Tyvek Under Roof Steel?
Reader BRANDON in MIDDLEBOROUGH writes:
“Greetings I’m writing to discuss the best practice for what to place under a metal roof. I am building a post frame, for shop use and will eventually be insulted. I had initially intended to install a WRB, like Tyvek, under the sheet metal roof. My thought was that the inside of the shop will have a vapor barrier and insulation, thus the warm air would not come in contact with the steel and create condensation. Is this thinking flawed?”
WRBs allow moisture to pass through outward and not return inward. Even with a vapor barrier between your ceiling finish and floor of attic insulation, a properly vented attic does allow air to flow in through eave vents and out through ridge.
According to building scientist and founding principal of Building Science Corporation Joe Lstiburek, “Plastic vapor barriers should only be installed in vented attics in climates with more than 8,000 heating degree days.” For extended reading on heating degree days, please see: https://www.hansenpolebuildings.com/2022/11/what-is-degree-day/
You will want to make certain to have intakes and exhaust in proper proportion. Provision of 1 square foot of NFVA for each 300 square feet of attic floor if both following conditions are applicable:
• A Class 1 (≤ 0.1 Perm) or 2 (> 0.1 to ≤ 1.0 Perm) vapor retarder is installed on warm-in-winter side of ceiling when the structure is located in climate zone 6, 7, or 8.
• At least 40%, but not more than 50% of NFVA is provided by vents located not more than 3 feet below roof’s highest point.
• Provision for a minimum 1 inch air space between roof sheathing and insulation in attic at vent location. Air flowing into and out of your attic space may very well contain more moisture than desired, some of which could pass through your WRB and become trapped between WRB and roof steel.
My recommendation would be to order your roof steel with an Integral Condensation Control factory applied (read more here: https://www.hansenpolebuildings.com/2020/09/integral-condensation-control-2/).
DEAR ROBERT: All of these recommendations are based upon meeting energy code requirements for conditioned buildings. Washington State will be a stickler for these.
DEAR POLE BARN GURU: I have an 18×20 metal building (my house) that’s covered with spray foam insulation at 1.5″ on the walls and 2″ on the inside roof. The entire space will be conditioned. I am now reading that I may need to ventilate the space. I thought the purpose of the spray foam was to make it air-tight as a closed envelope system. So, can you explain what I need to know about venting? Also I have a large attic fan I am wanting to install in the wall. It has louvers on it to hopefully seal the air and water out. I plan to use it to blow hot air out when I come home instead of cooling while at work. Then turn on the AC. Also plan to use on days that’s it’s not terribly hot as a method of cooling the whole house like my grandparents did. I live in East Texas so I won’t be able to do this too often. Is this ok or should I scrap this idea? Can I expect the louvers to seal good enough? With this would I need a mechanical ventilation system? Thank you for your awesome help. I’ve been reading your advice for years. CINDY in TYLER 
DEAR POLE BARN GURU: What size posts should be used for 30′ x 40′ x 10 with 6/12 pitch in southern lower Michigan??? Storage/ garage / shop. STEVE in MARSHALL 
DEAR POLE BARN GURU: I am planning to build a 48 x 48 Monitor style pole barn that I intend to use for RV storage in the middle and living space above the center section. Will 6″ x 6″ posts be adequate spaced 12 ft apart or should I use 8″ x 8″? TIMOTHY in MARION 
DEAR POLE BARN GURU: When vinyl backed fiberglass insulation is installed on a metal roof, and is enclosed with rib closures and the roof extends 2″ over the purlin, should water still be able to reach the fiberglass? If so, should the fiberglass be replaced? Would this be something the contractor would be responsible for? Thank you. MARY in GREENEVILLE
DEAR POLE BARN GURU: 28W x 30L pole barn, single 10’w x 9’t garage door, single entry door…need plans for county permit. Where can I get? DOUG in HAVRE DE GRACE 
Spent many a winter day on Mount Bachelor’s slopes back when I lived in Oregon (even 4th of July one year), so am familiar with your turf (my step-brothers also live in your immediate area).
