How Do I Insulate My Flat Roof or Cathedral Ceiling?
Last week, we talked about attic insulation. This week, we want to address those homes and buildings who may not have attics, such as flat roofs and homes with cathedral ceilings, and discuss the available insulation options for your roof.You see, if you have any attic space at all, installing roof insulation is essentially a waste of money. The air flow from your ventilation system undoes any benefit that roof insulation would have offered. But if you do not have an attic space, it is very important to insulate the roof.Insulation options include rigid insulation board installed above the wood deck, under the roofing material, batt insulation installed under the wood deck, between the rafters, and spray foam insulation installed under the wood deck. We put together some information about these options to help you make the best choice for your home.
Rigid Insulation
Rigid insulation boards are required to install on flat roofs to protect the roofing membrane from the roof deck, with nails and screw edges. If membrane was installed right on top of those, it would wear through the membrane.
However, most of these ‘recovery’ rigid board insulation is only ½ inch thick and does not offer a lot of R-value to the overall roof.
When it comes to rigid insulation board, I recommend iso (Polyisocyanurate) insulation. With this insulation, it is much easier to accommodate the height of the insulation to the drip edge or parapets because the R-value is greater per inch, therefore requiring less layers of iso board. The R-Value of iso per inch is about 6 after it has aged.
If you would like a roof with an R Value of 30, and decide to use iso board, you will layer the board until it is about 5 inches thick. This is a big deal, because with EPS, you will need about 7 ½ inches of insulation. This causes problems with installing skylights with curbs that are tall enough. All penetrations, side walls and curbs would need 8 inches of flashing in addition to being tall enough to accomodate the insulation.
One benefit to EPS is that they offer rigid insulation with a taper system. This is much less expensive. If 4’X4’ panels are put down, it is simple to acheive a slope from the high point to the low point. This is a good option for flat roofs, which are required to have a slope to be eligable for the maximum warranty on flat roof jobs.
Rigid insulation boards are required to install on flat roofs to protect the roofing membrane from the roof deck, with nails and screw edges. If membrane was installed right on top of those, it would wear through the membrane.
Traditional Rafter Insulation
If you are choosing to install insulation between your rafters, make sure it is a strong enough insulation to achieve the R-Value you need for your roof. You can view the average R-value
You can figure out what R-value is necessary for the area you live in using this interactive map from the International Energy Conservation Code (embed https://energycode.pnl.gov/EnergyCodeReqs/?state=Utah)
Cementitious Foam
One option would be to completely fill the rafters with cementitious foam. To achieve an R-Value of 38, the cavity would need to be 8-9.5 inches deep, which would make the possibility of skylights harder to implement. However, this insulation has absolutely no air flow, reducing the chance of moisture and mold, and so would reduce or eliminate the need for ventilation in the ceiling.
Loose Fill Cellulose
Cellulose is probably the most common way to insulate a cathedral ceiling or flat roof interior. It is inexpensive per the R-Value. It can be professionally blown into the rafters, packed to a density that will control moisture and air flow. This is becoming the preferred method of insulating cathedral ceilings. Also, if it is dense - packed, it is more likely to want to expand than settle.
Other Traditional Insulation
You can look over the chart above to determine what other insulation might be the best choice for you. However, some of these may allow air flow, so research to see what changes you may need to make to the ventilation.
A Combination of Rafter and Roof Insulation
It is a viable option to install a combination of rafter insulation under the wood deck and rigid foam board above.
For example, you could put three layers of iso board above the deck, and 6.5 inches of cementitious foam below for an R-Value of 44.
Either way, be sure to check with a contractor to be sure that all of your penetrations and ventilation system is correctly installed.
Contact us for a free roofing quote today. 801-487-5151