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Rigid (XPS) Foam Board Size for basement.


fastmodena's Avatar
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12-02-12, 05:42 PM   #1  
Rigid (XPS) Foam Board Size for basement.

Starting my basement finish. Just got done tearing down the CRAP blanket insulation the builder left behind. New home.

My plan was 2" PINK foam board against the concrete, followed by framed 2x4 walls, and R-13 fiberglass (non-faced) insulation in between the 2x4's.

After research and the fact that non-faced fiberglass can only be special ordered in my area, I found Roxul and the R-15 Roxul costs about the same as the special order fiberglass. Not to mention it's in stock.

I have searched google over and over (as well as forums) and my question still remains. Since the Roxul is R-15 would it be okay to go with 1" XPS board instead of the 2" that I was planning on? That would give me an effective R-20 which is okay for my area.

My concern is 1" being too small as an effective vapor barrier to prevent condensation on the poured concrete foundation wall. I have searched all over and can't really determine the difference between 1" and 2" except the R-value and the 2" is a substantial premium.

Your thoughts? Thanks!

 
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12-02-12, 08:14 PM   #2  
My thought is that the vapor barrier needs to be next to the heated area, immediately behind the drywall and in front of the insulation in the framing. keeping the insulation dry is the point.

Why did you not want to use FG with a vapor barrier?

How much of the wall is below grade?

 
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12-03-12, 02:41 AM   #3  
Vapor barriers can be a problem in a basement since there is no opportunity to dry to the outside below grade. XPS is actually rated as a vapor diffusion retarder and allows a small amount of moisture vapor to pass through, thus drying to the inside. But this is a very small amount and easily absorbed by the air inside.

But vapor diffusion is not water movement. All potential for leakage must be corrected before you build those walls and that is done from the outside.

If your code will allow, 1" of XPS will be fine in combination with the Roxul. The critical area is the exposed concrete above grade and about a foot below. And don't forget to air seal and insulate the rim above the concrete.

The latest debate on vapor barriers, even the experts can't agree.
Reviving an Old Debate on Vapor Barriers | GreenBuildingAdvisor.com
And a link on basements:
http://www.buildingscience.com/docum...lation-systems

Bud

 
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12-03-12, 07:14 AM   #4  
Thanks for the replies.

I am not going to use a vapor barrier. I was referring to the difference between 1" and 2" xps for vapor diffusion if you will.

Our code is stupid. They will allow basements to be r10 continuous or r11 and above. I'm doing at least r20. I am more concerned about condensation rather than rvalue.

 
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12-03-12, 08:28 AM   #5  
2" is obviously half the perm rating of 1", but for a dry basement either should allow some slight drying to the inside.

As for cost vs effectiveness, 1" plus the Roxul will perform well. The basement temperature would be the the same for both 1" and 2". The only difference would be a slight increase in heat loss through the 1" combination. That's where the calculations come in, estimating the added loss and cost as compared to the additional cost of the 2" insulation. Give me your HDD (heating degree days), area above grade, area below grade and intended set temperature down there and I can run those numbers. Also, your source of heat, oil, gas, electric and how new your system is.

Condensation is at a higher risk for the above grade area so tape the seams well to prevent any air seeping through with inside moisture. Roxul is very dense so it will help Below grade it is less of a problem as the ground temperature stays warmer the farther you go down.

Air sealing where the house rests on the foundation is an important fix, as normal (stack effect) air flow pushes cold air into the lower portions of a house and the warm air up and out the upper leaks. If you have an atmospheric draft heating system, you will need to consider your source of combustion air.

Bud

Vapor Barriers or Vapor Diffusion Retarders | Department of Energy

BSD-106: Understanding Vapor Barriers — Building Science Information

 
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12-03-12, 10:08 AM   #6  
Our heating degree days are 6301 (Live in Utah). The furnace is natural gas and home was built in August so furnace is brand new. 95% efficiency.

When you talk area above grade do you mean the main floor and basement sq footage? 1474 above and basement is 1424. Or do you mean parts of the basement foundation that are not covered by dirt outside? The whole basement except the right side is underground. 30% of the right side of basement is not underground due to the hill we live on.

If I am totally off how do I calculate the above and below grade area? I can provide pics if helpful.

