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Insulating a Flat Roof

I am currently a licensed contractor performing remodeling services for residential and commercial customers in Wisconsin.

Read my step-by-step guide for flat roof insulation.

Read my step-by-step guide for flat roof insulation.

Flat roofs have been a daunting task for contractors and homeowners alike since they were invented. They are hard to waterproof, hard to maintain, and hard to insulate. For today's purposes, we will stick with the insulation issue.

In order to properly insulate a flat roof area, you need to be very careful on the material and the methods used. I will break these into sections for organizational purposes:

Insulation Between Ceiling Rafters

There are only 2 types of insulation that should be used for this method. Spray foam (either closed or open cell) insulation or dense-pack cellulose insulation. The reason is that it is very difficult to properly ventilate a flat space because air does not generally move in a straight line. This invites moisture into an area that if trapped, can do a lot of damage and is hard to get rid of. It is much easier and more efficient to have a non-ventilated area and fill the entire cavity full with dense-pack cellulose, or seal the area with 4 to 5 inches of spray foam.

One consideration is rafter height. If you only have 2x6 rafters existing, you will only be able to use closed cell spray foam insulation to insulate with because of the R-value restrictions (closed-cell foam is R-6 per inch, open-cell foam is R-3.5 per inch, and cellulose is R-3.8 per inch). If you have at least 2x8's for rafters, then you can use dense-pack cellulose or open-cell spray foam.

The closed-cell spray foam option costs about $4/square foot. Open-cell foam costs about $2.75/square foot. Cellulose insulation costs about $1/square foot. FIBERGLASS INSULATION SHOULD NEVER BE USED IN AN AREA LIKE THIS.

Insulating on the Exterior Roof Surface

Insulating a flat roof from the outside is probably the most popular method for commercial roofs but can be utilized for residential roofs also. This is done by applying a strong adhesive to the foam board, laying it down on the roof surface, and securing it with washer head nails. Then the rubber waterproof membrane adheres to the surface of the foam board. Two or more layers of foam board are normally used in this application to achieve the needed R-value. Again, no ventilation needs to be present. In this application, the ceiling joist spaces may remain empty. Always remember that if this method is used, the rim joists still need to be insulated to keep the insulation continuous from roof to wall to avoid moisture and air infiltration issues.

Insulating the Ceiling

This method of insulating from the inside is not recommended because the ceiling height would have to drop at least 4 inches to meet code requirements. It also makes the future addition of lighting or hardware very difficult.

As you can see, there are limited options available that are truly correct ways to insulate and air seal without having moisture or air infiltration issues. This is why flat ceilings are not a popular construction method for residential use. However, if the proper insulation methods and materials are used, you can have a wonderfully insulated roof.

If you are unsure of what energy improvements to make, it is always a good idea to hire an energy consultant. They will walk through your building with you and show you the areas that can benefit from energy efficiency improvements.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.


John Massaro on March 08, 2019:

Should spray foam polyurethane insulation be applied directly to the underside of the plywood sheathing on a cold flat roof?

Newman954 on February 13, 2018:

I live in South Florida and am redoing my pitched roof (to metal from tile) and the smaller part of my house that has a flat roof (just a bedroom and bathroom). The roofers want to know if I want them to install one layer of 1.5" roof insulation boards over my flat roof for an R value of 8.5. This would add $1486 to the cost. Should I do this? I have noticed those rooms being warmer than the rest of the house but I assumed it was the lack of good air flow (closest air return in in the room next to these rooms). Should I pay for this upgrade? Thanks for the help.

Paul Haugstad on August 22, 2017:

Flat roof with 10 inch rafters for the subroof and space between rafters filled with insulation, about 16 inch airapace and then 12 inch rafters for the outer roof w/tongue/groove planking and two layers with water tight tar paper sheeting. There are air inlet/outlet around the overhang. Planning to fill the empty space w/blown insulation. Is this advisable, and if not, what is the risk. Temp summer about 20C/winter - 20C. Will moisture collect on planking and result in rot ? If moisture is a problem is there a way to "went off" moisture?

