# Building Height, Building on Existing Foundation, and Spray Foam

Today the Pole Barn Guru answers questions about calculating the height of a building, Building on and existing foundation, and Spray Foam Insulation.

DEAR POLE BARN GURU: I’m looking for over all height of a building with a 14’ eave?
Thanks. DOUG in PILOT ROCK

DEAR DOUG: The overall height determination starts with a clear understanding of how eave height is to be measured: http://www.hansenpolebuildings.com/2015/02/eave-height-2/.

With this in mind, the rise of the roof can be calculated by multiplying the distance from sidewall building line to the center of the building, in feet and multiplying this by the roof slope. Here is an example for a 36 foot width gabled roof with a 4/12 roof slope:  36′ X 1/2 (half the building width) X 4″ / 12″ = 6 feet. Adding this to the eave height gives an overall height of 20 feet, in this particular example.

DEAR POLE BARN GURU: Can they be built on a poured basement wall from a previous home? PAT in GREENEVILLE

DEAR PAT: As long as the concrete is structurally sound you should be able to utilize dry set column bases (ones designed specifically for post frame construction) to mount columns to the top of the foundation.

DEAR POLE BARN GURU: I have an existing Hansen pole barn 24×24 with a 9ft eave height and full length ridge vent, it has reflective roll insulation between the roof panels and the purlins. How can I further insulate it from Florida heat? I insulated the walls with rigid insulation. Can I add insulation under the existing reflective insulation at the roof? STEVE in ROSELAND

DEAR STEVE: I’d be contacting local installers of closed cell spray foam insulation. You will get close to R-7 per inch of foam (again, must be closed cell) and do not have the ventilation issues posed by using batt insulation between purlins. You will need to block off the eave and ridge vents for this to be an effective solution.

# Attic Insulation Types

Attic insulation has been a recent popular topic of discussion – so rather than reinventing the wheel, I’m sharing a relevant article written by Structure Tech Home Inspector Reuben Saltzman.
Originally published by the following source: Minneapolis Star Tribune — February 6, 2018
The following article by Reuben Saltzman was produced and published by the source above, who is solely responsible for its content. Hansen Pole Buildings is publishing this story to raise awareness of information publicly available online and does not verify the accuracy of the author’s claims. As a consequence, Hansen Pole Buildings cannot vouch for the validity of any facts, claims or opinions made in the article.

What’s the best attic insulation? That depends on your definition of “best”. What’s going to perform the best is definitely not the most cost-effective way to insulate an attic. But surely, you already knew that.
And I didn’t call you Shirley.
First, let’s discuss the most common types of insulation available for attics; spray foam, loose-fill fiberglass, cellulose, and fiberglass batts. Those aren’t the only types available, but they make up the vast majority of what’s used in Minnesota attics. For the listed R-values below, this refers to the material’s ability to resist the transfer of heat and is all per-inch. The higher the number, the better. The minimum R-value for a new Minnesota attic is R-49.

Fiberglass batts
Unfortunately, the easiest way to add insulation to just about any place in your home is to install fiberglass batts.  Fiberglass batts are typically the worst insulation for any job, but they’re easy to pick up in the store and easy to roll out, so people use them. The image below shows an atrocious installation at a two-year-old home in an upscale neighborhood of an inner-ring suburb of the Twin Cities. Yep, this passed the city inspection.

I won’t even discuss R-value because fiberglass batts have no place in an attic. Just don’t go there.

Cellulose

Cellulose has an R-value of approximately 3.5 per inch. The part that I love about cellulose is its ability to control air movement. While it doesn’t actually create an air barrier, it’s dense enough to stop most air movement to help control frost in attics. Not completely, of course, but it does a pretty good job. The same cannot be said for fiberglass.
If you check with the Cellulose Insulation Manufacturers Association, they’ll assure you that cellulose is definitely your best choice for insulation.

Loose-fill fiberglass

This seems to be all that’s ever used in new-construction homes and has an R-value of approximately 2.5 per inch. Like cellulose, you need a big machine to blow it in. You can’t simply buy it in bags and spread it around yourself. My biggest complaint with fiberglass is that it’s itchy and it’s a lung irritant. I’ve found that older fiberglass is way worse on your skin and lungs than the newer stuff, however. I have no scientific evidence to back this up, but I don’t need any. I’m completely sure of this based on personal experience.

