Fresh insulation rant

[jmarkwolf]

I am nearly complete with my 28ft x 30ft two story garage.

While trying to find the proper insulation for the ceiling of the ground floor (formed by 2x8 attic trusses), which is the floor of the 2nd story. And assuming that rolled and batt insulation must fill the wall and rafter cavity "properly" to achieve the rated R value (no compression nor air gaps), I'm finding the following dichotomy:

5 1/2" thick R21 insulation is too thick for 2x4, but not thick enough for 2x6.

6 1/4" thick R19 insulation is too thick for 2x6, but not thick enough for 2x8.

6 1/2" thick R19 insulation is too thick for 2x6, but not thick enough for 2x8.

8" thick R25 insulation is too thick for 2x8, but not thick enough for 2x10.

All the while, code dictates the R value that must be complied with, with no practical/economical way to get there?

Rather than making insulation that fits the various dimensions properly, ie 2x4, 2x6, 2x8 and 2x10 cavities, they would rather make insulation that fits nothing properly?

WTF Over!

Any insight gratefully recieved.

 

[FarmerPete]

I believe the no air gaps refers to the sides of the insulation. Thus those sizes are perfect for the cavities you said they're not thick enough for.

 

[James E]

May be a stupid question on my part but what code requires you to insulate the floor between the first and second stories of your house?

 

[Scott H in Wheaton]

Since you are insulating a garage, not a house, and you want to put insulation between the two floors, does that mean you wish to heat and cool the first floor but not the second floor?

A slight air gap above your insulation is okay, plus you will be sealing it with some type of flooring which will give you an additional R factor.

 

[pattenp]

You're in left field with your understanding. R13 3.5" is for 2X4 walls, R21 5.5" works in 2X6 walls, any R19/R21 will work with 2X8's, and R25 8" or R30 9.5" works with 2X10's.

 

[FarmerPete]

After looking at your numbers again, your main mistake is that you assume a 2x4 is 2" x 4". It's actually 1.5x3.5. A 2x6 is approximately 1.5" x 5.5".

 

[PugetDude]

I'm certain the problem here is not with the manufacturer of the insulation or the mill that produced the framing lumber.

 

[rlitman]

You will get a higher R value with compression. This has been covered in other threads.

 

[scottydosnntkno]

You will get a higher R value with compression. This has been covered in other threads.

You'll get fine insulation when you understand nominal sizing

 

[k p]

May be a stupid question on my part but what code requires you to insulate the floor between the first and second stories of your house?

Since you are insulating a garage, not a house, and you want to put insulation between the two floors, does that mean you wish to heat and cool the first floor but not the second floor?

A slight air gap above your insulation is okay, plus you will be sealing it with some type of flooring which will give you an additional R factor.

Likely he wishes to insulate between to stop the migration of hot/cold into the second floor from the opening/closing of the garage doors. Eg. In winter the top floor can be heated and kept warm even while having the garage doors open on the floor below.

 

[jmarkwolf]

May be a stupid question on my part but what code requires you to insulate the floor between the first and second stories of your house?

No code to my knowledge.

But I'm heating the downstairs and leaving the upstairs unheated, hence the insulation betwixt the two.

 

[jmarkwolf]

After looking at your numbers again, your main mistake is that you assume a 2x4 is 2" x 4". It's actually 1.5x3.5. A 2x6 is approximately 1.5" x 5.5".

And a 2x8 is actually 1.5" x 7.25". And as I said, 6.25" thick insulation is not thick enough to fill the cavity without 1" air gap, and 8" thick insulation will need to be compressed to fill the cavity, reducing R factor.

I followed the insulation threads on this, and other forums, but still come away less than confident with the best approach. Don't worry I'll muddle through it.

I think for this particular task, I'll use 2 layers of 3.5" x 23", removing the kraft facing from one layer. The resulting 7" will fill the cavity nicely. It just seems silly to have to go to the trouble and expense.

This is definately my "go to" forum when I need to navigate "uncharted" territory!

 

[ozyborn]

Fine, then spray foam the entire cavity.Problem solved

 

[jmarkwolf]

Fine, then spray foam the entire cavity.Problem solved

Got it quoted yesterday. $1200 for spray-in cellulose, for just the 1st floor ceiling. And the drywall has to be in place before he can do it. Cha ching.

