# Interior side of exterior sheathing temperature calculation. Does the calculation change when you are calculating for the roof?

In a REMOTE or PERSIST type assembly, does the calculation for determining the temperature at the inside surface of the exterior sheathing differ between walls and roof? Or is the temperature calculation the same? For instance if you used the same exact assembly at the roof as you did on the walls – would the temperature or condensation point be different at the interior side of the sheathing?

Obviously I will be using higher r-values in the roof (R-60)minimum, and (R-40) minimum in the walls.

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## Replies

Brad,

As far as I know, the same calculation method is used for walls and roofs. Here is an article on performing dew-point calculations: Are Dew-Point Calculations Really Necessary?

Of course, rafter cavities are generally deeper than stud cavities. When builders insulate on both sides of the exterior sheathing (for example, with rigid foam on the exterior of the sheathing, and fiberglass batts between the framing members), roof assemblies usually get more fiberglass. Because of that, you need thicker rigid foam on a roof than you do on a wall.

I have provided handy-dandy tables showing minimum R-values for wall foam and roof foam -- tables that allow you to avoid dew point calculations -- in this article: Calculating the Minimum Thickness of Rigid Foam Sheathing.

Brad,

Martin's right, there's no difference in the calculation, regardless of orientation. In theory, anyway. In reality, the different assemblies may see different temp & moisture levels of the air due to infiltration and stratification - but a good airtight, well-insulated envelope minimizes these effects.

Thanks for the answer, makes sense.

When I try to do the calculation using no inside insulation and only foam, my calculation seems to be off. I want to reverse the calculation, and figure for the amount of exterior insulation as X, based on my temp outside, inside, dew point, etc........ do you know the calculation for this? I used the calculations from the building science corp/build america document that highlights the calculation to figure out component and whole wall assembly r-values to come up with my value I used in my original post.

The answer I get now, when trying to to figure for only exterior insulation is skewed. I figured rather than trying to kept placing temps in to figure out my surface degree I would just try to calculate for the required R-value instead. I am in zone 6A, 8200 degree days, ave. cold temp is 3-6 degrees F. I will have 3" studs on the inside that I will just leave hollow, and insulate only on the exterior. How much insulation do I need to keep the dew away?????

Interior temp 70 degrees F, RH% 30, and dew point 37 degrees F.

R 40 @ wall, and R60 @ roof minimum.......help, help, help - my drawings are due in three days!

The article on calculating exterior foam is great - but it doesn't "really" address calculating it - it gives you a rule of thumb.....the article is great though - but is too generalized for my liking, and virgo blood - need more accurate and customized numbers.....so here my ADHD feeble mind struggles!

Build with a 50/50 split.

A 50/50 split will lead to condensation on the interior surface...............research has proven it - just read thru the REMOTE building guide, and the subsequent 10 year testing data of such walls found at cchrc.org

i don't need a generalization, I need a informed, educated and calculated answer based on scientific data.

Brad,

If all of your insulation is on the exterior of the sheathing, very, very little insulation is required to prevent condensation on the sheathing as the vast majority of the temperature drop will occur across the insulation layer, making your sheathing nearly as warm as the interior air.

If you are looking to also use some insulation inside the sheathing, then you need to calculate the temperature drop across the layers of your wall assembly. The temperature will drop across each layer as a direct function of the proportional r-value of the layer (ie. the layer r-value as a percentage of the total r-value of the assembly). This can be done using a simple spreadsheet. I can send you a sample if you would like.

Trevor,

Thanks.....and a copy of the spreadsheet would be awesome!!!!!!!!!!!!does the spreadsheet cost $? can you post it or do you need an email?

Hi Brad,

The spreadsheet is just a simplification of a school project that I did. It is not designed to be user friendly, but it will give you the information that you're looking for. If you can post your e-mail, I will send it to you with some instructions. - FYI,- it is in metric (SI) units.

Trevor