Side note: I wouldn’t dream of doing any type of insulation work without wearing a respirator. Heck, I won’t even enter an attic without one.

There was a widely publicized study conducted by Oak Ridge Laboratories in 1991 that said that loose fill fiberglass insulation lost a lot of its insulation value once temperatures dropped below 20 degrees, making loose fill fiberglass an inferior product when compared to cellulose.  I contacted Andre Omer Desjarlais at Oak Ridge Laboratories about this issue, and he said: “This was true 20 years ago but all fiberglass manufacturers have changed their products appreciably since then and this is simply no longer an issue.”  I also contacted several insulation manufacturers about this, and they said the same thing and sent me some great information, which I posted on my website many years ago; click any of these links to read the documents from Certainteed, Johns Manville, or Owens Corning.  Loose fill fiberglass insulation will still experience convection, but not nearly as much as old fiberglass used to.
If you check with the North American Insulation Manufacturers Association, they’ll assure you that fiberglass or mineral wool is definitely your best choice for insulation.

Spray foam
From a performance standpoint, the best type of insulation is spray foam. There are two types; closed-cell and open-cell, aka 2-lb and ½-lb, respectively. They have insulation values of approximately R-6.5 and R-3.6 per inch, respectively. When installed properly, both types of insulation will fill all of the nooks and crannies and make for a perfect air barrier. When air can’t move through it, you have zero heat transfer through convection. Oh, and by the way, Icynene® is a brand name of open cell foam.

With closed-cell foam, you also get a moisture barrier at over 2″ thick. Because of this and the higher insulating value per inch, most foam insulation used in Minnesota is closed-cell. To tell the difference between the two, try poking it with your finger. You can easily poke a hole in open cell foam, but not closed-cell foam. That stuff is way too hard.

The big downside to either type of spray foam insulation is the cost. It’s expensive stuff, and it shouldn’t be installed by the DIYer. Of course, that’s not to say it can’t be done, it just shouldn’t be done. Professionals already have a hard enough time getting it right. Check out this article for more on that topic: Avoiding Problems With Spray Foam. The image below shows a botched spray foam installation at the rim joist of a new-construction home that I inspected.

A concern with spray foam insulation is the off-gassing of toxic poisons. I’m no expert on that matter, so I won’t discuss. Just be aware that it’s a concern, and do your own research. After conducting my own research, I concluded that I was comfortable putting it in my own home.

# Insulating a Post Frame Home Crawl Space

One of our clients has been erecting a post frame home in Colorado Springs, which is over a crawl space. Here is our discussion in regards to insulating the crawl space.

“While I have your ear, I had asked you a question earlier about getting the code required R30 in the 2×6 floor joists of my raised floor. I looked into your suggestion of spray foam and I got some quotes from local companies and I was shocked! One company quoted me \$16,000 to do 3″ in the floor and walls with some performance charts showing that the 3″ would satisfy the code (I’m a bit skeptical). I got another general quote of \$1.10/board foot and that it would take 5″ (i.e. \$5.50 / sq ft)  or \$12,650 just for the floor. The second company did suggest though that El Paso county allowed the R30 around the crawl space perimeter and no insulation in the floor… which leads me to my question…

What would be your thoughts of a non-vented crawl space using something like 15 mil plastic on the ground and up the sides the 18″ to the floor and the R30 spray foam from the ground to the floor level?  I could get that done for around \$2500. I’m still haven’t completely decided if I will used dense pack cellulose or BIBs for the walls but I’m pretty sure I don’t have the budget for spray foam in the walls.”

Mike the Pole Barn Guru Responds:

When I moved to Oregon from Eastern Washington in 1979, I was amazed at how different the construction techniques were from what I had grown up with. Eastern Washington was the land of full basements, whereas Western Oregon was predominately crawl spaces. The typical crawl space would have 6 mil black visqueen draped down the sides of the foundation and covering the ground, with R-19 unfaced batts used to insulate directly beneath the floors.

A variant of this was to use the crawl space as an air plenum, eliminating the need for heat ducts, and placing the unfaced insulation against the foundation.

This variant is basically a very slight spin away from what you propose to do.

