This part of the build diverges a little from what you’d consider a normal shed. We’re going to insulate this roof like it was a tiny house. You’ll learn about unvented roofs, figuring out how much rigid foam you need and some tips to make sure your insulated roof sheathing panels go smooth.
Below is the corresponding video with this article.
This shed is going to be where I build things. That means I’ll be out there working in the hot Las Vegas summers and very windy winters. I’m going to treat this structure like a small house when designing and building it. If you just want to skip all this fanciness, then jump to the section on installing the first layer of roof decking and call it good.
Your everyday home typically has a vented roof, except in cathedral ceiling areas. This means you have an attic where air can enter through vents in the soffit and exit in several ways like
- Ridge vent
- Attic vents
Simply put, unvented roofs get rid of the vents and move the thermal, air control and moisture boundaries to the roof deck.
I know this is a very basic example so if you want to read more on the topic, I’ve put some additional reading for unvented roofs at the end of this article.
Figuring Out How Much Foam is Necessary
As I mentioned above, we live in the Las Vegas desert where it’s not uncommon for summers to be 115F and up. I mostly wanted to have a thermal break between the layer of asphalt shingles and the inside of the structure. So, I’ll be doing rigid foam above the roof sheathing with air-permeable insulation between the rafters.
What’s Your Climate Zone?
Before we look at how much rigid foam you’ll need, we need to figure out what climate zone you’re in. You can use the map below to help you do that.
How Much Insulation Do You Need?
Next, we need to figure overall out how much insulation you need in your climate zone for a ceiling. For this we’re going to use the table below from the 2018 IECC.
How Much Rigid Foam Goes on Top of the Roof Deck?
Last thing we need to figure out is how much rigid foam will go on top of the first layer of OSB. For that we’re going to use the 2015 IRC table from section R806.5 Unvented attic and unvented enclosed rafter assemblies.
|Climate Zone||Minimum Rigid Board Insulation R-Value|
|2B and 3B tile roof only||0 (none required)|
|1, 2A, 2B, 3A, 3B, 3C||R-5|
You might have some questions about the three different types of rigid foam out there.
- Expanded polystyrene (EPS)
- Extruded polystyrene (XPS)
- Polyisocyanurate (polyiso)
Fine Homebuilding has a good article that explains the differences between them all.
Let’s Put This Roofing On
Installing the First Layer of Roof Sheathing
We’re going to install the first layer of 16/32 (½ inch) OSB using the diagram below as a guide. If you are only installing one layer of OSB for the roof then I suggest using something thicker.
Remember: This is just a guide. Always measure because wood will dry and twist and it takes less time to measure something than it does to go buy a new sheet of OSB. 🙂
Nail every 6 inches on the outside and 12 inches in the center areas.
Cutting the Top Layer of OSB
I know it seems backwards but we’re going to cut the last layer of OSB now. We’re doing this because when we put on the layer of foam we won’t be able to easily see the rafters.
Lay down chalk lines on the first layer of OSB at the center of the rafters.
Once you’ve got the chalk lines you can measure out the panels to cut. Use the diagram and mark your measurements.
I found it easier to mark the panels so I wouldn’t get confused.
Installing the 2×4 Edging
For my climate zone I need to install a layer of rigid foam with an R-5 value. Lucky for me 1 ½ inch thick expanded polystyrene (EPS) rigid foam insulation is R-5.78. The rigid foam needs to have a border, and because it’s 1 ½ inches thick standard 2x4s can be used.
For my situation I’ll cut 16 foot 2x4s to fit the outside border of the first layer of OSB.
Installing the Rigid Foam and Top Layer of Roof Sheathing
My original plan was to install the layer of rigid foam first and then do the sheathing on top. Only thing is it was windy that day. The glue I was using didn’t set fast enough to hold the rigid foam panels in place, so I ended up installing the foam and OSB sheathing at the same time.
If you’re installing multiple layers of rigid foam, try not to overlap the seams.
Lay down a full sheet of rigid foam. Optionally use some adhesive designed to use with rigid foam.
Cut the next piece so you have a full row of rigid insulation in place.
You might have noticed there is aluminum foil faced up on the rigid foam, and ask if you should install the rigid foam with foil up or down.
It doesn’t matter. In order to receive the benefits of the reflective foil, you need to have an air gap (usually ¾” ) between the foil layer and the one above it.
Now that we have a full row of insulation, it’s time to use the diagram and pre-cut top OSB sheets. If you’re using adhesive for your panels, put it on the bottom of OSB sheet.
Put down an entire row of OSB and shifted it around to make sure everything is lined up. Then nail every 6 inches on the outside edge that’s on top of the 2x4s.
If you can easily figure out where the rafters are for any inside seams then use 3 ½’ exterior rated screws there.
Now that all the sheets are in place and the edges are either nailed or screwed in, we need to screw the centers of the top OSB sheets. You’re going to snap a chalk line where the rafters run.
I found that I had to use a countersink bit when installing the screws.
Sealing the Seams and Screw Holes
Now that everything is assembled we want to seal it up. The ultimate way to do this is to put a layer of adhesive backed ice and water shield roll roofing underlayment. I wasn’t able to find this so the next best thing was to make sure all the seams are sealed up.
With that in mind use some 6 inch peel and stick flashing membrane on all of the seams.
Give yourself a couple inches of overlap on the edge.
After you do the long strips move on to the short ones.
I was using the small roller, that probably wasn’t designed to do this, I had. It definitely took longer than if I had one of these babies 😊
Use some 9 inch peel and stick flashing membrane along the edge.
At first, I thought I would use one continuous piece all along the border. I soon figured out that wasn’t happening. It was too cumbersome going around the corners.
- Cut the length about 6 inches past the corner edge
- Sliced the seam line from the corner
- Folded the two pieces over like you would a present
Last thing we need to do is seal up the screw holes using some roof and flashing sealant. It may seem a little overboard, but hey… we just spent the time to seal the seams. Plus, it’s way cheaper and faster to use caulk for this.
At this point there are a couple small details that need to be taken care of.
- Painting and installing the fascia before shingling the roof
- Putting OSB on the upper wall area
This was the most expensive part of the project so far. If you wanted to cut some costs, you could not use the foam adhesive and save yourself $50.
- Building Science Corp – Unvented Roof Systems – https://buildingscience.com/documents/reports/rr-0108-unvented-roof-systems/view
- Unvented Roof Assemblies for All Climates – https://buildingscience.com/documents/digests/bsd-149-unvented-roof-assemblies-for-all-climates
- Field Testing of an Unvented Roof with Fibrous Insulation,… – https://www.nrel.gov/docs/fy16osti/64999.pdf
- Insulating Unvented Roof Assemblies – https://www.finehomebuilding.com/2015/11/11/insulating-unvented-roof-assemblies
- Residential Provisions of the 2018 IECC – https://www.energycodes.gov/sites/default/files/becu/2018_IECC_residential.pdf
- 5 Unvented attic and unvented enclosed rafter assemblies. – https://www.demilec.com/documents/Tech-Library/2015-IRC-R806.5.pdf