Scientists agree that 2016 was the warmest year ever recorded globally. The 2016 summer in Boston was the driest ever recorded and August, the hottest month ever measured. While this may still not be enough to convince everyone on climate change it is worth our attention as home owners, builders and designers. Higher outside temperatures mean higher air-conditioning costs. A hot-roof helps in keeping the heat out of the house reducing your energy bills.
Incidentally 2016 was the year that our insulation team partnered up with the University of Massachusetts to test out spray foam insulation being installed in attics and we did definitely learn a lot from it.
Temperatures in an unconditioned attic in Massachusetts can exceed 140° during the summer. There are several benefits of turning an inhospitable, unconditioned attic into a conditioned space. Many houses have HVAC, electrical, water, & air-ducts running through the attic. Maintenance or repair work on electrical junction boxes can be very difficult in such an environment. Expansion & contraction associated with temperature change may cause water lines to freeze or spring a leak. Putting the ducts inside the conditioned space, not only keeps the air from getting superhot or cold, it also ensures that any duct leakage is not being lost outside, but kept inside the house. If the structural design of your roof permits, you can store sensitive items in your conditioned attic. A properly conditioned & sealed attic also helps eliminate ice-dams.
Historically, building codes have required that insulated sloped roofs be ventilated via channels under the roof sheathing. Many builders still follow this technique. These codes were however, based on misconceptions rather than scientific principles or research and have therefore generally failed to produce the desired results. Builders and designers must understand a few basic principles before deciding whether to build a vented or an unvented ceiling. First of all, roof ventilation cannot be used to lower indoor humidity levels. Secondly, the design should not encourage migration of water vapor through the ceiling. Thirdly, roof ventilation does not significantly lower the external temperature of the roof during the summer. If the ceiling is not airtight, roof ventilation can do more harm than good. The air movement in rafter bays can cause indoor air to leak through the ceiling cracks. The primary focus should therefore be on building an airtight ceiling. Accomplishing this design goal can make roof ventilation a nonissue.
A hot roof design omits attic or under-roof ventilation entirely. The term "hot roof" refers to the fact that the absence of ventilation causes the outside roof temperature to be comparatively higher than a vented roof. In reality, a hot-roof is not much hotter than a normal roof. Studies have shown that the temperature is only 1 - 5° higher during the hottest part of the day, while at night the surface cools faster than a vented roof. This takes a bit of getting used to but the benefits of insulating like this without ventilation are tremendous.
There are two main options for creating a hot roof. The first approach uses spray polyurethane foam (SPF) on the inside surface of the roof. The second approach utilizes foam board on top of the exterior roof sheathing. The choice depends on your roof geometry, ease of access and budget.
Use of SPF for creating a hot roof is the most popular approach for both new construction as well as retrofitting projects. For complex roof geometries SPF is the only practical approach. SPF application is also quicker and can help you complete the project in the shortest amount of time. Closed cell spray foam is installed against the underside of the roof sheathing. Care must be taken to ensure that the thickness of SPF is sufficient to meet the minimum code requirements and especially here, in Massachusetts, prevent ice dams. The rest of the rafter cavity can be filled with an air-permeable insulation. This type of assembly is designed to dry to the interior and therefore must never include an interior polyethylene vapor barrier. This is a common misconception on the use of a vapor barrier in an unconditioned attic and the Massachusetts building code around unconditioned attic insulation and ventilation as well.
Use of open cell SPF can be risky in our climate. If used, the underside of the open cell SPF should be covered with gypsum drywall and/or painted with a vapor-retardant paint. One area of concern when using SPF insulation is loss of heat from thermal transfer. Unless the rafters / trusses are fully covered with foam, heat will transfer through the wood, to the sheathing outside reducing your energy efficiency. Another concern might be the loss of head room from inside application of SPF. Hybrid insulation systems are fast becoming an insulation system of choice in unfinished attics that do not need to be drywalled where 2"-3"of closed cell spray foam insulation is applied, followed by 6" open cell spray foam insulation as well. This allows homeowners to achieve R-values with the Massachusetts attic insulation building code as well as have Continuous Insulation (C.I.).
When retrofitting, it is important to carefully inspect the condition of the existing roof and sheathing before SPF application. If the roof has spongy areas, leaks or the sheathing shows signs of water damage or mold, these issues must be resolved first. It’s better to invest in your roof first, with a solid roofing contractor and then have the insulation installed. Any existing venting must be removed or sealed up, as it is no longer required. Make sure you allow sufficient time for the fumes / VOC’s to be exhausted before reoccupying the house. Our spray foam insulation has no CFCs, HCFCs and no VOCs and uses water as a blowing agent making it a very good residential foam insulation.
A hot-roof design is also great for cathedral ceilings that are increasingly common in modern living rooms, bed rooms and even kitchens. If you are building or renovating a house, consult a professional to find out if the hot-roof design is suitable for your project.
For more information on the benefits of closed cell spray foam insulation unconditioned attics in your Massachusetts home or upgrading your home installation to the most current Massachusetts Building Insulation Code, and the ways in which this can be achieved, call Mass Energy Lab Insulation at 617-902- 2744 or visit our insulation services page. We’re home insulation geeks and would love to hear from you!