Rigid Insulation Boards

Foam boards—rigid insulation boards—can be used to insulate almost any part of your pole barn, from the roof down to the foundation. They provide good thermal resistance. Foam board insulation sheathing reduces heat conduction through structural elements, such as wood wall girts and roof purlins.

There are some who would feel the best application of these products is to the outside of wall girts and purlins, directly beneath the wall and roof steel. Doing so can create a huge structural problem with pole buildings.

A properly designed pole building utilizes the steel skin to create a diaphragm. The steel skin functions like the shell of a uni-body car or modern jet aircraft.

When rigid insulation boards are applied to the exterior framing, the screws must be ordered appropriately longer to attach the siding or roofing. When the pole building is then subjected to racking loads from wind or seismic forces, the shank of the screw between the steel skin and the underlying wood – which passes through the rigid insulation boards, may bend or break, resulting in a compromise of the building’s structural integrity.

The most common types of materials used in making foam board include foam building insulationpolystyrene, polyisocyanurate (also known as polyiso) and polyurethane.

Molded expanded polystyrene (MEPS) is a closed-cell material which can be molded into many everyday items (think coffee cups and shipping materials), or into large sheets of foam board insulation. MEPS foam board insulation is commonly known as beadboard.

To make beadboard, loose, unexpanded polystyrene beads containing liquid pentane are mixed with a blowing agent and poured into an enclosed container. The mixture is heated to expand the beads many times their original size. The beads are then injected into a mold. Under more heat and pressure, they expand to become foam blocks, which are shaped as needed.

The physical properties of MEPS foam board vary with the type of bead used. It’s manufactured at various densities, depending on the application. Beadboard for roofing materials has to be dense enough to walk on without damage; wall insulation foam boards are several times less dense than roof boards. R-values range from 3.8 to 4.4 per inch of thickness.

MEPS foam board is available with a variety of facings. Since spaces between the foam beads can absorb water, a vapor diffusion retarder is needed if water transmission through the insulation might become a problem. MEPS foam board also is often used as the insulation for structural insulated panels (SIPS) and insulating concrete forms (ICFs).

One of my personal pole buildings, at my home, utilizes rigid insulation boards, ICFs, on 2-1/2 walls of the lowest level.

Extruded expanded polystyrene (XEPS) is a closed-cell foam insulation similar to MEPS. To make it, the polystyrene pellets are mixed with various chemicals to liquefy them. A blowing agent is then injected into the mixture, forming gas bubbles. The foaming, thick liquid is then forced through a shaping die. When cooled, the panel is cut as required.

XEPS is more expensive than MEPS. Like MEPS, the R-value depends upon the density of the material and is generally about R-5 per inch. It’s also much more consistent in density and has a higher compressive strength than MEPS, making it better suited for use on roofs or for wall panels. Extruded polystyrene also has excellent resistance to moisture absorption.

Like MEPS, XEPS is available with a variety of facings and is also often used as the insulation for SIPs and ICFs.

Polyiso and polyurethane are very similar, closed-cell foam insulation materials. Because both materials offer high R-values {R 5.6 to R 8} per inch of thickness, a thinner foam board can be used to achieve the required thermal resistance. This can be an advantage in limited space applications.

Polyiso foam board insulation is available in a variety of compressive strengths. Compressive strength refers to the ability of a rigid foam board to resist deformation and maintain its shape when subjected to a force or load. Also, polyiso remains stable over a wide temperature range, making it good as roofing insulation. When used with a laminated aluminum foil facing, polyiso foam board provides an effective moisture or vapor barrier. These foam boards can also be used to make SIPs.

The maximum performance of rigid insulation boards depends heavily on proper installation. If you want to install it yourself, obtain and carefully follow instructions and safety precautions from the manufacturer. Also check local building and fire codes to see if it’s allowable to use foam board insulation in your new pole building. If so, are there any special requirements for using it?

Hold these thoughts until tomorrow – and I will delve into how to use foam board in foundations, using a fire barrier and a whole lot more more.  Stay tuned!