Ron on October 30, 2015:

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Energy Guild (author) from Ripon, WI on July 07, 2013:

The likely hood of condensation buildup will depend on how greatly the temperature differential varies in your climate. Anything more than 25 or 30 degrees variance will produce some sort of condensation. But if you are experiencing summer to winter type temperatures, you either need ventilation, or need no ventilation and closed cell spray foam insulation directly against the roof deck all the way down to the rafter ends to prevent air infiltration and to allow for enough R-value in insulation depth. 3 inches or roughly 7.5 cm should be enough of an insulation, vapor, and moisture barrier for you to not worry about condensation. I hope that helps.

dug1972 on June 23, 2013:


I have a flat roof with alot of insulation in and wanted some advice. The roof has various levels. There is a tin top section with 40mm kingspan below it which sits onto an older felt. below the felt level is a cavity which has been fully filled with cavity bat insultion, no air gap, completely filled. Then there is a 200mm cavity below that which has 150mm kingspan and a mulitfoil layer before the plasterboard. I am concerned about the build up of condensation.

Energy Guild (author) from Ripon, WI on November 21, 2012:

Yes you should insulate the roof joists against the wall. Where the two roofs meet at the area where the roof joists are parallel I am having a hard time visualizing how the roof lines come together. Depending on how the roofs come together and the space that is below the other roof will determine how to handle that portion.

Thank you.

lkswv on November 21, 2012:

More explanation of last question: My question involves insulating only the flat roof so there is no air flow. The flat roof on 2 sides meets house walls which have an open and unfinished attic on the other side. Roof & attic floor are at the same level. Should we insulate the roof joists against the wall the same as described in your drawing? What about the side of the roof with the joists parallel to the wall?


Energy Guild (author) from Ripon, WI on November 20, 2012:

Yes, the spray foam should be wrapped down the rim joists to create a total air seal. If the side with parallel roof joists has an attic space below it and is accessible, then insulating with blow in cellulose would work well. The key is what kind of access you have to that area. As for the wall space, the rule of thumb is if the wall separates attic spaces only, it does not need to be insulated. However, if the wall separates the attic from a living space wall, then it should be insulated. The best method is to apply 3 inches of closed cell spray foam to the wall wrapping the spray foam over the studs of the wall (on the attic side) to create a monolithic thermal blanket and also air seal the wall and prevent air from moving down the wall. I hope that helps answer your questions. Thank you.

lkswv on November 15, 2012:

Thank you for the article on flat roof insulation, which is very helpful. I would appreciate your consideration of my question. If using the insulation method in picture 3, should the closed cell spray foam be used in the rim joists that meet a wall of the house? The other side of the wall is an unfinished and uninsulated large attic space. Another side of the roof which is against the same attic space is parallel with the joists. How is this side handled regarding insulation? Thank you.

Energy Guild (author) from Ripon, WI on August 16, 2012:

I appreciate the reccomendation, but you will have to tell me the problem before I can help you. If you wish to reamin anonymous, send me a private message. Thanks.

martinnitsim on July 23, 2012:

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filling address , they have a deal on at the mo ,tell them m netsims recommened you

Robert Gonzalez on March 20, 2012:

Hi, I am looking at buying a 1959 home with a flat roof. It has 3 foot overhangs and no gutters, just little u shape spouts at spots on edge. Inside I see laminated beams and tung and grove pine or cedar planking as roof surface. Outside on the roof I see white gravel and some black material. First would a home of this age have an insulation layer under the black material and on top of the tung and groove? I can't really see if there is. Second, is an insulation layer needed? This house is in NY state. I am wondering what I have and if it is sufficient for a home. Are the heating and AC cost going to be crazy since there is no insulation? If I wanted to add insulation on the top what would you recommend?

sumit (lucky) on February 07, 2012:

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Bigodrywall on January 10, 2012:

