2" air gap in the walls and if yes, should the gap be against the wall board or the outside wall.

I have access to foil faced fiberglass from a friend who has a bunch of overstock. What's the deal with foil faced as opposed to kraft faced? Any benefits or drawbacks?

Thanks

Z

 

[matt_i]

Just a reality check here...you spent all that money in framing lumber to get 2x6 and are going to leave it half-full ??

 

[bloomingtonmike]

Sounds like room for two layers to me.

 

[bczygan]

The proper, and only way to insulate those walls, is with 6" batt R=19.

 

[dfiler2]

The main reason people build using 2 x 6 walls is for the extra insulation, otherwise a 2x4 has plenty of strength. R-19 is what most use, however I have seen some 5.5 inch insulation claiming R-21, might be worth checking into.

 

[bczygan]

The main reason people build using 2 x 6 walls is for the extra insulation, otherwise a 2x4 has plenty of strength. R-19 is what most use, however I have seen some 5.5 inch insulation claiming R-21, might be worth checking into.

Sorry,..........Do THIS instead!

 

[Steevo]

You can actually get R21 in those walls using unfaced batts. (This is what I used in mine).
But if you want to get the "now" savings of cheap R13 vs the "forever" discount of higher R-value, by all means get the cheap R13 stuff.

 

[Zogman]

Let me put it another way gents. Will R13 be rendered useless with 2" air gap? I'm in Southern California. Not in Minnesota or Michigan and this is just a stand alone ga rage. Originally I wanted insulation for sound only, not heat or cold retention. 6" walls were used because of the size of the open area of the garage without posts and for seismic reasons. Y'all have snow drifts, we have earthquakes. And thanks for restating my question Matt.

 

[dfiler2]

It won't be useless you will still have near R-13, the air gap will allow some air movement within the wall cavity which will lower the R value some, although probably not a lot.

 

[Aerogt01]

No, it will not be rendered ineffective. You will have R13 walls. If it's free and you have enough I would do 2 layers.

Kraft facing on batts is flammable. Foil is not. The facing acts as a moisture and vapor barrier which is why I cringe when I see unstapled faced batts when they could have been.

Edit: If the inside of the walls is open, staple the facing to the stud faces.

 

[DC73]

Let me put it another way gents. Will R13 be rendered useless with 2" air gap? I'm in Southern California. Not in Minnesota or Michigan and this is just a stand alone garage. Originally I wanted insulation for sound only, not heat or cold retention. 6" walls were used because of the size of the open area of the garage without posts and for seismic reasons. Y'all have snow drifts, we have earthquakes. And thanks for restating my question Matt.

No, the air gap will not hurt the R-value. The air gap actually has a bit of R-value itself especially if the wall assembly is air tight. Insulation is most effective when air cannot move through it.

Fiberglass insulation isn't all that great for sound proofing. You should look into Roxul mineral wool insulation. Excellent fire resistance and great for sound proofing. Around here, Lowe's carries the R15 Roxul mineral wool batts (3.5") and can order the R-23 5.5" batts. Home Depot can also order it. I am using the R15 mineral wool (installation underway) in my workshop and I actually found it cheaper at a lumber yard that caters to builders.

Good luck.

DC

 

[cat06]

if your getting a deal on the r13, install it in two layers. on the first take off the foil facing and install it as unfaced insulation with the foil faced on top. it will be crushed alittle and the r value will be a little less than 19 because its packed but its better than r13.

you also could try peeling it in two equal thicknesses and getting double the coverage.

this only would apply if your getting it so cheap, its cheaper than buying r19

 

[MScott]

Let me put it another way gents. Will R13 be rendered useless with 2" air gap? I'm in Southern California. Not in Minnesota or Michigan and this is just a stand alone ga rage. Originally I wanted insulation for sound only, not heat or cold retention. 6" walls were used because of the size of the open area of the garage without posts and for seismic reasons. Y'all have snow drifts, we have earthquakes. And thanks for restating my question Matt.

Don't forget that insulation will also keep heat OUT of your building so it will be cooler and more comfortable during your "warmer" seasons. It will probably do little to protect you from earthquakes.