DEAR POLE BARN GURU: Hello Sir, hoping you can help answer a question I cannot get a straight answer on. Currently building a 30x40x14 building and have the walls and roof house wrapped with Kelly Clark Block It. Steel is going on the building now. I just ordered steel for interior ceiling and trying to figure out if I should add a vapor barrier to bottom of truss first. Thoughts? MATT in ILLINOIS 
DEAR NICK: Properly pressure preservative treated columns (UC-4B) should outlast anyone alive on our planet today, especially in climates (such as yours) not prone to termite infestations. As for improving ability to heat – dig a trench around outside of building at least two feet wide and two feet deep. Invest in 2′ x 8′ (or 4′ x 8′ to be cut in half lengthwise), R-10 EPS insulation boards. Attach vertically to exterior side of pressure treated splash plank with top of insulation even with top of interior concrete slab. Run another 2′ horizontally out away from building at bottom of vertical. Any portion of vertical insulation above backfill will need to be protected from UV rays. This should keep your slab from getting so cold, as well as help to avoid frost heave. If you are in an area prone to burrowing rodents, you should further protect insulation 
DEAR POLE BARN GURU: I am trying to decide whether to go with open cell or closed cell spray foam on my bare metal pole barn walls. I will be enclosing the walls with some material, most likely plywood. What are your opinions regarding the pros/cons (is one or the other worse for corrosion, condensation, other pertinent factors, etc.) of the two foam approaches? Thanks so much! TERRI in CHESTER 
When it comes to insulating a building envelope, there are various methods that can be used depending on the building’s purpose and the required level of insulation. However, combining metal roof and wall panels with spray polyurethane foam insulation (SPF) is widely considered one of the most effective ways to achieve secure, strong, and long-lasting insulation. Utilizing this method of insulation offers numerous benefits including the sealing of panel joints, creating a vapor barrier, providing thermal insulation, and producing air barriers.
The thickness of the initial pass fillet during picture framing should be at least 0.5 inch and should not exceed the maximum pass thickness recommended by the foam manufacturer. The minimum thickness of the initial pass is specified to provide enough material to activate the blowing agent and initiate the cure. After picture framing the perimeter of the area, the applicator fills in the center of the cavity using the maximum thickness recommended by the foam manufacturer. Maximum pass thickness varies by foam formulation and is listed in the technical data for each SPF product. Excessive pass thickness can result in inferior quality due to the increase in foam temperature during curing. For most closed-cell SPF, the maximum pass thickness is 1.5 to 2 inches. It is important for applicators of closed-cell SPF to follow manufacturers’ installation instructions on pass thickness limits and proper cooling times between subsequent passes made if they wish to develop the desired total thickness of the insulation.
Reader ERICA in WEST COLUMBIA writes: “Is there a specific bubble insulation brand you recommend? We will be using this as our vapor barrier in the roof. I’ve seen posts about this type of insulation disintegrating, so I’m wondering if the claims are exaggerated or if in fact it could be a certain brand. Also we are wanting to have cathedral ceilings throughout our building. Our roof is made with metal trusses and wood purlins. We are using bubble insulation and some type of batt insulation. What is the best method to vent if we are not going to have an attic space?”
Instead, look at spraying two inches of closed cell spray foam directly to underside of roof steel (between purlins), then fill balance of purlin cavity with unfaced rockwool batts. This will get you a higher R value and save on material and labor for a lot of 2×4. You should not vent either eaves or ridge in this case.
Making sense of Building Codes can be a daunting task, even for Building Officials. Post-frame construction is vaguely mentioned, at best, within Building Codes, leading to at best head scratching and at worst total confusion.
DEAR SONNY: Your radiant barrier acts as a vapor barrier. In order to prevent moisture from being trapped within your wall cavity you will want to use unfaced batts. I would recommend rock wool, rather than fiberglass or cellulose, as it is unaffected by moisture. Do not add an internal vapor barrier or retarder (such as clear poly).
