Tag Archives: attic ventilation

Unvented Post Frame Attics

Unvented Post Frame Attics

Energy efficiency concerns have literally become a “hot” (pun intended) topic in new construction, and post frame construction methods are no exception to inclusion. Traditionally buildings have had insulation placed or blown into dead attic spaces, directly above a ceiling. Unvented attics have entered fray as an alternative.

 

To construct an unvented attic, air-impermeable insulation (think closed cell spray foam) will be applied in direct contact with steel roofing (or sheathing) underside and gable end walls so as to tie roof insulation into wall insulation below. Moving insulation boundary to the roof deck underside allows temperature and humidity conditions in the attic to be reasonably close to those of the conditioned building interior.  No attic floor vapor retarder or insulation should be installed with an unvented attic assembly.

Closed cell spray foam insulation products meet code requirements for use of an air-impermeable barrier applied to underside of roof. This prevents air infiltration and limits accumulation of airborne moisture in the attic. Using closed cell spray foam insulation applied to underside of roof deck eliminates a need for alternative methods of condensation control such as reflective radiant barriers or CondenStop (https://www.hansenpolebuildings.com/2014/07/condenstop/).

In hurricane or wildfire prone areas wind-driven rain or embers cannot enter an unvented attic assembly, as there are no vents.

Spray Foam

Vented attic designs originated in cold climate areas. In these cold climates, attic ventilation is commonly used to remove warm, humid air from attic spaces. Air leakage from conditioned spaces below a ceiling greatly increased likelihood of moist air entering attics. Without adequate attic ventilation, the underside of the roof deck can have condensation form and interior heat can cause roof surface snowmelt leading to ice damming.

Use of venting to control moisture in cold climate attics comes with some inherent challenges. In high snowfall areas, snow accumulation and drifting can often block ridge vents. This limits venting and increases potential of damage due to ice damming, roof leaks and condensation.

Closed cell spray foam does not come without added upfront investment costs, however some of these can be mitigated in materials and/or labor savings.

Considering climate control of your new post frame building? If so, an unvented attic may be a viable solution worth investigating.

 

My Pole Barn Needs Ventilation

My Pole Barn is a Sauna in Summer- and needs ventilation!

“Hey there Pole Barn Guru, got a question about ventilation.

Just bought a house with a pole barn on the property. I believe it’s only about a year old. 30 x 32.  It has no soffits or windows, only a standard garage door and walk-in door.

Metal siding and roof, and the underbelly of the roof has a vapor barrier. There are also two ceiling fans in here.

I don’t care that it’s cold inside the building in winter, but it’s like a sauna now in the summer.  I was thinking of an exhaust fan to pull out the heat, but I don’t know if that’s waste of money. How does one ventilate this thing without having to bulldoze it and start over?

Thanks.

Dezy”

Mike the Pole Barn Guru responds:

Since you cannot increase the amount of venting in your soffits (as you have none), you’ll need help from power vent fans.

Attic vent fans can be hard-wired and equipped with a thermostat and/or humidity sensor so they automatically cut on at a preset moisture level or temperature. You could also install solar-powered attic vent fans, though it has been found most solar models aren’t powerful enough to be very effective.

To determine what size power vent fan(s) you need for your attic, you first need to know the size of your attic in square feet.

Attic Size

To determine the size of your attic, multiply the width by the length of the attic floor in feet. In your case 30′ wide x 32′ long = 960 square feet of attic space.

Vent Fan Size

Next, multiply the square feet of attic space by 0.7 to get the minimum number of cubic feet of air per minute the fan should be rated to move. 960 sft x 0.7 = 672 CFM minimum fan rating.

Add an additional 20% (CFM x 1.20) if you have a steep roof, and 15% (CFM x 1.15) for a dark roof. Attic vent fans are commonly rated from 800 to 1,600 CFM, making one fan suitable.

Vent Fan Location

Install gable mounted fans on the gable vent at end of the building facing away from the prevailing winds.

Intake Air Vents

It’s also important to have plenty of soffit or gable vents for the fan to draw air into the attic. To find out if you have enough vent space, divide the cubic feet of air per minute the fan(s) is rated for by 300 to come up with the minimum number of square feet of intake vent space needed for that size fan. 672 CFM ÷ 300 = 2.24 sq. ft. intake vent area

If you prefer the answer in square inches rather than square feet, multiply the answer by 144 and round to the nearest inch (2.24 x 144 = 322.56 sq. in. vent area).

