how much R value

Quebecnewf · February 24, 2007, 06:06:44 AM

How much R Value in the following roof

1" rough board/tar paper/1 1/2" air space /2 layers 2" Styrafoam / Steel sheeting.

Quebecnewf

Don P · February 24, 2007, 08:10:42 AM

Just a stab, about R-25. Which foam? Is it the beaded expanded polystyrene (EPS) or the smooth extruded polystyrene (XPS). I think you can take about R1 for the board and another R1 for boundry air layers. So foam + R2

Norwiscutter · February 24, 2007, 10:53:08 AM

I am sure you probably already know this, but the airspace should be between the foam and the metal. From the order you state in your post, I was not sure if that is where you already intend the airspace to be. EPS foam carries a lesser rating per inch then the XPS. Consider using a foam with a radiant heat barrier.

Raphael · February 24, 2007, 11:57:34 AM

It's a little complex as R values/inch of solid material can't be added without adjusting for conductive loss. If an inch of a given foam has an R6 value, two inches of the same foam will usually be rated something like R11 so two layers of that same 2 inch foam is closer to R21. One inch of continuous sheathing has an R value of 2.5, but that drops if one face is exposed to moving air. I think Don's estimate of R1 is a good conservative number for your rough boards but actual perfomance may be better (the roughness works in your favor here).
Theoretically your 2x material that's creating the air break has a value of ~1.5, but this is ignored. Unmoving air has an R-value of up to 5.7 per inch (I'd use R5 as you can usually get experts to agree it's at least that much). Conductive loss in air is quite low but convection can really kill the R value/inch, you'll get better perfomance out of your airgap if you sandwich it between the layers of foam.
Built as you describe I'd call the air gap an R3 infill insulation, move it between the foam layers and it's an R7.5 and any radiant barrier (foil backing facing down into the gap) added to the foam is protected from picking up dirt so it's going to perform well and add to that value. I'd also space the metal off the upper layer of foam with firring strips to kill conductive loss and allow any moisture to escape.

Not knowing the foam I'll vote for R25 as well. :D

mike39chef · February 24, 2007, 03:35:28 PM

This site has all the values for most construction materials. Its a great resource, i hope it helps you. Go HABS Go

http://www.coloradoenergy.org/procorner/stuff/r-values.htm

submarinesailor · February 24, 2007, 07:41:55 PM

About 8 years ago, I took a Certified Energy Manager" course from the Association of Energy Engineers (AEE), and as part of the course they addresses the issue of adding "R" values – Raphael got it right. You can't just add R values. You must first convert them to a "U" factor first. IF I REMEMBER IT CORRECTLY, you divide the "R" factor into 1 to get the "U" factor and then you can add them. After adding them together, you divide the "U" factor total into 1 and that gives you the total "R" factor. Please remember it's been 8 years and in my current Energy Manager job I don't get to use anything like this. Just move natural gas around to military installations.

Bruce

Quebecnewf · February 25, 2007, 10:38:21 AM

my plan was to have the airspace after the 1" rough board but i could have the air space after the 4" of styfoam before i get to the steel cladding. What way would be best and why????

board /tar paper/air space /foam/steel
board/tar paper/foam/air space/steel
Quebecnewf

beenthere · February 25, 2007, 11:01:05 AM

I'd think the air gap under the steel would give good ventilation. The steel will sweat when the temps are just right, and venting that moisture seems would keep the steel from early deterioration. Shouldn't have that temp differential between the wood side, so venting isn't usually done there. Just like a house, vent above(outside) the insulation (at least when the outside is colder than inside. This reverses when cooling the inside and the outside is warmer)

Raphael · February 25, 2007, 11:54:54 AM

Given the choice of those two I'd put the air space just below the steel to allow accumulated moisture to escape and preserve the steel... It depends in part on the steel roofing you are going with some have air channels built in.

How about: 1"board/tar paper/2"foam/1"air space/2"foam/½"air space/steel.
Same thickness and better R value... More boards but the same BF... Shorter length for the longest nails as you don't need to go through 4" of foam anywhere.