DEAR POLE BARN GURU: I have a pole barn and need the outside sliding doors replaced. There are two doors and they’re each 10′ high, 8′ wide, and 1-2/3″ thick. Do you do this kind of work? ANDY in MIAMISBURG
DEAR MARK: You can easily check your concrete slab for a vapor barrier – place a wrench on floor overnight, next morning remove wrench and if a dark spot is seen where wrench was placed, you have no vapor barrier. If so, seal slab with a high quality sealant (here is an example 
DEAR POLE BARN GURU: Do you do 6 bedroom Barndo’s? I’m looking for a 6 bed, 4 bath barndominum floor plan. I have an idea of what I like in my head but 6 bedrooms one story are difficult. HEATHER in CLEBURNE
DEAR POLE BARN GURU: I would like to build a 20 foot wide x 40 foot long pole barn house in Fredericktown, Ohio. Do you know if the local building department would give a permit for a residential pole barn house, & what are the steps to obtaining that permit, who to call, & etc.? I’m at the beginning of that building process & would like to begin this summer. Thanks. RENEE in MOUNT VERNON
DEAR OMER: Thank you for being a loyal follower. Although I certainly have the ability to do your structural design, I prefer not as it would place me in a position of liability as well as practicing engineering without a license. Even though you are building a permit exempt agricultural building, I would encourage you to build from engineered plans – as you say you want it to be “rock solid for generations to come”. Only having it fully engineered is going to give you this sort of assurance. Sawing your own wood is also problematic 
DEAR POLE BARN GURU: I have a 40″ side wall pole barn with rough cut red oak 6″ x 6″ posts on 8″ centers. The posts are bolted to engineered U braces that are secured in the concrete. The top of the posts have 2″ x 8″ rough cut red oak inside and out secured with through bolts. The bottom of the posts where they are secured to the u braces seem to me as a pivot point. Would “Y” bracing from the top down to about 4 foot to the center of each post be sufficient, or do I need to come down to the bottom near the U braces and run a board all the way across the 40 foot span and then 1 in the middle and “W” brace the side walls. I was thinking of keeping it open with the oak sealed with linseed oil or equivalent for looks, but could close in the sides. Thanks RUSS in STUARTS DRAFT
DEAR JACK: A great place for info is the 
Now, I’ve been told to do closed cell insulation on my walls 1” thick. I wanted an opinion on whether to go every inch of the walls top to bottom all the way to the roof? Would I benefit from the insulation at all by just going to the 10’ mark (my lowest wall height) because that’s as high as my interior osb is going anyway. I guess what I am asking is it any benefit to insulate closed cell up to 10’ mark from the floor and then just bubble foil the main shop above 10’ to give a finished look? THOMAS in PLEASANT HILL
DEAR POLE BARN GURU: We are looking to build a 40×90 pole barn, with 1/3rd being used for storage and 2/3rds for a vehicle maintenance. Both will be heated and storage cooled. Concrete floor, shingled roof, insulated, vertical metal siding, no windows, 5 overhead doors on the same side. Can you span the trusses from wall to wall and provide the above with no center columns? Thank you, ROD in CLEVELAND
DEAR POLE BARN GURU: I am building a 24X48 pole barn, but instead of using a slab, I would like to have a conventional foundation. Is there any advice you can give me on layout and construction using this method? ROBERT in FRENCH CAMP
Thank you. HAROLD in WELCH
DEAR JOSHUA: Having a continuous footing and foundation will not act as a vapor barrier (but will add to your expense). Building Codes require a minimum 6mil vapor barrier under any concrete slab poured in a conditioned building (and we recommend using one under any interior pour). We normally recommend using thicker material (ideally 15mil) to help prevent damage during pouring slabs on grade.