Performance charts always frighten me as they general require some hocus pocus involving dead air space. Closed cell spray foam is R-7 per inch, so a claim of three inches in a system making R-30 sounds bogus unless the balance of the cavity is going to be filled with something like unfaced fiberglass batts. There is no question about closed cell spray foam being expensive, even the work I had done when we added the elevator shaft on the back of our home ran \$2.80 per square foot for four inches thick.

Personally, I have no issues with what you propose to do and would probably take it a step further and utilize the principles of Frost Protected Shallow Foundation insulation below the base of my wall steel. http://www.hansenpolebuildings.com/2016/11/frost-protected-shallow-foundations/.

# How to Insulate My Post Frame Garage

How to Insulate

I fear “how to insulate my post frame (fill in the blank)” is going to be my most answered topic for the next decade. Energy efficiency is the “hot” topic right now and sadly there are more folks trying to solve what they already have, than there were those who planned for it correctly in the beginning.

“Hello, I am wondering what the best and most cost effective way to insulate my post frame garage would be. I recently erected a 32’x48’post frame garage with glulam posts on 8’ centers, girts and purlins on 2’ Center’s, trusses on 4’ centers with 1’ overhangs with center soffit and ridge vent. Walls and roof are steel with double bubble between purlins and roof steel and tyvek between hurts and the wall steel. I will be building a wall to separate one of the bays as a metal shop for welding and fabricating. This will be the only bay that is heated and is 32’x21’. I am on a budget but my biggest concern is moisture. I installed the tyvek and double bubble hoping to positively effect the problem but am still hesitant to put fiberglass in the
walls but spray foam is out of my price range. I have seen some people cut 1 1/2” foam board to fit between the wall girts and either stop there or then frame traditional walls between the posts and add R13-R19 faced insulation. Is this an adequate way to insulate and will the foamboard keep moisture from the fiberglass? I will definitely be framing between posts and then covering with painted OSB regardless of the insulation method I choose. Also, I was leaning toward using fiberglass batts in the ceiling and then using the white liner steel under the trusses. They are 2×6 top and bottom cord trusses and rated for a ceiling. Fiberglass in the ceiling gives me the same moisture
concerns however. So I guess my question is, now that you know about my building, what is the best abs most cost effective way to insulate the portion of the building and avoid moisture? Spray foam is out of the question due to costs. I have been doing a ton of research but get different answers everywhere I go. Any help will be greatly appreciated. Thank you.”

Mike the Pole Barn Guru
As long as the Tyvek is well sealed, you will not be gaining moisture from the outside on the walls. What you need to create is a dry wall cavity. Completely fill the wall with unfaced fiberglass (you might consider using BIBs http://www.hansenpolebuildings.com/2011/11/bibs/)
and cover the interior of the wall with a well sealed vapor barrier (clear visqueen will do nicely). Cutting foam board is an exercise in
futility unless you can figure out how to completely seal it, if you
stop at this point.

For your ceiling – there is a good chance you will experience
condensation on the underside of the steel ceiling liner panels. With
your vented eave and ridge, blown in fiberglass is probably the best
answer. If you do not have raised heel trusses, you should probably look
at spray foaming the first couple of feet of the ceiling area in order
to reduce heat loss from not being able to gain full thickness of the
fiberglass.

# Condensation Solutions, A Ceiling the Right Way, and Timing

DEAR POLE BARN GURU: My deck roof is metal panels on 2×4 purlins, rafters are 2×6, like a pole barn. I am enclosing it, and need to stop the condensation. I spray foamed it with closed cell, but there is some condensation on the foam in a few places. It will be covered with drywall. Would a 6 mil plastic vapor barrier on the conditioned side work? MICHAEL in FRAZIER’S BOTTOM

DEAR MICHAEL: Provided you are able to reduce the moisture content within the building so as no vapor is being trapped between the vapor barrier and the foam, it should take care of the problem. In all reality, as long as you have no holes in the gypsum drywall, once it is painted you should have eliminated the problem of condensation against the insulation.

Now getting to the real problem – you have too much moisture in your building. If you did not place a well sealed vapor barrier under your concrete slab floor, you need to seal it. Walls also need a vapor barrier (without holes) on the conditioned side to prevent moisture from passing through.

DEAR POLE BARN GURU: I have a 40 x 80 pole barn with 8 foot truss spacing. I will be installing faced rolled insulation between each truss. What is the recommended ceiling product to install on the inside? Wood, metal, that will be lightweight and easy to install?? Thanks JEFF in SYCAMORE

DEAR JEFF: I see problems in your future….