7 thoughts on “Rigid Insulation Boards

  1. I am planning on insulating my 36x38x12 pole barn. With 6×6 poles. Thinking about using 1-1/2″ thick R-7 pink foam board between the girts. Then 6-1/4 thick R-19. Then covered with 6-mil poly. Will the foam board touching the steel cause problems? I live in Minnesota. The building will be heated with in floor heat.

    1. Jeff ~

      The foam board touching the wall steel will not be a problem, what may pose a problem would be creating a space with two vapor barriers with insulation trapped between (properly installed the foam board will act as a vapor barrier). You would be better served to install a quality housewrap between the framing and the wall steel, then completely fill the wall cavity with insulation.

  2. I have a ’70’s built wood-frame home on stilts, located on the Louisiana Coast. Yes, it has survived the many hurricanes and I’m not sure how. I have decided to remodel and have chosen to remove our current sheetrock/popcorn ceiling. We discovered beautiful old rafters, ceiling joists and some interesting ‘beer-drinking’ support decisions. I would like to have them exposed to 1) increase the height of our living space (pre-ceiling demo ceiling height was ~7’…yes, I at 5′ 4″ could reach up and touch that nasty popcorn ceiling). 2) increase the ‘character’ of the home. Wow! with no ceiling, we gained an additional 6′ at gable peak! We won’t even go into how I’d like to now add loft areas (that’s a structural question for a later date. Now, for my inquisition: How do I appropriately insulated my now exposed rafters while still allowing my rafters to be exposed. I have 24″OC rafters that are 2×6’s. The home is a 34×34 square, supported entirely by the ceiling joists (which are also 2×6/sistered in the center). I can literally see the roof nails from the shingle installation. My thoughts are to install a vapor barrier directly to the roof decking, glue foam board to the vapor barrier, leave a 1″ air-gap and then add furring strips/purlins to allow me to brad ‘ship-lap’ to the purlins. Oh, and you should know there are no whirly-birds or a ridge vent on our roof, however, we do have soffit holes for air circulation. My assumption is I would now have to close those soffit holes up.

    1. I’d probably look towards using closed cell spray foam insulation – it will give a superior R value, seals completely and will take care of the need for a vapor barrier. You would need to close the soffit holes.

  3. Hi, long time reader here. First time commenting. I’m in the process of building my first pole barn and things are going very well, thanks to your blog and much of the information I have found searching and reading many of your articles and engineering documents.

    We just finished putting up the trusses and purlins, and the bookshelf grits are getting framed up within the next week. My steel is currently on order, and I was planning on doing a continuous exterior insulation with 1.5” 25psi polyiso. I completely understand the “unibody“ issue that you mentioned here. My question is, with a proper number of fasteners to the steel what is the realistic impact of structural rigidity in my case. I understand there are a lot of variables here so I don’t expect a detailed answer. Just curious if there was any real data.

    Lastly, if exterior continuous insulation isn’t an option, can it be placed on the interior walls with drywall over it as long as the walls are designed to dry to the outside?

    1. Placing your polyiso on the exterior of your wall girts will pretty much negate the shear strength of your wall steel, regardless of how many fasteners you use. Yes, it can be placed on the interior. Use a WRB (Weather Resistant Barrier – Tyvek of similar) between girts and siding, then unfaced batts (I recommend rock or mineral wool as they are unaffected by moisture), then glue on your polyiso to inside face of girts, sealing all of the seams.

      1. Ok great. Thanks for confirming. So, I will use 2×6 bookshelf girts between my 6×6 posts. This will allow me to offset the girts 1.5” to the outside while leaving a 1.5” inset space between the posts. This way, I can run my polyiso on top of the girts inside and still be flush with the posts (so I don’t loose the additional floor space). I plan to attach my drywall directly to the girts through the taped and sealed rigid foam. I’m not certain if I want to use furring strips or not. I’m unsure whether they are of any real benefit since my foam is 25psi.

        In addition, I had settled on unfaced r19 rockwool between the girts. My whole goal is to limit thermal bridging and improve air tightness. I think all of the above will meet that need. Thoughts?


Leave a Reply

Your email address will not be published. Required fields are marked *