Insulating 2x8 floor on 16" centers


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Old 06-20-17, 04:51 PM
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Insulating 2x8 floor on 16" centers

From what I'm reading, Hamilton County Tennessee only requires R19 for the floor, but I can achieve R25 with 2x8's.

That said, I'm having major issues finding R25, faced, rolled insulation. I found one supplier at *WAY* higher pricing than I'm willing to pay.

In the meantime, I found some R30, unfaced rolls on the cheap, but are 9.25" thick. Will I achieve at least R25 if R30 is compressed down to 7.25"? Is there a supplier of R30 faced rolls somewhere? I assume facing in my region would be important.
 
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Old 06-20-17, 05:12 PM
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Is it over a basement or crawlspace or ?
 
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Old 06-20-17, 05:14 PM
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Open crawlspace, it needs to be skirted, but open for now.
 
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Old 06-20-17, 05:43 PM
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The facing is on the heated side of the floor, so unless you have plywood on the bottom of your joists, there won't be anything there to compress it.
 
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Old 06-21-17, 06:38 AM
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So, I'll need to insulate, then cover with plywood?
 
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Old 06-21-17, 08:15 AM
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Not necessarily. Metal strapping can be used to support the insulation to prevent it from falling through. But you should know that mice and other vermin may like to make their home in the insulation if its just a dirt crawl space.

You could cover the bottom of the joists with Typar or an insulation fabric like this. Insuloc insulation fabric May not keep the mice out but it looks good and provides better support for the insulation, keeps dirt out.
 
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Old 06-21-17, 08:54 AM
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If the crawlspace will remain somewhat open then whatever you install should be covered with something critter proof. As X said, mice and rats love insulation.

You could use high density fiberglass insulation (R-21) intended for 5.5" walls and just leave the space above open. If you air seal those floor cavities that open space will provide some insulation due to the air gap.

Bud
 
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Old 06-21-17, 09:31 AM
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What kid of covering is recommended? I'm fairly new to insulation, sorry about all the questions.
 
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Old 06-21-17, 09:45 AM
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I prefer 1/2" plywood, but osb is less expensive. The plywood will last a lot longer.

bud
 
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Old 06-21-17, 10:23 AM
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So that said, should I even bother with a vapor barrier and go with R30 attic insulation then?
 
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Old 06-21-17, 11:49 AM
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Below is a link which also contains many other links related to vapor barriers. More than you will ever need.
I would hesitate to stuff that r-30 into a 7.25 cavity unless you strip off some to more closely match the depth. Fiberglass insulation is more correctly referred to in terms of r-value per inch (approximately 3.5 per inch) and you have 7.25" no matter how much you stuff in there. Plus, too much compression and that r-value per inch begins to decrease. It is also a pain to install.

A complete fill of a 7.25" cavity would be about r-25 and add in a good air sealing job and the performance will be excellent.

As for vapor barriers I would follow the modern trend to omit any plastic but would not be concerned about using a faced batt to help with the installation, just don't tape the seams. A plywood covering will serve as an air barrier and the assembly will retain a small amount of permeability to allow it to dry in either direction.

Bud
The Return of the Vapor Diffusion Bogeyman | GreenBuildingAdvisor.com
 
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Old 06-21-17, 12:57 PM
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Ok, unfaced, with plywood/osb enclosure.

But how do you fill a 7.25" cavity? As stated in the original post, finding R-25 insulation is extremely difficult.
 
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Old 06-21-17, 01:25 PM
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I've always found it fairly easy to peel off a layer with fiberglass. Then buying the r-30 would fit easier. A little gap at the top or a bit of compression is not enough of an issue to worry about. It is primarily the difficulty of stuffing the r-30 in and then trying to cover it with plywood.

Insulation values are more for comparison of products. It is the air sealing and neatness of the install that are more important. If you were to use either r-19 or r-25 you wouldn't be able to feel the difference. From a comfort point of view they would be equal. The real measure would be the energy cost and that is where we try to determine the cost effectiveness of upgrading.

In TN, I doubt there would be a $50 in yearly cost due to r-19 vs r-25. Give me the area and I can give you the theoretical numbers.

Bud
 
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Old 06-22-17, 08:37 AM
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Chattanooga, TN - Hamilton County
 
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Old 06-22-17, 09:16 AM
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You are going to like this.
Don't know the floor area so I'll use 100 ft².
Your area has about 3,600 Heating Degree Days (HDD)
I'll calculate heating only and we can just double that for an approximate year around cost.
The equation is
Q = u x a x HDD x 24
Q = total btu's
u = 1/R where R equals either 19 or 25
a = area and we will use 100 ft²
HDD = 3,600
the 24 is hours/day
Source is Residential Energy by Krigger and Dorsi

For R= 19 we get 455,000 btus
For R= 25 we get 346,000 btus

The difference is = 109,000
I don't know what fuel you use or the efficiency of your system so I'll use mine, fuel oil @ 85%. I would guess your energy costs less than mine. There are roughly 100,000 btus of delivered heat from each gallon of oil so I divide the 109,000 by 100,000 and we get roughly 1 extra gallon of heating oil would be used with R= 19 vs R= 25 for each 100 ft² of floor area.

To include cooling my wild guess would just double that number.

So, for a floor area of 1,000 ft² the r-25 would save you 20 gallons of oil/electricity or about $45 per year.

Recalculate for your floor area (half the area would be half of my number).
We could fine tune this with a significant increase in details but 5this is a reasonable ballpark number for guidance. As a note, these equations are not specific for floor vs wall or ceiling so I would expect the actual energy loss to be even less being a floor over a crawlspace.

Bud
 
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Old 06-27-17, 10:59 AM
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I have about 315 sq feet.

I plugged that formula into an excel spreadsheet and got about ~1.814 million BTU for R15 and ~1.088 million BTU for R25. Also the numbers seem a bit off compared to yours when I plugged in the 100 sq foot room. (576,000 & 345,600 respectively)

Here's how I'm calculating it, if I understood you correctly -
Q = (1/R)*a*HDD*24


This room will be cooled by a brand new Mitsubishi 220V minisplit.

It has R13 in the walls, and spray foam in the ceiling (open cell in 2x6 studs).
 
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Old 06-27-17, 11:27 AM
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Your math is correct. I used R-19 where you used R-15.
We can drill down your cost of energy with the mini-split, but basically it should be less than my oil. I didn't get into the cooling costs I just increased the HDD a bit for your area. A lot od variables but the general answer is minimal savings for upgrading that insulation.

Now, I elected to do so on my own home as every btu counts when you are trying to get as low as possible. But energy is my profession so I'm allowed to be fanatical. But when I advise others the reality has to play a roll and most people have other places to invest that energy money that will yield a greater return.

Bud
 
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Old 06-27-17, 12:10 PM
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Ahh, you're right. So I also found this: Home Heat Loss Calculator

Looks like it's about a $12 difference annually in the R25's favor. Which really cannot viably offset the cost of R25 vs R19, not for a 315 sq foot room anyway. Thank you for the help.

Final synopsis. R19, with vapor barrier against the floor, then sealed with plywood.
 
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Old 06-27-17, 12:56 PM
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In reality I suspect the actual difference would be less because the crawlspace is not subject to the winds and temperatures reflected in the HDD number.

I like that calculator and used it often. It allowed me to easily manipulate the input numbers to true up my calculations to match the home owners actual use. If I remember his online version doesn't allow the print. He had a link back then where I could download the program and he even helped this computer dummy get it going. Once downloaded I could print as needed. That program is based upon that equation.

Bud
 
 

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