I would like the set temp anywhere from 69-74 depending on how hot my main floor will get due to the thermostat being on the main floor. I am aware that this is not the best setup for the heat source in the basement but a second furnace is out of the question (cost), and I looked into zone systems but those seem like they would be expensive as well. Not to mention the dampers seem to break on people a lot.

 
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12-03-12, 11:35 AM   #7  
What we use is the exposed area of the foundation, plus a bit to go down an extra foot. But what you gave me is close enough. These numbers don't need to be anywhere near exact as the answer we want is the difference between the two calculations. So, here we go.

Roughly, a 1,500 ft� house and same for basement. We will call it a 30' x 50' basement with 3 plus feet on average of exposed concrete. 160' perimeter times 3' would be 480 ft� so call it 500 ft� of heat loss area.

The equation is: Q = (U x A x HDD x 24)/ 90,000 which will give you the Therms per year through the area specified.

Q is the heat, normally in BTUs, but I divide that by BTUs per Therm to convert to Therms.
U = 1/R
HDD is the 6,300
24 is hours per day to correct the terms for the HDD
90,000 is BTUs per Therm. NG has about 100,000 BTUs per Therm, discounted to 90% for furnace efficiency and some delivery losses (leaky ducts, warm up, and such).

It gets easier. Since A, HDD, 24, and the 90,000 will remain the same, we calculate those first and get 840 for a multiplier (500 x 6300 x 24)/90,000 = 840. We then divide that number by any R-value we want to evaluate for the wall. Dividing by R is the same as multiplying by U.

Before your improvements, your basement walls are using 840 divided by 2 or 420 Therms.
With R-20 walls (the 1" foam) 840 is divided by 20 for 42 Therms.
With R-25 walls (the 2" foam) 840 is divided by 25 for 33.6 Therms.

At $1.00 per Therm, you would be paying $420 per year to heat the basement to 70� with no improvements. HDD is based upon 70�.
$42 per year to heat it with the 1" option and $33.60 per year to heat it with the 2" option.

Going to 2 inch foam will save you $8.40 per year.

In this case, 1" will breathe a little better than the 2" and do the job just fine.

If you want me to sharpen my pencil and fine tune these numbers I can, but this is a very good look at the range of savings.

If your concrete walls have some imperfections you want to smooth out before attaching the rigid foam, an angle grinder works great. If you want it even better, use a diamond cup. Thought I would mention those just before xmas .

Bud

 
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12-03-12, 07:29 PM   #8  
Bud,

WOW! Your the man. Thanks for that.

The difference in cost from 1" to 2" is $10 per board in my area. Total cost difference around $400. With your calculation it would take roughly 47 years to break even! Hardly worth it. So I think I will just go with the 1" XPS and in between the 2x4 wall the r-15 Roxul.

I am also going to do the rim joists. The builder has r11 fiberglass up in those areas. Should I stick with 1" XPS here as well or up it to 2" XPS for the rim joists? Once I put in the foam I am going to use great stuff (I wonder how many cans that will take) to seal the edges of the foam. Should I throw the r11 fiberglass back up in there to take up the cavity space, use Roxul to take up the space, or just leave it?

 
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12-03-12, 09:48 PM   #9  
You can play with that equation anywhere in the house. There are just a few guesstimates necessary to get all of the pieces to add up to your total energy use. I like, walls, windows/doors, ceiling, foundation, and air leakage, as the 5 major groups. When you see those add up and come close to what you are actually spending you gain confidence in the numbers you put into the math.

Not sure what your NG prices are, but when they were headed to $7 a Therm, the decisions were different. My current oil prices, adjusted for BTUs, are 3.5 times that $1.00 per therm. If we only knew what fuel prices were going to be 10 yrs from now. I'm adding 3.5" of foam over the entire exteriors walls of my house, I think prices are going up in time.

As for the rim joist, I would use Roxul for all of it, R-20 or better. Use caulking or foam to seal first. I find caulking less expensive and gives a better seal with small gaps. You can run your finger over it to smooth it out and push it into place. You can't touch that foam until it drys and then it is a pain to trim. You can use up and scraps of leftover 1" foam, just double or triple them up. A bale or two of 6" Roxul makes for a nice quick job, it is already cut to width.

Enjoy and let us know how it comes out.

Bud
PS, use pressure treated wood anywhere it will contact the concrete.


Last edited by Bud9051; 12-03-12 at 09:51 PM. Reason: addition
 
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