Norwiscutter · February 25, 2007, 12:01:00 PM

Just based on what they allow us to use in figuring the r value for log walls, most pines net a value of 1.4 per inch. As was stated previously by smarter people than myself, two air spaces would be ideal. If only one, it has to be on the outside of your insulation.

Kevin K · February 25, 2007, 09:57:57 PM

For peace of mind, please consider swapping the tar paper for Ice & Water Shield. You get cold winters and hot summers, and you haven't mentioned a vapour barrier.

It isn't much more expensive (in the big picture).

Tar paper isn't a vapour barrier.

Raphael · February 26, 2007, 02:41:01 AM

You could also leave out the tar paper and tape the seams on your bottom layer of foam to achieve the same result, assuming it's one of the impermeable foams. The EPS and XPS used in SIPs constitute a vapor barrier when properly sealed.

Quebecnewf · February 26, 2007, 06:16:03 AM

Interesting responses

My thought on the tar paper was as a water barrier in case a small leak in the steel should develop it would be a second barrier against the leak.

I like the idea of two air spaces. The STY I would be using would be Styrofoam SM or Clademate. I think with proper taping of the seams it would be a vapour barrier. I am also looking at maybe putting 5 ml poly down on the roof before or instead of the tar paper.

As I said earlier this is for a heated workshop so moisture in this region should not such a problem

Quebecnewf

beenthere · February 26, 2007, 01:00:46 PM

Quote from: Quebecnewf on February 26, 2007, 06:16:03 AM
Interesting responses
...................
As I said earlier this is for a heated workshop so moisture in this region should not such a problem
........

Wouldn't the heat in the shop be the main reason that moisture is a problem in the walls? Condensation occurs when there is warm air/cold air interface. Least that is how I'd learned it. :

Quebecnewf · February 26, 2007, 01:47:19 PM

This is true warm and cold = moisture . A workshop should never have the moisture in the air you would find in a home per say.

Quebecnewf

beenthere · February 26, 2007, 04:11:14 PM

Think I could ~~argue~~ discuss that issue, but if'n you go in the shop and breath, or take lumber in there to work on, seeems to me you will introduce/have plenty of moisture. My point is only that moisture is gonna condense on the cold wall.
Up to you how you want to address the issue.

Quebecnewf · February 26, 2007, 04:34:44 PM

Here is a bad drawing of what I plan to do

The blue is the STY

I will try to explain later any questions sorry for the drawing still learning Sketchup

Kevin K · February 26, 2007, 06:00:34 PM

--Photos MUST be in the Forestry Forum gallery!!!!!--/img376/5944/layerspv1.png

Here's how I'll be doing it.

Quebecnewf · February 27, 2007, 05:56:10 AM

Kevin

How will you fasten the 2x4 on their flat . Will you nail down through 4" of Sty. ?

If so what type of fasterner?

Quebecnewf

Don P · February 27, 2007, 08:30:16 AM

We've used Oly panels screws for screwing thru strapping and foam, into the ceiling deck or rafters. They are a long hardened screw with a ~3/4" flat head with a square or star drive.
These are a couple of shots from my gallery of one we did. This had 5" of foam and 2x4 sleepers.Ventilation channels are formed by the sleepers from bottom to top.

I remember from some time ago being told that a cavity deeper than about 3/4" allows the upward and downward flow of a convection current to set up, staying shallower than that the currents block each other and insulate. That was a long time ago, someone might know better.

Raphael · February 27, 2007, 10:20:17 AM

The one discussion on wrap and strap enclosures I was in on they were saying two one inch air gaps was superior to a single two inch gap (because of convection)... I didn't think to ask if we were discussing a true 1" or a nominal 1" (ie: ¾").

Quebecnewf · February 28, 2007, 07:22:52 PM

Where can I find out more info on these Oly screws. Do the co have a web page or Canadian Dist.

Quebecnewf

Don P · February 28, 2007, 09:57:49 PM

These folks have them, the headlock fasteners are the ones we used.
http://www.loghelp.com/fastener.html
I've had good dealings with that company in the past.

The Oly's are high end fasteners, it might pay to shop around online and compare specs and prices.