Building A Modern Shed – Part 7 – Insulated Roof Sheathing Panels

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.

Unvented Roof

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

 

Vented Roof Diagram

Vented Roof Diagram

 

Simply put, unvented roofs get rid of the vents and move the thermal, air control and moisture boundaries to the roof deck.

 

Unvented Roof Diagram

Unvented Roof Diagram

 

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.

 

IRC Climate Zones

IRC Climate Zones

 

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.

 

Ceiling R Values

Ceiling R Values

 

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
4C R-10
4A, 4B R-15
5 R-20
6 R-25
7 R-30
8 R-35

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. 🙂

 

First Layer of Roof Sheathing Dimensions

First Layer of Roof Sheathing Dimensions

 

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.

 

Laying Chalk Line to Measure Top Layer of Roof Deck

Laying Chalk Line to Measure Top Layer of Roof Deck

 

Once you’ve got the chalk lines you can measure out the panels to cut.  Use the diagram and mark your measurements.

 

Top Layer of Roof Sheathing Dimensions

Top Layer of Roof Sheathing Dimensions

 

Measured Top Layer of OSB

Measured Top Layer of OSB

 

I found it easier to mark the panels so I wouldn’t get confused.

 

Marking the Layout Diagram for Installation of Top Layer

Marking the Layout Diagram for Installation of Top Layer

 

Marking Top Layer of OSB for Installation Later

Marking Top Layer of OSB for Installation Later

 

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.

 

Truck Full of Rigid Foam and 2x4s

Truck Full of Rigid Foam and 2x4s

 

Cutting 2x4 Border with a Japanese Handsaw

Cutting 2×4 Border with a Japanese Handsaw

 

Gluing 2x4 Border Before Screwing It In Place

Gluing 2×4 Border Before Screwing It In Place

 

Screwing 2x4 Border for Rigid Foam in Place

Screwing 2×4 Border for Rigid Foam in Place

 

2x4 Border Ready for Rigid Foam

2×4 Border Ready for Rigid Foam

 

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.

 

Putting Rigid Foam Adhesive on Roof Deck

Putting Rigid Foam Adhesive on Roof Deck

 

Putting Rigid Foam In Place

Putting Rigid Foam In Place

 

Cut the next piece so you have a full row of rigid insulation in place.

 

Measuring to Cut Rigid Foam

Measuring to Cut Rigid Foam

 

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.

 

Cutting Rigid Foam with Japanese Ryoba Handsaw

Cutting Rigid Foam with Japanese Ryoba Handsaw


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.

 

Putting Adhesive on Bottom of Top OSB Layer

Putting Adhesive on Bottom of Top OSB Layer

 

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.

 

Nailing the Outside Edge of the Top Layer of OSB

Nailing the Outside Edge of the Top Layer of OSB

 

If you can easily figure out where the rafters are for any inside seams then use 3 ½’ exterior rated screws there.

 

Screwing OSB Seams with 3 1/2 inch Screws

Screwing OSB Seams with 3 1/2 inch Screws

 

Installing Last Panel on Roof

Installing Last Panel on Roof

 

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.

 

Snapping a Chalk Line to Install Screws in the Center of Sheets

Snapping a Chalk Line to Install Screws in the Center of Sheets

 

I found that I had to use a countersink bit when installing the screws.

 

Countersinking Before Driving Screws in Top OSB Layer

Countersinking Before Driving Screws in Top OSB Layer

Driving Screw into Center Sections of the Panel

Driving Screw into Center Sections of the Panel

 

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.

 

Installing the Long Strips of Membrane FIrst

Installing the Long Strips of Membrane First

 

Give yourself a couple inches of overlap on the edge.

 

Flashing Overlapped Over the Edge

Flashing Overlapped Over the Edge

 

After you do the long strips move on to the short ones.

 

Rolling the Flashing Membrane with a Roller

Rolling the Flashing Membrane with a Roller

 

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 😊

 

Powertec J-Roller

Powertec J-Roller

 

Use some 9 inch peel and stick flashing membrane along the edge.

 

Installing Flashing Along the Edge of the Roof Deck

Installing Flashing Along the Edge of the Roof Deck

 

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.

Instead I

  • 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

 

Cutting the Corner of the 9 Inch Flashing

Cutting the Corner of the 9 Inch Flashing

 

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.

 

Sealing Screw Holes with Roof and Flashing Sealant

Sealing Screw Holes with Roof and Flashing Sealant

 

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

 

Insulated Roof Panels Budget Totals

Insulated Roof Panels Budget Totals

 

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.

Additional Reading

Unvented Roofs

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