I have a flat roof ranch house. I am currently remodeling. 2x10 roof joists 1x8 plank decking torch down modified roofing on top of that. Cavities are open to soffit around perimeter of house with soffit vents. There originally was r13 fiberglass with large air gap above, rock lathe and plaster ceiling with foil backing for moisture barrier. I finished a room already and filled the roof cavity with r30 bats, then someone told me that was wrong. Can't blow in because it would just shoot outside thru soffit vents. My idea was foam baffles on underside of decking then pack in r30, then I would filling the cavity but still leaving 2" air gap for ventilation. I don't want to get rid of the ventilation because of how hot the roof get in the summer. Any thoughts?

Energy Guild (author) from Ripon, WI on December 21, 2011:

In your situation, I actually would recommend dense packing cellulose insulation in the rafter cavities. The R-value per inch of cellulose is R-3.8 per inch so you can have an R-value of R-26.6 but more importantly, you can air seal the cavities very successfully. Moisture will not be an issue with this system either because the cellulose allows for any moisture to move from high concentration to low concentration. This means that any moisture accumulation in the cavities will move to the perimeter of the building. I have done this many times and it works very well. Do not try dense packing fiberglass though as fiberglass is not a true insulation material like cellulose and will cause massive moisture problems. Any insulation already in the cavity will compressed filling the cavity with cellulose. This is the best and least expensive method for your situation. Good luck.

HHummel on December 10, 2011:

This article and the dialogue in the comments is very helpful. I still am stumped about my situation, though.

I have a 7" tall cavity between 16" spaced 2x8's that both hold up a (very slightly sloped) flat roof on the top side and drywall for the ceiling on the bottom side. There is currently one layer of R13 fiberglass batt laid into each cavity, but the cavities themselves are very drafty. There must be vents at the ends of channels, but they are not visible/accessible. As a result, I basically live in a place with a continously refrigerated or heated ceiling, which makes it expensive to maintain comfortable conditions.

Because I cannot take down the ceiling, whatever insulation choice I make must involve injection through holes cut through the ceiling into each channel. That means that spray foam is out but injection foam might work. (I've already excluded blown in dense pack cellulose and fiberglass due to the risk of moisture damage to the roof deck.) I understand that closed cell foam could provide both an air and vapor barrier (true?) as well as additional thermal insulation if installed on top (or beneath?) of the existing fiberglass batt. I saw your recommendation of a combination of foam plus blown in fiberglass for another commenter, but I don't think it's possible through an injection application to do anything but the whole 7" with foam.

Although I am confused by the various closed-cell foam types on the market (and I haven't found a good website for comparison), my question is more general: Would you advise this kind of 'cathedral ceiling' type approach to retrofiting a (basically) flat roof with insulation?

richard on November 05, 2011:

Thanks! I hadn't thought of batts and foam. Your comment reminded me of a recent Fine Homebuilding article suggesting that idea as a way to cut costs compared to using foam alone.

I will follow your advice. Thanks again.

Energy Guild (author) from Ripon, WI on November 05, 2011:

Hey Richard,

Applying closed cell spray foam to the underside of the roof sheathing is the best solution. In this case, you have a lot of room so I would apply 3 inches of closed cell polyurethane spray foam directly to the underside of the sheathing and then install fiberglass batts underneath this or dense pack cellulose insulation. This will give you an extremely high R-value and also provide superior sound attenuation qualities.

As far as masking a leak, almost all leaks are only detected after they have been going on for a long time. One recommendation I would consider is to take part in a preventative maintenance plan. Basically, this entails a very detailed inspection of the property with a full report that outlines the condition of the property and its components. Certain items will need immediate attention and others will need attention in the future. The entire point is to know what is going to fail and when. This will help you plan and save for the items that will need replacing. I have a few clients that I do this for and it is well received and all of my clients have said it was a valuable service. Hope this helps.

richard on October 31, 2011:

I have a flat roofed house built in the late 80's. The roof assembly consists of 2 x 10's with 2 x 4's running perpendicular across the top of them to create an air gap of 3 to 5" (varied to create drainage) below the sheathing and roof membrane. The 2 x 10's have pink fiberglass batts in between them measuring about 8 to 9" deep. i have much of the drywall down as part of a renovation and am now considering whether to invest in changing the insulation. If I go with spray foam, do I apply it to the underside of the sheathing and if so, will this cause any problems with respect to over heating the roof membrane?