Faced insulation is the absolute wrong product to use for insulating your ceiling. Any insulation placed at the truss bottom chord level should be unfaced. The best bet would be to blow insulation in above the finished ceiling.

In any case, you must adequately vent the attic space.

Now, on to the ceiling.

I am hopeful you have trusses designed with a minimum of a five psf (pounds per square foot) ceiling load, with 10 psf being even better. Confirm with your RDP (Registered Design Professional – architect or engineer) who designed your building, however 2×4 #2 ceiling joists at 24 inches on center between the bottom chords with joist hangers should adequately support a ceiling.

My choice of ceiling product?

5/8” Type X gypsum wallboard. It is affordable, weighs under three psf and provides fire resistance.

DEAR POLE BARN GURU: I’m putting up a building with a 3/12 pitch single sloped roof. radiant reflective polyethylene, vapor barrier insulation between the purlins and the metal roof sheathing. Probably rock wool batts under the 1-3″ draped barrier. Do you think the roof has to be vented, and how would this work? CHRIS in BROOKLINE

DEAR CHRIS: Yes, it would need to be vented and it is my feeling you are going about this entirely in the wrong direction. Your question is well timed, as I have just written an article on how to properly insulate between purlins, which will be posted soon. The basic gist is your best solution is to use closed cell spray foam applied directly to the underside of the roof steel.

# How to Properly Insulate between Roof Purlins

How to Properly Insulate Between Roof Purlins

Efficient climate control is becoming the buzz term for post frame construction. A challenge occurs when clients look to insulate between their roof purlins.

“I am building an all wood pole building. The purlins are 2x8s. I want to insulate the walls and up at the purlins to keep as much usable space as possible with an insulated building. How do I insulate and properly vent between the insulation and the underside of the plywood roof underlayment. If I use R19 it is 6 inches so there would be 2 inches of space between the insulation and the wood. Do I use something like a house bird block at each end of the building for each purlin space?”

Mike the Pole Barn Guru Writes:

The best solution lies in creating an unvented roof

It is quite possible to design an unvented insulated roof assembly which performs well, as long as you get the details right. In recent years, most building codes have begun to allow the construction of unvented insulated sloped roof assemblies. Many such roofs have failed over the years, however, so don’t get creative. Follow the rules.

For sake of brevity, I will limit this discussion to only as it pertains to post frame buildings with widely spaced trusses and purlins on edge.

First of all, you can’t use air-permeable insulation (for example, fiberglass batts, mineral wool batts, dense-packed cellulose, or blown-in fiberglass) to insulate an unvented roof assembly unless the roof assembly also includes a layer of air-impermeable insulation (spray polyurethane foam) directly below the roof steel or sheathing.

The 2009 IRC (International Residential Code) defines air-impermeable insulation as “an insulation having an air permeance equal to or less than 0.02 L/s-m² at 75 Pa pressure differential tested according to ASTM E 2178 or E 283.” Although spray foam insulation and rigid foam insulation meet this standard, fiberglass batts and dense-packed cellulose do not.

If you want to use just one type of insulation in unvented bays, you are limited to spray polyurethane foam. Another possibility, of course, is to build your roof with structural insulated panels (SIPs), which in most cases is cost prohibitive.

The code restrictions on the use of air-permeable insulation between purlins were developed to prevent the purlins or roof sheathing from rotting. When fiberglass batts are installed in unvented bays, the batts allow moist indoor air to reach the cold steel roofing or sheathing. That leads to condensation or moisture accumulation, followed eventually by rot. Since spray foam prevents air movement, it almost eliminates this problem.

It’s important to note, however, recent research suggests closed-cell spray foam is much less risky than open-cell spray foam in this location.

To summarize, there are really two practical ways to build an unvented roof assembly:

Install closed-cell spray foam against the underside of the steel roofing or roof sheathing, and no other type of insulation. Be sure the thickness of the spray foam is adequate to meet minimum code requirements. Remember open-cell spray foam is risky in all climate zones, and if open-cell spray foam is installed in this location in a cold climate, the underside of the cured foam must be covered with gypsum drywall which has been painted with vapor-retarder paint. Vapor-retarder paint is ineffective if it is sprayed directly on the cured foam.