DEAR POLE BARN GURU: Combination business and residential? Square footage of 7000. Can it be done? KEVIN in FLORISSANT
On the walls I plan to cut-to-fit the 1.5″ closed cell rigid foam board and install between the girts. After foam board is installed I intend to line all interior walls with painted OSB. 
DEAR POLE BARN GURU: 
Here is how we did it for my Sales Manager, Dan, in my past life when I was a post frame building contractor. Dan wanted a 30′ x 50′ monitor style building for a garage/shop and then an office above. We had engineered a clearspan roof/floor truss combination for support of wings and second floor. This system had a double truss every 10 feet. At eight feet from each outside wall, we mounted columns to these trusses to support roof of raised center. Joists we placed for ceilings and floor system. With an 8/12 roof slope, upper level used scissor trusses with a 5/12 interior pitch.
DEAR POLE BARN GURU: 
DEAR MATT: 
If you are getting condensation inside of your WRB it is due to excess moisture in your building. You need to eliminate or minimize sources of water vapor (seal any concrete slabs-on-grade if a well-sealed vapor barrier was not installed beneath). Proper ventilation from eave to ridge will also help to alleviate this challenge.
DEAR POLE BARN GURU: 
You have only a couple of realistic options – first one is ugly, remove roof steel and place a thermal break between conditioned space and roof steel. This could be as simple as adding a Reflective Radiant Barrier. It never comes back together as well as it did originally, and when all is said and done, option number two will be less expensive.
DEAR THERON: Thank you for your inquiry.
DEAR DAN: You should have some sort of barrier between wall framing and wall steel to prevent condensation within the wall cavity. Wall cavity moisture can lead to a plethora of challenges – premature rusting of steel siding, rot, mold and mildew on wood framing and lack of performance of fiberglass insulation.
DEAR MAX: I happen to live in a two story post frame shouse (shop/house) with a partial third story. Back in the great state of Washington, I also have a three story post frame building with roof top deck! We can provide any low rise building with up to 40 foot tall walls and three floors (or 50 feet and four floors with sprinklers).
As much as we would like to believe otherwise, not every client or contractor is morally trustworthy. We have provided builder information to potential clients and had these same wonderful clients try to get builders to go around us and cut a better deal for their building materials. Conversely, we have had builders tell our clients to buy everything direct from them and they will get a better deal.
Absolutely, you should be concerned about attic condensation. Warm moist air from inside your conditioned space will rise into attic and when it comes in contact with your building’s roof steel’s cooler surface it will condense (even with ventilation). For sake of ease of construction I would recommend an integrated Condensation Control Membrane (
Properly sealed Radiant Reflective Barrier, Steel with Condenstop or Dripstop factory applied, 30# felt or Ice/Snow Shield over plywood or OSB, or (if none of those previously mentioned) two inches of closed cell spray foam. Me, I’d vote for Condenstop/Dripstop as it is a relatively low investment and easy to install. Blow in insulation on top of your ceiling.
DEAR POLE BARN GURU: Should I put plastic down under the stone floor in a steel building? BOB in WYALUSING
LOAD (in psf – pounds per square foot) X (½ building width plus sidewall overhang in feet X 12”) X Distance spanned by beam squared (in feet)
Without some method of condensation control beneath your building’s roof steel you are going to have moisture problems. Blown in fiberglass or cellulose insulation will lose their effective R value once they get wet. A practical solution will be to have closed cell foam insulation sprayed upon roof steel underside. Normal recommendation would be two inches thick however your local applicator(s) can give you their best input from experience. Make sure spray foam does not block either eave air intakes or ridge exhaust points. You can create a “dam” at eaves to keep blown in insulation from filling soffits, by use of ripping high R closed cell insulation boards. Again, make sure not to block incoming airflow (you need a minimum of least one inch of free area above insulation boards). 
In doing my research on the whys and why-nots I found apparently there are some spray foam insulation contractors who will not spray foam against house wraps, apparently from not being able to guarantee their product would properly adhere to the house wrap.
