Insulation For Roof Without Attic – Spring is here! As we welcome the long days and beautiful NC weather (minus the pollen!), we also know that heat and humidity are just around the corner. High-performance homebuilders in North Carolina are uniquely challenged by our humid mixed climate to deliver healthy, energy-efficient and weather-resistant homes.
Building with a closed ceiling is an option that many explore for many reasons: tighter envelopes, duct placement in a planned space, lower ceiling temperatures, but it presents potential problems. In this post, we explore attic sealing best practices and strategies to implement to avoid rot, condensation, and moisture issues.
Insulation For Roof Without Attic
To give or not to give – that is the first question. While I will cover the best practices for a sealed ceiling, it should not be assumed that sealing the ceiling is always the best way to go. Properly ventilated rooms can provide more insulation in the attic, still provide a very tight building envelope, and with the addition of a radiant barrier, the temperature can be controlled to store those Christmas decorations. Cost also plays a role, as creating a sealed ceiling is usually more expensive than a ventilated one. So what makes a project a good candidate for an enclosed attic? Here are some questions to consider to help weigh the pros and cons:
When & How To Use Spray Foam Insulation
Will your HVAC be located in the attic, or is there another way to get your equipment and ductwork into a conditioned space (like a well-enclosed crawl space you usually provide)? What? Is your attic an unfinished extra room that you plan to insulate in the near future? Is attic storage at the top of your client’s “wanted” list or is it a nice-to-have? The cost of sealing the ceiling may not see a return on investment if not much is done in the space.
Does your house plan have large vaulted or cathedral ceilings? What? Do you have long open hallways or ceilings of varying heights that may be difficult to insulate with battens or built-in insulation? If so, attic insulation may be the best option for you. Conversely, if you have a flat roof plan that is easily insulated with battens or a cottage with a large amount of attic space, insulating the roof deck can be very expensive when insulating the attic.
What? do you have your jam again and can’t live without at least a dozen of them on the ceiling between the conditioned and unconditioned space? You may laugh, but I’ve seen it, and even gasketed lights contribute to air leakage when the sheet metal is your air barrier. There are fixed lights that can be shaped like a closed light without large holes, but the main thing here is that if you know that you will have larger than average diameters through the paper that can make it difficult to seal the air, move the air. barrier for the roof covering may be something to consider.
Have you decided that a set of drop ceilings is the way to go, but did you know that they can easily fail in a year or two if not done right? It would be a shame to get some extra space and a nice HVAC environment, only to replace a rotten roof due to the low ceiling. So what causes this condensation and decay, and how can you prevent it?
To Vent Or Not To Vent: Deciding What Is Best For Attic Applications
Unfortunately, the fiberglass insulation you’re used to using in your attic can’t be “moved” up to the eaves without venting or a way to keep the roof warm, otherwise you’ll get condensation. Why? space Outer Space: It’s Cold! (Like -450 degrees Celsius cold!) And since the laws of thermodynamics say that heat always flows from warmer things to cooler things, placing a ceiling in an opaque space will reflect (lose) heat to the cold wonder of the space.
Studies show that due to this phenomenon, called night sky radiation, a roof facing the night sky can be between 6°C and 22°C cooler than the surrounding air temperature. In our humid North Carolina climate, that’s enough to drop the temperature to the dew point, the temperature at which water vapor condenses on the surface, depending on the temperature and humidity of the ceiling air.
Two things are necessary for condensation to occur: air with water and a surface cold enough to remove the water (ie reach the dew point). Therefore, to avoid condensation you need to control the surface temperature of the material, the amount of water vapor in the surrounding air, or both.
A miracle of miracles: the principles of construction and construction science agree in this sense. Four code approved unopened attic insulation strategies prevent drafts by ensuring that attic air does not come into contact with a cold surface. They are listed in Chapter 8 – Section R806.5 of the 2018 NC Housing Code and are as follows:
Insulating An Attic Room
Hermetic insulation (air cannot pass through it) is applied in direct contact with the underside of the roof to meet minimum roof insulation requirements (R38). This is most successful with spray foam insulation. Both open and closed cell spray foam are acceptable, but additional precautions must be taken with open cells. More on that later.
Hermetic insulation is placed directly over the roof sheathing of the structure to maintain the minimum requirements (R38). Rigid foam insulation installed on the roof, while not common, is the preferred method among the building science community. This approach should be considered during design as it can pose a challenge for encryption and plaintext.
Air-tight insulation is installed above the roof to the minimum R-value requirement of Table R806.5, and air-permeable insulation (such as fiberglass or cellulose) is installed directly under the roof sheathing to a total of R38 required to meet . The minimum R-values in Table E for the ratio of rigid insulation to breathable insulation must be maintained or it will not retain enough heat to prevent condensation.
Airtight insulation is installed in direct contact with the underside of the roof to the minimum R-value requirements of Table R806.5, then air-permeable insulation is installed below and in contact with it to minimum R-values reaching the ceiling (often .called flash and batt). Again, the spray foam must meet the minimum R values in the table AND the ratio of spray foam insulation to permeable insulation must be maintained or it will not retain enough heat to prevent condensation.
Insulation On No Attic Roof.
If you plan to exceed code isolation standards or cannot install isolation to meet the requirements in the instructions above, please call us so we can come up with a custom solution that will still work.
To ensure infiltration control, airtight insulation must meet some or all of the R-value (R38/R30ci) of the roof insulation code required for the climate zone. The minimum R-values required for hermetic insulation for each NC climate zone are listed in the chart below:
B. Alternatively, sufficient continuous insulation shall be placed directly over the structural roof to maintain the monthly average temperature of the underside of the structural roof above 45 degrees F. For purposes of calculation, an indoor air temperature of 68 degrees F outside is taken. the air temperature is considered the average outside temperature of the three coldest months.
The most common method we see used for closed rooms is open cell spray foam placed on the underside of the roof. Over the past few years, reports of high moisture levels in spray cell foam rooms have become more common, and the building science community is struggling to understand why and where this moisture is coming from.
Under Insulated Homes: What Problems Can Occur?
I’ve been following the Building Science Association and Green Building Advisor (a couple of recommended technical resources) on this topic and if you want to dig deeper, this article by Allison Bailes does a great job of taking a breather. science together For the rest of you, here’s a quick overview of what causes high moisture levels in spray foam ceilings:
Because water vapor moving through cellular spray foam escapes into the building envelope, deliberately controlling attic air can lower humidity to acceptable levels. Dr. Joe Lstiburek of the Building Science Corporation recommends supplying air at a flow rate greater than or equal to 50 cfm per 1000 ft2 of ceiling using one of the following methods:
The key to a successful closed attic can be summed up in two words: warm and dry. If you are planning to build an open roof assembly, we would love to be part of your high performance team.
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What Is Insulation & How Does It Work?
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