Install a layer of closed-cell spray foam against the underside of the steel roofing or roof sheathing, and fill the rest of the purlin cavity with an air-permeable insulation. This type of assembly is designed to dry to the interior, so the assembly should never include an interior polyethylene vapor barrier.

# Post Frame Insulation in the South

Post Frame Insulation in the Hot and Humid South

Reader RICK in LUCEDALE writes: Dear Pole Barn Guru, I am in the planning stage for designing a post frame house. I live in a “Hot and Humid” climate in the southern US. Joseph Lstiburek, a building science guru, suggests having an unvented roof for my climate zone with the HVAC in the conditioned air space. The metal roof would have a layer of single bubble vapor barrier under it with BIBS insulation installed in the roof purlins. The walls would have a building wrap behind the metal siding and BIBS insulation. The walls would have a vapor barrier between the drywall interior and the insulation. Does the roof assembly need another vapor barrier on the inside? What happens at the intersections between the single layer bubble vapor barrier, the building wrap, and the sub slab vapor barrier? I assume I can use non venting closure strips at the ridge and closed non venting soffits? What size should the purlin be to get an R value of 30+ ? What would be your recommendations? Thanks.”

Mike the Pole Barn Guru writes:

Unless your post frame house will have extremely large purlins, you will not be able to get sufficient depth of insulation using BIBs. Energystar.gov recommends roof insulation values of R-30 to R-60 for your part of the country. BIBs provides an R value of 4.23 per inch, so to achieve a minimal value of R-30 would require at least a 2×8 purlin and would realistically not provide the insulation value I would personally be looking for.

I’d be looking at the use of closed cell spray foam insulation, which would give you R-7 or better per inch of depth. It also completely seals everything, eliminating the need for a reflective barrier below the roof steel. With 2×6 purlins, one could spray eight inches of foam completely filling the space between the purlins as well as covering the underside of them (and the underside of the roof truss top chords).

The goal here is to achieve a complete envelope seal of your building’s perimeter. You will not want a vapor barrier between the living space and the attic. The building wrap is not a vapor barrier, it is a weather barrier. The vapor barrier on the inside of the walls should be installed so as to be sealed into the roof plane spray foam and sealed tightly to the slab on grade (although I prefer living over a crawl space).

Soffits should be non-ventilated and closed cell foam closure strips should be used at the top and bottom of all steel panels.

# Insulating an All Wood Gambrel Barn

A reader writes: “Dear Guru.I have an all wood gambrel style pole barn that I’m converting to a shop.  I’ve installed forced air heat and am getting ready to insulate.  My exterior walls are Tyvek wrapped osb and vinyl sided.  I am wanting to use rigid board to help deter rodent nesting.

My questions are: for the walls should I cut and fit 1 1/2 inch board to fit all of the spaces between my girts before I layer rigid over the girts or can I layer over the girts to start?   The ceiling I was planning on installing 2″ rigid on top of the 2×4 truss bases and then applying a closed cell poly “Prodex” brand white faced on the bottom leaving the 3 1/2 inch air gap between the two. Prodex is claimed to be r16 and the rigid r10. Or is there a more suitable way to do the ceiling like cutting board to fit between said trusses and using Prodex on bottom with no air gap?

Thank you for your help.  I’m finding hundreds of articles and advice on metal buildings which mine is not and trouble finding a solution for my project.  Oh, and I live in northern Ohio”

Good move having Tyvek in your walls to prevent weather from seeping into your insulation cavity. If your walls are tightly framed (which they should be) the possibility of rodents getting into your wall cavities should be zero. Cutting and fitting insulation board to fit between framing members sounds like a mountain of labor, as well as pretty near impossible to be able to get a tight fit against every stud. I’d be inclined to use either closed cell spray foam insulation or BIBs insulation for walls.

Prodex is a radiant barrier and your chances of getting a measurable R value out of it more than one and change is not good. In a thorough 2010 study by the Canadian National Institute for Research in Construction, their conclusion: In a perfect state (with no dust on the surface), a radiant barrier with an air gap increased the efficiency of insulation in a wall by 10%. In other words, if the wall was already R6, adding ‘miraculous’ foil bubble wrap added .6, for a total of R6.6.