 

[boobag]

insulation doesnt really cost much. i'd just buy the r19. it will fill better and not sag or collapse due to no air gap.

 

[Zogman]

Thanks for the info guys. That Roxul stuff looks great DC73. Fire proof makes it that much more attractive.

 

[PT Doc]

What is the price difference for the r13 vs r21 for the whole garage.

 

[jabin]

I would really recommend looking into the Roxul mineral wool. I'm going to use it in my bed room walls since it did amazing in the TV room.

 

[NUTTSGT]

Let me put it another way gents. Will R13 be rendered useless with 2" air gap? I'm in Southern California. Not in Minnesota or Michigan and this is just a stand alone ga rage. Originally I wanted insulation for sound only, not heat or cold retention. 6" walls were used because of the size of the open area of the garage without posts and for seismic reasons. Y'all have snow drifts, we have earthquakes. And thanks for restating my question Matt.

Insulation is just like a thermos, keeps the heat in/ cold out in the Winter months and keeps the cold in/ heat out in the Summer months. It'll also prevent temperature swings that result in stuff rusting in your garage.

If you're getting a deal, do the double layer of R13 after peeling the facing. If you're not getting the deal from a friend, spend the money and buy the R19/R21. Insulationis an investment, it'll continually pay for itself during your lifetime.

 

[larry4406]

In the houses my company builds, we fill the cavity 100% to try and preclude convective currents, regardless the requirement. So we would fill the 2x6 to R21 even if your region allowed R13.

 

[pablo94sc]

R13 is about 92% reduction in heat transfer. R19 is about 2-3% better than R13. It's an exponential decay in heat transfer, so you'll be fine with it for a garage. Use the thicker stuff for the attic to reduce transfer from the attic to the garage.

 

[Zogman]

Thanks Pablo. That's some great info!!!!

 

[theoldwizard1]

"you pay for insulation once. You pay for heating and/or cooling every time the thermostat call for it !"

 

[Zogman]

That would be true if I had a heater or an AC unit in my garage.......

The gist is, I can insulate the **** out of this thing but with (4) 10' x 10' garage doors, I think I will lose or gain a bunch more heat out of the garage doors even if they are insulated.

I thought this was Garage Journal..... Not Home Journal!

 

[DC73]

R13 is about 92% reduction in heat transfer. R19 is about 2-3% better than R13. It's an exponential decay in heat transfer, so you'll be fine with it for a garage..

I'm very skeptical of this. I'm not sure exactly what this chart represents. If R19 insulation is really only 2-3% better than R13, there would be little need for the Department of Energy to recommend R25 in walls and R60 in ceilings for cold climates and yet they do.

There is a diminishing return on insulation where at some point it is no longer an effective payback to add additional insulation. The DOE numbers take that into consideration.

Perhaps the graph is referring to convective heat transfer (ie air movement) instead of conductive heat transfer which is what most insulation is designed to circumvent?

I can tell you with certainty in my hot climate that doubling attic insulation from R19 to R38 will make a huge difference in air conditioning consumption. A difference that doesn't make sense based on this chart.

I do agree that for the OP's purpose, R13 in the walls is fine. Since he is dealing with 2x6 construction, if cost is not an issue, I'd fill up the cavity.

DC

 

[CNGsaves]

+1 on above comments to fill cavity with insulation as next owner of garage will surely WANT and EXPECT that you went "all the way" and fully insulated, if you insulated at all. You may later want to cool space in garage in Southern Cal as party area to hang out with friends, as your garage looks mighty nice.

Another +1 for the roxul, instead of fiberglass insulation which is cheap for a reason (it doesn't do much for insulation, and also crappy for fireproofing).

Thus, overall vote is fully insulate cavity with Roxul (added benefit of sound proofing that OP wants).

 

[bzinsky]

R13 is about 92% reduction in heat transfer. R19 is about 2-3% better than R13. It's an exponential decay in heat transfer, so you'll be fine with it for a garage. Use the thicker stuff for the attic to reduce transfer from the attic to the garage.

That is a very misleading post. It's like owning a gas station and gas X has a 5% profit margin and gas Y has an 8% profit margin and saying the difference is almost meaningless.