...or $275 for 1 layer of 6-1/2" fiberglass rolls of R19. Can do the drywall later.

...or $385 for two layers of 3-1/2" fiberglass rolls totalling R26. Nearly 40% improvement over R19. Can do the drywall later.

Answering my own question. Thanks for being the sounding board.

 

[ddawg16]

I had the same issue with the ceiling joists in the upstairs of the 2 story addition of my house. 2x8 just like you....but a vaulted ceiling. We used 2 layers of R19 with the second layer not having the craft facing.

 

[signcrafter]

Got it quoted yesterday. $1200 for spray-in cellulose, for just the 1st floor ceiling. And the drywall has to be in place before he can do it. Cha ching.

...or $275 for 1 layer of 6-1/2" fiberglass rolls of R19. Can do the drywall later.

...or $385 for two layers of 3-1/2" fiberglass rolls totalling R26. Nearly 40% improvement over R19. Can do the drywall later.

Answering my own question. Thanks for being the sounding board.

1200 for spray in cellulose? I didn't read how much you're going to need but based on your price for batt insulation I would say that is pretty high. You can buy the bags of cellulose from home depot and they will let you use the blower for free to do it yourself.

But the poster you quoted was telling you to use spray foam, different then cellulose. Spray foam gets sprayed in the cavity before drywall and will fill the whole cavity. Then scrape flat with the joists before you hang drywall.

 

[rlitman]

...or $275 for 1 layer of 6-1/2" fiberglass rolls of R19. Can do the drywall later.

...or $385 for two layers of 3-1/2" fiberglass rolls totalling R26. Nearly 40% improvement over R19. Can do the drywall later.

Huh?!? NO, you cannot add R values like that.
Two layers of R13 is NOT equal to R26. The R value is calculated for a wall system, not just the insulation. Using two batts that add up to the same size as one gets you the same thing as the one thicker batt. The additional compression increases the density, which increases the R value, but probably not more than R20. Not worth the effort over the single R19 batt that belongs in the space.

 

[signcrafter]

Huh?!? NO, you cannot add R values like that.
Two layers of R13 is NOT equal to R26. The R value is calculated for a wall system, not just the insulation. Using two batts that add up to the same size as one gets you the same thing as the one thicker batt. The additional compression increases the density, which increases the R value, but probably not more than R20. Not worth the effort over the single R19 batt that belongs in the space.

What do you mean by "compression"? If you compress insulation you actually decrease the R value. That is why you don't stuff R19 made for 2x6s in a 2x4 cavity.

 

[gpflepsen]

It's amazing how the extra space acts like a ventilation duct when insulating vaulted rafters with material that is 1" too thin...

 

[jmarkwolf]

Huh?!? NO, you cannot add R values like that.
Two layers of R13 is NOT equal to R26. The R value is calculated for a wall system, not just the insulation. Using two batts that add up to the same size as one gets you the same thing as the one thicker batt. The additional compression increases the density, which increases the R value, but probably not more than R20. Not worth the effort over the single R19 batt that belongs in the space.

If that's true, then I would agree, but two 3-1/2" thicknesses will fill my 7" cavity pretty nicely without any compression nor gaps.

As for totalling up thicknesses to increase R value, it was merely intuitive, but I will defer to those more knowledgable.

 

[jmarkwolf]

It's amazing how the extra space acts like a ventilation duct when insulating vaulted rafters with material that is 1" too thin...

"ventilation duct" being good or bad?

 

[gpflepsen]

"ventilation duct" being good or bad?

Good on a roof. Indifferent on a ceiling.

 

[rlitman]

What do you mean by "compression"? If you compress insulation you actually decrease the R value. That is why you don't stuff R19 made for 2x6s in a 2x4 cavity.

Correct, and incorrect. Compressing fiberglass from it's factory loft decreases the overall R value, but INCREASES the R per inch.
So an R19 batt stuffed into a 2x4 cavity will not yield R19 any more, but will yield more than the R13 batt that correctly fits the space. Probably somewhere between R14 and R15.