The best way to insulate your ceiling is to blow in cellulose or fiberglass to at least R45, if not R60. Do not place a vapor barrier under blown in insulation. Make sure the attic space above the insulation is adequately vented.

Mike the Pole Barn Guru

# Spray Foam Advantages Over Batt Insulation

Once again – confession time. I’ve never personally used spray foam insulation.

My oldest stepson, Jake, teaches high school chemistry and physics. He is one smart dude, as he has a master’s degree. When he added onto what was formerly his grandparent’s home, in the Browns Valley, MN area, he utilized closed cell spray foam insulation.

Not only is Jake smart, but he is also frugal, which tells me he did his research and compared costs of not only the original installation, but also savings over time.

Polyurethane foam insulation is available in closed-cell and open-cell formulas. With closed-cell foam, the high-density cells are closed and filled with a gas which helps the foam expand to fill the spaces around it. Open-cell foam cells are not as dense and are filled with air, which gives the insulation a spongy texture.

Polyurethane and isocyanate foams are applied as a two-component mixture which comes together at the tip of a spray gun, and forms an expanding material. While open-cell foams typically have R-values of 3.5 per inch, closed-cell foams can attain R-values of 7 per inch. Closed-cell foam is very strong, and structurally reinforces the insulated surface. By contrast, open-cell foam is soft when cured, with little structural strength. However, it provides superior sound resistance and allows timber to breathe. It is also fire-resistant and won’t sustain a flame.

Spray foam insulation costs more than batt insulation, but it has higher R-values. It also forms an air barrier, which can eliminate some other weatherizing tasks, such as caulking. This plastic insulation goes on as a liquid and expands to fill the available space, sealing all gaps and cracks and stopping any air leaks (This can also keep out bugs or other vermin). Another advantage is foam can fill wall cavities in finished walls without tearing the walls apart (as required with batts). It also provides acoustical insulation and increases structural stability. When building a new post frame building, this type of insulation helps reduce construction time and the number of specialized contractors, which in turn saves money.

The cost can be high compared to traditional insulation; however, open cell foams provide a better economical ratio. Open-cell foam is \$1 to \$1.20 per sq. ft. while closed-cell foam is \$1.75 to \$3 per sq. ft. (for a 2-by-4-framed wall). Both require professional installation.

Here is an earlier example of the investments into each: http://www.hansenpolebuildings.com/2014/02/insulation-foam-fiberglass/

Although for moisture control closed-cell foam is non-porous, open-cell requires a vapor barrier; however, the added cost of closed-cell foam may not be as advantageous as vapor barriers are usually required by building codes, regardless of the type of insulation used. Also, closed-cell polyurethane insulation levels can drop over time as some of the low-conductivity gas escapes and air replaces it in a phenomenon known as thermal drift.

In summary, DIY people use fiberglass as it is readily available, maintains a reasonable price ratio and is easy to install. Although it is not as easy to sustain the higher performance required by today’s insulation standards over time. It is also a health hazard. Most tract homes also use fiberglass insulation for the same reasons. However, the installation experience of the contractors can improve the overall performance.

Higher-end tract homes and custom homes tend to use the cellulose and foam solutions. They provide a superior insulation level and a number of other advantages, including air and vapor blocking, noise reduction and insect minimization.

As with all things, you get what you pay for and you can pay up front or pay later. There is no shortcut to energy efficiency and saving money.

We will be adding an elevator shaft to our own post frame building home later this summer. Although it is not a large footprint area, it will be over 30 feet tall and keeping it the temperature of the rest of the building will be a high priority – so I will be investigating spray foam myself.

# Insulation Between Roof Purlins

From the number of “Ask the Pole Barn Guru Questions” I receive and the new pole buildings I see being constructed, climate control is of very high importance. When I first entered the post frame industry 35 years ago, no one cared about it as virtually all ‘pole barns’ were farm buildings or small private garages. Not the case anymore! Pole Buildings run the gamut from heated shops, airplane hangers with living spaces and custom designed homes or year around lake cabins.

For heating and cooling, it is most efficient to have to control the least amount of space. Reducing the height of the area to be heated, will result in more comfortable temperatures in the area humans typically occupy.