 

Pole Barn Insulation, Part II

Continued from yesterday’s blog:

(1) Storage – if you ever believe anyone might ever in the future desire to climate control then provision should be made for making it easiest to make future upgrades.

At the very least a reflective radiant barrier (single cell rather than wasting the money for the extra approximately 0.5 R from double bubble), an Integral Condensation Control (https://www.hansenpolebuildings.com/2017/03/integral-condensation-control/) or sheathing with 30# felt should be placed between the roof framing and roof steel to minimize condensation.

If a concrete floor is poured (in ANY use building), it should be over a well sealed vapor barrier.

For now we will assume this building is totally cold storage. If it might ever (even in your wildest dreams) be heated and/or cooled include the following in your initial design: Walls should have a Weather Resistant Barrier (https://www.hansenpolebuildings.com/2016/01/determining-the-most-effective-building-weather-resistant-barrier-part-1/) between the framing and the siding. Taking walls one step further would be ‘commercial’ bookshelf wall girts (https://www.hansenpolebuildings.com/2011/09/commercial-girts-what-are-they/).

In the roof – have the trusses designed to support a ceiling load ideally of 10 pounds per square foot (read about ceiling loaded trusses here: (https://www.hansenpolebuildings.com/2016/03/ceiling-loaded-trusses/). Trusses should also be designed with raised heels to provide full depth of future attic insulation above the walls (https://www.hansenpolebuildings.com/2012/07/raised-heel-trusses/).

Make provision for attic ventilation, by having an air intake along the sidewall using enclosed ventilated soffits and exhaust with a vented ridge.

Any overhead doors should be ordered insulated – just a good choice in general as, besides offering a minimal thermal resistance, they are stiffer against the wind.

(2) Equine only use: Same as #1 with an emphasis upon the ventilation aspect.

(3) Workshop/garage and (4) Garage/mancave/house are going to be the same – other than whatever the client is willing to invest in R value, being the major difference.

Adding onto #1 for the walls the low end would be unfaced batt insulation with a 6ml visqueen vapor barrier on the interior. Other options (in more or less ascending price and R values) would be Mineral wool insulation as it is not affected by moisture (https://www.hansenpolebuildings.com/2013/03/roxul-insulation/),  BIBs (https://www.hansenpolebuildings.com/2011/11/bibs/), closed cell spray foam in combination with batts and just the closed cell spray foam (https://www.hansenpolebuildings.com/2016/07/advantages-spray-foam-over-batt-insulation/).

For added R value and a complete thermal break, add rigid closed cell foam boards to the inside of the wall.

Once a ceiling has been installed, blow in attic insulation.

For (4) a Frost-Protector Shallow Foundation (https://www.hansenpolebuildings.com/2016/11/frost-protected-shallow-foundations/) with sand on the inside rather than a thickened slab is an excellent and affordable design solution.

For insulation solutions which follow the roof line, the best bet is going to be the use of closed cell spray foam, as it solves the potential condensation on the underside of the roofing and does not require ventilation above.

In most cases, the steel trusses fabricated for post frame buildings are either not designed by a registered engineer, are not fabricated by certified welders or both – so it makes it difficult for me to recommend them as part of a design solution.

With scissor trusses, they can be treated the same as a flat ceiling would be, provided the bottom chord slope is not so great as to cause blown in insulation to drift downhill.

 

Insulating an Attic Bonus Room

Attic bonus rooms seem to be the rage – Hansen Pole Buildings does more than a few of these and the trend seems to be increasing in popularity. With this comes how to properly insulate an attic bonus room.

There are more than a few challenges when it comes to utilization of attic space for a bonus room. Highest amongst them are the space is neither free nor inexpensive. The lack of accessibility becomes another factor. At our home outside of Spokane, Washington, my lovely bride and I have our office in a bonus room above our garage. Now a paraplegic, due to her motorcycle accident in 2015, my wife will probably never be able to access our office there again. Our offices have moved to a handicapped accessible space in our home, and the attic space now becomes a catch all for overflow of what I endearingly call “stuff”.

With all of this said, there are still going to be clients looking at attic bonus rooms as a design solution. In an earlier article, I had written about how to insulate the knee walls of bonus rooms: https://www.hansenpolebuildings.com/2013/09/bonus-room-3/.

There are other attic bonus room areas which need to have attention paid to them, however.

Enter closed cell spray foam (read more about spray foam insulation here: https://www.hansenpolebuildings.com/2016/07/advantages-spray-foam-over-batt-insulation/).

