Possibly stupid insulation question

[Jason Morgan]

This may be a stupid question, but am I better off with R-15 insulation in my 2x4 shop walls or compressed R-19? I realize that loft is everything, but I was wonders if there was any benefit to the R-19 in a 2x4 wall?

thanks

[Brandon Shew]

R-15 in the 2x4 walls. Insulation is all about trapping air pockets. Squeezing R19 in there (if it will even fit) will push out most of the air and replace it w/ fiberglass.

[Paul Girouard]

Use the R-15 or call some one who does BIB (Blown In Batts) or the spray foam

http://www.polarproducts.org/images/main-right.jpg

Both provide better / higher R value , at a higher price.

But compressing the R -19 or R -21 the now more "normal 2x6 wall fiberglass insulation would result in LESS R value as it forces the insulation to be to tighly packed.

[Chris Padilla]

Just to reiterate, insulation should not be packed into a space. It should remain "light 'n fluffy" when installed.

[David Cramer]

To re-reiterate, exactly what Chris said. Make sure it fills the cavity and is touching both sides of the wall so the air is actually trapped and not getting by.

From my school physic courses, I also believe when you compress the batts too much, you are simply making a solid versus trapping the air. The air will transfer much more readily through a solid (compressed fibers) versus loosely installed fibers which will again, trap the air. This is my thinking anyways and I've been :eek: wrong** before.

Dave

**see wife for proof/verification

[Russ Filtz]

David, I think you mixed a couple of heat transfer scenarios there! With the cracks or openings you are simply letting the drafts through, for a more direct heat transfer. With insulation, the intent is not "trapping" the air from moving (although that is one intention of creating a "tight" house), you are creating heat "non-transfer" zones to create the insulation effect. Heat pretty much only moves via conduction, convection, and radiation.

Conduction and convection are typically more efficient than radiation. With insulation (outside of a pure vacuum, which is the best insulator) you are trying to maximize the path of conduction (hence the typical nonwoven fibers) and/or trap the air so it has to resort to radiation, or micro convection, to move the heat through. (yes I just said trap the air, but this is within the insulation itself, not preventing gaps with the studs! Yes gaps affect the overall system, but we're just talking about R-value of the insulation itself)

Of course material choice is also relevant and each material will have its own coefficients of heat conduction, absorptivity, and emissivity. My thermo and heat transfer classes start getting fuzzy after those thoughts!

[David Cramer]

Russ, I defer to you and stand corrected. I told you my wife knows I'm not always right, or rarely at best:) . Sorry for the misinformation and thanks for clarifying.

Honestly, I thought it trapped the air and slowed down the loss of heat. By compressing, it would be a solid and transfer more easily. Even though I got an A in physics, I am obviously not up to par.

Again, thanks for the clarification and detailed response. I put it in my favorites so that I'll always be able to find it.

Dave

[Russ Filtz]

Well I think we're BOTH right! It does trap the air, and preventing drafts is a HUGE plus in preventing heat loss. However when it comes to heat moving through the actual insulation, it's better to compare it to heat moving though a solid like the 2x4 next to the bat insulation.

As stated, the heat can conduct through a solid easier than by radiation or convection across a space (hence your analogy of compressing the deeper bat, is "kind-of" correct!). By compressing the bat you are doing two thing, one is reducing the actual depth which minimizes the distance the heat has to travel through the wall and (importantly) reduces the size of the air spaces between strands of the fiberglass (small convection currents would be more efficient in the trapped air spaces).

Radiation in this case may be a wash since it really is not affected by distance, and the total number of air spaces is the same, just smaller (unless the bat is crushed so much that it actually does become more of a "solid" by eliminating some of the air spaces). If it is crushed and totally closing some air spaces, then conduction would trump radiation/convection, proving your point!

[Jim Becker]

R-15, not compressed R-19. If you want to add a little more R value, put 1/2" high density foam board sheathing on the inside before you drywall. (assuming you don't have access to the outside of the walls) If you add the foam board, be sure to account for the extra depth when you place your electrical boxes so you don't get forced to use extensions.

[David G Baker]

R-15, not compressed R-19. If you want to add a little more R value, put 1/2" high density foam board sheathing on the inside before you drywall. (assuming you don't have access to the outside of the walls) If you add the foam board, be sure to account for the extra depth when you place your electrical boxes so you don't get forced to use extensions.
Jim,
Do you mean adding the foam board as well as the fiberglass?

[Eric Gustafson]

I told you my wife knows I'm not always right, or rarely at best:) .

In my house I have learned a responce for those rare occasions that I am wrong. It never fails to calm down SWMBO:

"YOU are right, I am wrong, I am sorry, I will never let it happen again as long as I live!" :D

[Jim Becker]

Jim,
Do you mean adding the foam board as well as the fiberglass?

Yes, I'm sorry if I was not clear on that. We had to put the R15 behind a pre-formed shower in the new guest bath since it will be impossible to shoot spray foam in that area later. For most of that area, we'll add a layer of foam sheathing on the outside to bring the area back to R-19. The R-15 is designed to fit in a 2x4 wall and then you use the sheathing to bring it up to code which in many areas is now R19 for walls. Obviously, not as critical for a stand-alone shop, but something to consider anyway.