Tag Archives: heating degree days

Condensation Control, Ventilation or Insulation

Most Builders Do Not Understand Condensation Control, Ventilation or Insulation

Crucial to proper performance of climate controlled buildings of any sort is condensation control, adequate ventilation and insulation. Sadly, most builders do not understand how to come up with a proper design solution.


“Hello, I was wondering if you had a minute for a quick question ? I have read posts for countless hours regarding vapor barrier between trusses and roof steel and I can’t find the answer I’m looking for. I’m building a 44×96 that will have in floor heat in South central Wisconsin (by Madison) so we have cold winters and humid summers. Walls will be spray foamed. I will be applying a vapor barrier (poly) to the bottom of roof trusses and then putting a metal ceiling on and spraying fiberglass insulation above the ceiling. Fully vented ridge cap and 2’ overhangs with vented soffits around the entire building. The builder did not put any vapor barrier down between the roof steel and the trusses so the steel is directly on the trusses. Could this end up creating a moisture issue or will I have enough air flow in the attic space that I do not need the vapor barrier between the roof metal and trusses ? Thank you very much for your time.”

Thank you for reaching out to me Aaron, I am always available to answer questions.

Your builder sadly did you no favors in not having a means to prevent condensation on your roof steel underside. It also requires having an actual thermal break, not just a vapor barrier. Your best solution now is to have two inches of closed cell spray foam applied directly to the underside of your roof steel, making certain they do not block ridge vents. Without this thermal break, expect to end up with damp insulation.

If your builder installed vented soffits on your building’s end overhangs, you need to find a method to block them off – otherwise your attic will not vent properly.

In regards to ceiling vapor barriers, Joe Lstiburek (building scientist and founding principal of Building Science Corporation) says plastic vapor barriers should only be installed in vented attics in climates with more than 8,000 heating degree days. Even in South central Wisconsin, you are not to this point.

You’ll want to verify a correct ratio of air intake from soffits to air exhaust at ridge (chances are good your builder did not). At least 40% and no more than 50% of your attic’s net free ventilating area (NFVA) should be at the ridge. You may need to block off some of your sidewall eave vents to get the ratio correct.

I would also recommend you blow in rockwool insulation in your attic rather than fiberglass. Fiberglass insulation loses R value when outside (attic) air temperatures are low and is also affected by even small amounts of moisture.

Does My Pole Barn Need a Vapor Barrier?

Does My Pole Barn Need a Vapor Barrier Above the Metal Ceiling?


“I have a new pole barn with bubble vapor barrier under roof metal. I am installing a metal ceiling with R-38 cellulose in the attic. Do I need a vapor barrier above the metal ceiling? The barn will be heated somewhat in winter.”

From Mike the Pole Barn Guru:

Kudos to you for recognizing bubble wrap products advertised as ‘insulation’ are, at best, a good vapor barrier if properly sealed!

My son Brent is working on his doctorate at UMass Amherst, so I utilized Building and Construction Technology information from their Department of Environmental Conservation in penning this response.

My concern is with blowing in cellulose above a steel ceiling. From UMass:

“Wet insulation of any stripe is bad. But cellulose is hygroscopic. It’s able to soak and hold liquid water. Undetected leaks can wet cellulose causing it to sag within framing cavities. Water leaks can compress the blanket of fiber and in extreme cases, can create a void space, degrading its thermal value. Another concern is that chemicals used to protect cellulose from fire make it potentially corrosive in wet environments. Tests conducted by the Oak Ridge National Laboratory show chemical treatments used to treat cellulose can cause metal fasteners, plumbing pipes and electrical wires to corrode if left in contact with wet, treated cellulose insulation for extended periods of time. “

I’d much prefer to see you blowing in rockwool as it remains unaffected by moisture. Last thing you want is to have damp cellulose insulation chemicals eating holes in your ceiling.

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.”

As your site has under 8000 heating degree days a year, you should not have a ceiling vapor barrier. You do need to ensure your newly created non-conditioned dead attic space is adequately ventilated at eave and ridge (https://www.hansenpolebuildings.com/2018/03/adequate-eave-ridge-ventilation/).

For extended reading on heating degree days, please see: https://www.hansenpolebuildings.com/2022/11/what-is-degree-day/

What is Degree Day?

What is a Degree Day?

In an article penned a year ago (https://www.hansenpolebuildings.com/2021/11/insulated-ceiling-vapor-barrier/), I referenced building scientist and founding principal of Building Science Corporation Joe Lstiburek.

Joe stated, “Plastic vapor barriers should only be installed in vented attics in climates with more than 8,000 heating degree days.”

However my bad was, I did not define what a degree day is.

Degree days are measures of how cold or warm a location is. A degree day compares mean (high and low average) outdoor temperatures recorded for a location to a standard temperature, usually 65° Fahrenheit (F) in United States. More extreme outside temperature, higher degree day number will be. A high number of degree days generally results in higher levels of energy use for space heating or cooling.

Heating degree days (HDD) are a measure of how cold temperature was on a given day or during a period of days. For example, a day with a mean temperature of 40°F has 25 HDD. Two such cold days in a row have a total of 50 HDD for a two-day period.

Cooling degree days (CDD) are a measure of how hot temperature was on a given day or during a period of days. A day with a mean temperature of 80°F has 15 CDD. If the next day has a mean temperature of 83°F, it has 18 CDD. Total CDD for these two days is 33 CDD.

What do people use degree day data for?

People study degree day patterns to assess climate and to assess heating and cooling needs for different regions.

What are population-weighted degree days?

Degree day data can be weighted according to the population of a region to estimate energy consumption. U.S. Energy Information Administration (EIA) uses population-weighted degree days to model and project energy consumption for United States and for U.S. census divisions. Learn more about EIA’s degree day modeling and forecasting methodology.

Where can people find degree day data?

Newspapers may publish degree day information in weather sections. Electric and natural gas utilities may publish degree day information on their websites, and some utilities include degree day data in customer utility bills. Several weather data-related websites publish daily high and low temperatures and degree days for specific locations. National Weather Service Climate Prediction Center is a source for historical United States’ temperature and degree-days.

Historical monthly (from 1973) and annual (from 1949) population-weighted degree days data for United States and U.S. census divisions are available in Monthly Energy Review Tables 1.9 and 1.10.

Historical monthly and annual population-weighted degree days for U.S. census divisions for 20 years and a forecast for one to one-and-a-half years are available in Short-Term Energy Outlook (STEO) Data Browser.