There are two perfect areas of the attic bonus room which are ideal for closed cell spray foam as a solution.

The most important of these is any sloped area which has become part of the bonus room. As heat rises, it is most likely to escape through the narrow areas of the slope, or the edges of the area above the “cross tie” (the flat ceiling at the center of the bonus room). At R-7 per inch, a 2×6 truss top chord can be filled to R-38.5 with closed cell spray foam, as opposed to R-19 with fiberglass batts!

Bonus room floors can also be cold. Often attic bonus rooms (such as my own) are above unheated garage or storage space. By utilizing spray foam directly to the underside of the floor sheathing, cold floors can be minimized.

In any case, be sure to provide adequate ventilation in any enclosed attic spaces. These would include the area above the cross tie and the spaces outside of the knee walls.

Will My Trusses Hold Added Ceiling Dead Load?

Understanding the Needs of Load Bearing Finish

I sadly hear this story all too often. A brand new post frame building which quite possibly will not meet the load needs of the owner due to lack of due diligence upon the part of whomever sold the building. Here is the story and my response:

DEAR POLE BARN GURU: I have a newly constructed 30 x 40 pole barn, truss spacing 8′ o.c., 2×6. 5 PSF. I am wondering if I can cap the ceiling and insulate without over loading the bottom chord? Recommended material used for this? In laymen’s term how do I determine how much dead weight can be applied to bottom chord? At the time of construction I did not understand the load bearing needs of interior finish. Thank you – JUSTIN in MONROE

Dear Justin: Yours is one of the two most frequent issues following completion of a new post frame building, the other being insulating. Most post frame builders and building suppliers are afraid to have this discussion with potential new building owners – for fear the increase in price will scare them off! In my humble opinion, part of delivering “The Ultimate Post Frame Building Experience” is to discuss important issues such as this with clients BEFORE the building design process gets too far down the line. Shame on whomever you invested hard earned dollars with for not having had this discussion with you.

You should have been furnished with engineer sealed truss drawings for your building. If you were not, call whomever you purchased the building from, and request them. On the truss drawing will be a section which outlines all of the live and dead loads which the trusses are designed to support. If the number next to BCDL (Bottom Chord Dead Load) is less than five psf (pounds per square foot) then the trusses and the building are not designed to support a ceiling.

Take heart, if the design BCDL happens to be less than five, you can contact the truss manufacturer and for a nominal fee they can usually (especially with smaller truss spans like yours) get an engineered repair (or fix) to upgrade the trusses to support the load of the ceiling. This is never as inexpensive as having it done right to start with.

In order to install a ceiling in your building, you will need to ventilate the dead attic space you will be creating. More reading on ventilation is available here: https://www.hansenpolebuildings.com/2014/02/pole-building-ventilation/

Once past the truss loading and ventilation stages, the adequacy of the footings for the building columns to handle the extra load could pose a challenge – IF your new building was not designed to support a ceiling load to begin with. You should consult with the engineer of record who sealed the plans for your building, to verify the ability of the footing to properly transfer the loads from your building to the supporting soils. If you are unable to contact him or her, a competent engineer should be contacted to confirm what you have works, or to design a repair if not. Don’t overlook this step, or assume what you have will handle the load – we all know what assuming ends up causing – nothing but grief and having a column settle due to the added weight is not a problem you want to have to solve.

Ceiling joists will need to be installed between the bottom chord of the trusses. To support 5/8″ gypsum drywall, #2 (not standard & better) grade 2×4 or 2×6 can be placed 24 inches on center supported at each end with 2×4 joist hangers.

Planning a new post frame building? If you feel you or anyone after you who uses your new building will ever have the desire to install a ceiling (trust me – it happens a lot), at the very least have the trusses designed to support a ceiling load, as well as make provisions for adequate ventilation. The headache you solve, may very well be your own!

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 https://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

Dear Pole Barn Guru: What is the Proper Wind Shear Bracing?

New!  The Pole Barn Guru’s mailbox is overflowing with questions.  Due to high demand, he is answering questions on Saturdays as well as Mondays.

Welcome to Ask the Pole Barn Guru – where you can ask questions about building topics, with answers posted on Mondays.  With many questions to answer, please be patient to watch for yours to come up on a future Monday or Saturday segment.  If you want a quick answer, please be sure to answer with a “reply-able” email address.