This area can be reduced by finishing off the ceiling (with my personal preference being gypsum wallboard) and blowing in insulation above the ceiling – along with having a properly ventilated attic space. To give a rough idea of the volume of space differentials on a 40 foot wide by 60 foot long by 12 eave height building, having the most typical roof slope (4/12), about 25,000 finished cubic foot of area is to be conditioned with the ceiling, as opposed to nearly 34,000 cubic feet otherwise.

A significant amount of some sort of fuel is going to be used to heat or cool the extra one-third volume of space!

Some people prefer to insulate between the building roof purlins, however this can be fraught with potential challenges if not done properly.

The easiest solution, however possibly not most cost effective, is to utilize spray foam insulation. For most people, this is just not an affordable solution (read more at: http://www.hansenpolebuildings.com/2014/02/insulation-foam-fiberglass/).

Unfaced (typically fiberglass) insulation can be placed between the purlins. The purlin dimensions can be increased to allow for thicker insulation – which will be required (in most cases) if energy code requirements need to be met. By Code, airflow must be provided above this type of insulation. As roof purlins run the longitudinal direction of the building, 2×4 material can be placed flat on top of the purlins, running from eave to ridge. In order to utilize the space created by these 2x4s, the eaves and ridge will need to be ventilated.

A vapor barrier will need to be installed above the air flow area, if the roof is through screwed steel over purlins. An ideal solution would be a reflective radiant barrier, with another flatwise layer of 2x4s placed on top of it (in the same direction as the roof purlins). This creates another dead air space which improves the efficiency of the reflective radiant barrier.

Seriously looking to insulate between roof purlins while conserving energy? Design it right in the beginning!

# Dear Guru: Why Vapor Barrier?

DEAR POLE BARN GURU: I constructed a pole building with the help of Iowa based Amish group. They put up the main structure including metal roof. Due to city codes, I enclosed the 40x60x12 structure using 1/2 osb, house wrap and then vinyl siding. I want to use paper faced 4x8x4″ Styrofoam sheets on the walls, and roll insulation for the ceiling. My question is, do I use a vapor barrier on the walls after putting in the Styrofoam or none at all? And for the ceiling I would assume I would attach a vapor barrier to the bottom side of the trusses and lay the R-25 unfaced insulation on top of that. I have ridge vent and soffit vents. Thanks for your help! Curt in Center Point, IA DEAR CURT: For a properly performing system, your building should have a vapor barrier on the inside of all walls. The paper facing on the Styrofoam™ panels should be a vapor barrier. In order to perform properly, you need to make sure all edges and joints are tightly sealed, to prevent moisture from entering the wall cavity.

A vapor barrier should NOT be placed across the bottom of the roof trusses. If your building has steel roofing, I am hoping some sort of thermal break (like a reflective radiant barrier or similar) has been installed between the roof purlins and the roof steel, otherwise you are in for a plethora of problems. Warm moist air from your building needs to be able to pass through the ceiling and into the non-conditioned dead attic space, where it can be properly vented out of the ridge vent. You also should consider a greater R value in the attic. According to the North American Insulation Manufacturers Association http://www.naima.org/insulation-knowledge-base/residential-home-insulation/how-much-insulation-should-be-installed.html a minimum of R-38 should be installed in Iowa.

Mike the Pole Barn Guru

Dear Pole Barn Guru: We had hail damage to a post frame office building last Summer. Several months prior to the storm we had the side walls spray foam insulated (closed cell) and then framed and dry-walled. We have finally settled up with the insurance company and are ready to “re-skin” the building. The spray foam insulation was a significant expense and if we take off the metal siding the insulation will come off too. Here is my question: Can we simply install another layer of 29 gauge metal siding over the existing siding? Or can we fur out and install a different type of siding? Your input would be greatly appreciated!   KEN in Ft. Collins, CO

DEAR KEN: Although hail damage to steel siding and roofing is unusual, you have now found the downside to spray foam insulation applied to the inside face of it. If you place furring strips on the outside of the existing siding, you are most likely going to end up with the siding on the eave sides extending past the typical steel roof overhangs provided with most pole buildings. Plus, anything other than pre-painted steel siding is likely to come along with a lifetime of having to maintain it. In all probability, your best solution may very well be to install siding of the exact same profile over the existing steel. Screws will need to removed from each panel as you work your way down the wall, and replaced with screws of a larger diameter, as well as longer – in order to properly hold both layers of siding in place. With some patience, the results should turn out satisfactory

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