The point is that fiberglass insulation is fluffed as much as practicable at the factory to make it the least expensive to install. Fluff it any more and you get too much airflow between the fibers and the R value drops dramatically. Compress it, and you need to use a lot more fiberglass to get the job done, so it gets prohibitively expensive (and then there would be more effort to get the sheetrock flat).

So is isn't worth getting thicker batts and compressing them to insulate better. If you wanted more R per inch, you should just switch to something other than fiberglass.

 

[RedBKM]

I would look into rock wool aka stone wool, mineral wool etc. It has multiple advantages over fiberglass.

Lowes carries a brand called Roxul and it is so much easier to install. Try it and you will wonder why anybody would buy fiberglass for anything.

 

[#1SomeGuy]

I would look into rock wool aka stone wool, mineral wool etc. It has multiple advantages over fiberglass.

Lowes carries a brand called Roxul and it is so much easier to install. Try it and you will wonder why anybody would buy fiberglass for anything.

+10000000000

Did my basement with Roxul...it's R14 for 2x4 framing (more than pink fiberglass), doesn't cost much more, is quite fire resistant, easier to handle and doesn't itch, doesn't hold water (so getting it wet doesn't destroy it's R value), doesn't mold either. Better product in every single way.

 

[FarmerPete]

I just bought some to insulate a spot in my attached garage wall. I like that it's higher R value and fire resistant. The Lowes I went to had a guy with a demo table. He had a torch setup to show that it wont catch fire. Very neat stuff. Significantly more expensive than fiberglass though.

 

[BigGMC]

Very neat stuff. Significantly more expensive than fiberglass though.

I thought so too!
Maybe its cheaper in Canada, but here it's more than double the $$/per sqft compared to FG batts.

 

[#1SomeGuy]

It's about double here as well (same package cost but twice the square footage in a package), but for R12 (pink) vs R14 (roxul) it kind of skews things a smidge. The doesn't cost much more is when you compare pink to roxul to spray foam. If pink cost $1, roxul would cost $2 and spray foam would cost $8. I got a quote of $2500 to spray foam my basement (Installed), but it only cost me about $500-600 for roxul + vapor barrier (DIY)

 

[Zeke]

Whatever you do, don't peel the paper off insulation. Buy unfaced batts and lay them on top of the installed insulation if you are installing craft face down. Around here we don't use faced insulation in floors. Just hold it in place with wire until the ceiling is installed. Or if the ceiling is coming in soon, just leave it in compression between the joists. In the case of no ceiling, we just depend on the wire.

 

[pattenp]

jmarkwolf, look at this compression chart and don't go off of assumptions and guesses. By this chart you'd get the highest R value by compressing R-38C into your 2X8 cavity.

http://www2.owenscorning.com/litera...ul Compressed R-Value Chart Tech Bulletin.pdf

 

[RedBKM]

I just bought some to insulate a spot in my attached garage wall. I like that it's higher R value and fire resistant. The Lowes I went to had a guy with a demo table. He had a torch setup to show that it wont catch fire. Very neat stuff. Significantly more expensive than fiberglass though.

I had some drops left over that I tossed on a campfire just for fun. Nothing happened and it eventually smothered the fire.

Lowe's has a special knife now for cutting the stuff. It looks like something you would see advertised on TV at 3AM but it works great.

 

[Jim_No_Garage]

There is a cheap Farberware steak knife tucked into the walls of my family room. I was using it to cut Roxul and never saw it again.

Roxul is VERY DUSTY compared the the new Itch Free FG batts.

I have 2x6 ceiling joists so I ran faced R21 in the bays and unfaced R30 cross-wise.

I need to roll out a layer of r30 across the rest of the attic - should have done it 15 years ago. I HATE blown in cellulose - we had some walls done and the air leaks in unrenovated spaces bring leak cellulose into the bedroom. Dusty . . .

Jim .

 

[01ss]

Worst thread ever. Talk about wasted brain power.

 

[jmarkwolf]

Worst thread ever. Talk about wasted brain power.

Then don't follow it. Move on.

I was able to correct some misconceptions I had on a subject that I had no previous experience in, while building my new garage, by "conferring" with the kind folks on this forum.

Thank you to those helpful posters.

 

[01ss]

There was a time when I had never built a wall in my life. And whoop whaddya know right on the insulation package it says "for 2x4 wall". Doesn't get much simpler.