Email all questions to: PoleBarnGuru@HansenPoleBuildings.com

Dear Pole Barn Guru: What is the proper wind sheer bracing for a 60’W x 80’L by 20’H monitor pole barn with a 20′ center aisle and a second story? The raised center portion has 20 foot walls, then another 6 feet to the center at the ridge.

The 2 sheds on either side are 10′ at the edge and intersect the center at about 15′. The entire structure is made of poles on 20′ centers, Lvl beams and ladder trusses for floor and trussed roofs. It is unprotected from the wind. We are in central Texas. TEETERING IN TEXAS

DEAR TEETERING: This is why it is such an excellent idea to order complete pole building packages from a company who can run all of these calculations accurately in advance. Then buildings are designed to resist the proper loads, including wind shear, without having to search for solutions in the middle of the game.

Provided all of the columns are adequately sized and embedded …..

Steel roofing should be 3′ width, minimum 29 gauge, with high ribs every 9″, attached to 2x purlins on edge spaced no more than 28″ o.c. Roofing should attach to purlins with #12 x 1-1/2″ diaphragm screws at 9″ o.c. at each purlin. At eave and top edges of panels place screws on each side of every high rib.

Provided you attach the endwall steel the same as the roof steel, you should be able to have up to 19 lineal feet of openings on each endwall without further reinforcement.

All of this should be reviewed by a Registered Design Professional (RDP – engineer or architect) for structural adequacy.

DEAR POLE BARN GURU:  Is it possible to get a quote to have it assembled? MINDFUL IN MORRIS

DEAR MORRIS: We are not contractors, however fair market value for labor is typically about 50% of materials costs.

I recommend placing an ad on Craigslist under “labor gigs” such as:

Contractor needed to assemble pole building kit package on my clear level site in Morris County. 24’x40’x14′ includes 12″ overhangs, a 12’x12′ overhead door, entry door and wainscot. I will provide all materials except for nail gun nails. Willing to pay $4000-5000 depending upon experience and references.

DEAR POLE BARN GURU: I have a metal building constructed of 2×4 square tubing and 4×4 metal posts. 3″ × 1.5″ c purlin is welded with the c side down spanning across my 2×4 square tubing roof beams. I have a metal roof screwed to the c purlin. The building wraps around an existing building and looks like an L from a top view and has a 2 on 12 shed roof with a hip. The building is “stand alone” and attached to the other structure only with a side wall transition piece. The metal roof slides under the eve of the other structure with the side wall transition on top. I also used the vented enclosure under the sidewall transition for ventilation. I also have a ridge vent at the hip with the vented enclosure under the hip cap. I have a 12″ soffit around the outside perimeter of the metal structure with soffit vents every 5 ft. I have a 4 ft cedar picket half wall around the outside perimeter with the remaining height of the wall in screens….basically a screened in party room but I plan on switching out the screens for heavy plastic sheeting in the winter. I have a hot tub inside and I want to insulate the roof and install a ceiling. 1st question: do I have to install a vapor barrier IF I am using 3/8 marine plywood for the ceiling and the attic side of the sheathing is covered in heavy vinyl….sheathing was once election signs. I am attaching the ceiling directly to the bottom of the 2×4 roof beams following the slope of the 2 on 12 pitch.

2nd question…..because I only have the 4″ depth of the 2×4 sq tubing + 1.5″ depth of the c purlin = 5.5″ total for insulation AND vent space, how would you insulate? Spray foam is too expensive for me. I want to reduce the radiant heat in the summer and I am concerned with the humidity from the hot tub especially in the winter. Any help would be greatly appreciated. TOASTY IN TEXAS

DEAR TOASTY: My guess is you are going to be creating an inadequately ventilated dead attic space. You need to have 1/150th of the “foot print” of your space, as ventilation, equally divided between the eaves and the ridge. In your case, you have no ability to adequately vent the high side of your roof, as it abuts another building.

Even though the best solution might well be to tear everything down and construct a new building, chances are you would not look favorably financially upon it as the end all.

Probably your best bet is to install an A1V reflective radiant barrier beneath the roof purlins (https://www.buyreflectiveinsulation.com) with the reflective side up. Make sure every seam is tightly sealed. This should help reduce the thermal gain in the summer. If you are going to create a dead attic space between a ceiling and the reflective radiant barrier – powered attic vents in each end could be a good investment.

As for humidity from your hot tub, install air inlet vents near the floor and exhaust vents near the ceiling line. You may also need to have one or more powered vents in the walls of this area.