Or to make it even easier to understand. Lets say you're entire home has an r-value of pretty much 0. Your heating bill is $1000 a month. Then you perfectly cover your home with r13, and thats keeping in 92% of the heat, your heating bill would be $80/month. With r19, which is keeping in 95% of the heat, your bill would be $50/month.

There is a significant difference.

 

[matt_i]

I think the biggest thing saying the chart is a falsification is the fact that the X-axis can be read in either inches OR cm ...there's no way that can possibly be. How can 2.54 cm have the same Y-value as 1 inch...

The insulation can be thought of as a thermal resistor and you get another 1-3/4" of insulation with 2x6 framing over 2x4 framing. A 50% gain in insulation.

The resistor, just like an electrical resistor, works in relation to the "pressure" on it which equates to voltage or, in heat transfer, the temp difference between inside and out. As you'd expect, if inside and outside are equal temps, then you might as well not have insulation (or walls! for that matter). However, aren't there some times in dead of winter or summer where heat or AC would help? To me the value of insulation would mean you could probably heat the place literally with incandescent bulbs in the winter, and a really small window-type AC unit in the summer....depending on if you can keep the doors closed or not.

I asked the reality check not to be a smart a$$ but just to clarify, you could have probably framed with 2x4, saved money in the build and been very good for your climate.

However, at this stage, I would not want to leave a gap in a wall, to me its an invitation for some animal or insect to make an easy home, they might as well have to scratch themselves to death from that itchy pink stuff as a penalty for burrowing in

 

[30cal]

It's about air. If you have 2x4 walls, 13 gets the job done. 2x6 walls, 19 gets the job done. 2x4 studs with r19 stuffed in,compressing the air out, yields r-13. Of course, I'm talking fibre-glass bats..

 

[Angelfire]

No, the air gap will not hurt the R-value. The air gap actually has a bit of R-value itself especially if the wall assembly is air tight. Insulation is most effective when air cannot move through it.

Fiberglass insulation isn't all that great for sound proofing. You should look into Roxul mineral wool insulation. Excellent fire resistance and great for sound proofing. Around here, Lowe's carries the R15 Roxul mineral wool batts (3.5") and can order the R-23 5.5" batts. Home Depot can also order it. I am using the R15 mineral wool (installation underway) in my workshop and I actually found it cheaper at a lumber yard that caters to builders.

Good luck.

DC

I used Roxul (2x6, R23) on my addition to my home a few years ago. I'm a convert and will never go back to fiberglass. If I could have afforded it, I would have gone with the spray foam but the mineral wool seems to be a pretty good alternative. Love the sound attenuation it gives and was much nicer to install. That all being said, don't leave your cavities with air in them. It won't lower the R-Value but you can get more by increasing the amount of insulation.

 

[pablo94sc]

That is a very misleading post. It's like owning a gas station and gas X has a 5% profit margin and gas Y has an 8% profit margin and saying the difference is almost meaningless.

Or to make it even easier to understand. Lets say you're entire home has an r-value of pretty much 0. Your heating bill is $1000 a month. Then you perfectly cover your home with r13, and thats keeping in 92% of the heat, your heating bill would be $80/month. With r19, which is keeping in 95% of the heat, your bill would be $50/month.

There is a significant difference.

The graph is accurate enough to illustrate the point being made.

Second, your math is wrong. Insulation slows heat transfer, not eliminate it so a 95% reduction is heat transfer does not equal a 95% reduction in heating costs. It'd be more like $1000 with none, $285 w/ R13 in your attic, and $250 w/ R38 in attic, and $220 with R60.

I don't remember all the formula as I haven't studied thermodynamics since I took physics in HS, but more insulation means it will take longer for the heat to "escape", but there is a point of diminishing returns.

As for why code is for more in colder climates, that's because of the temperature differential. It's -40F and you want your house at 65F. That takes a LOT more energy to maintain than 35F to 65F, so the longer it takes for that heat to transfer out the better off you are and less fuel you use.

 

[PT Doc]

The insulation can be thought of as a thermal resistor and you get another 1-3/4" of insulation with 2x6 framing over 2x4 framing. A 50% gain in insulation.

2x6 framing has 2" more depth than a 2x4. That difference accounts for 57% increase in insulation if filled similarly.