In addition to my roofs, I have a number of decks over heated spaces that have a similar construction. They have just have new membranes installed as there were some leak issues. the insulation has been removed as part of the repair process. What insulation should I install and if I choose closed cell spray foam, what happens if i get another water leak? Will it mask the source leading to greater problems later?

Thanks in advance for your advice.

Energy Guild (author) from Ripon, WI on September 04, 2011:


Adding insulation to the 6 inch cavities will help, but keep in mind that the maximum R-value that you will gain with ventilation is 4 1/2 inches because of the necessity of a minimum 1 inch air space throughout the rafter cavities. Using closed cell spray foam insulation to fill the cavities and have ventilation throughout the roof will give you roughly an R-31.5. However, using closed cell spray foam negates the need for ventilation at all and will operate better if you simply do not ventilate the roof and have closed cell spray foam insulation installed. While this is the most expensive method, it will be the least problematic.

E-parttimeaddon on September 03, 2011:

Enclosing an attached porch with a flat roof that

vents part of the house attic across the 6 inch rafters to the vents in the eve. I want to insulate, mainly to keep the cold in and the heat out (house is in central florida). What would be the best method to use. Should I fill the six inches between the roof and the ceiling or should I leave an inch or two away from roof for ventilation to eve?

mantolama on July 17, 2011:

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izolasyon on July 17, 2011:

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rickhra2k5 on June 18, 2011:

I have a flat concrete roof about 6 inches thick. I'm considering spray roofing or laying foam sheathing with asphalt/tar to stick the sheathing on. I thought maybe a 3/4" sheathing with an r5 value might be good. What would be the best way to insulate this type of roof?

Darrel on April 12, 2011:

Great information on this page. My situation is a little different. I live in Ontario, Canada and R-45 is the mimimum recommendation for attic insulation. I have a built up roof that has been covered by a conventional peaked roof. For the built up part from the warm side I have drywall, paper backed batt insulation (not sure of the r value but it is only about 3 inches thick and the paper is towards the drywall), then a 5" airgap (I don't see any venting to the outdoors ) then the built up portion. The peaked roof is traditional and has all kinds of venting, eaves, peak and through the sides.

Now for the question. How would I go about adding a vapor barrier to this system? Is the built up roof a vapor barrier in itself? The paper backed insulation is obviously not working. Could I remove the drywall and existing paper back insulation, replace and completely fill the cavity with new batt insulation then cover that with a poly vapor barrier and re drywall? If going the foam route could I leave the existing batt insulation in there and just fill the cavity through holes in the drywall with low expanding closed cell foam?



Energy Guild (author) from Ripon, WI on December 20, 2010:

Thank you very much. I have made a lot repairs on flat roofs myself and, contrary to popular opinion, they are always the hardest to fix.

Energy Guild (author) from Ripon, WI on December 12, 2010:

Hey Wheatkingq,

What you are thinking about doing with ventilation will unfortunately not work very well in increasing your efficiency, especially using fiberglass batts. Since you currently do not have ventilation and it sounds like you have access to the attic, the most cost effective insulation technique you can use is to spray closed cell foam using spray foam kits around the perimeter of the roof between the roof surface and the ceiling to a nominal depth of at least 2 inches to prevent air infiltration around this area. Then staple netting to the bottom of the ceiling joists so it is tight and blow cellulose insulation over the netting to create a nice thermal blanket of insulation. Get at least 8inches in there, but ideally 12 inches equals and R-45 and is an awesome insulation depth. The spray foam kits are around $650 per 600 board feet. Cellulose can be blown by renting a machine which is free with the purchase of the cellulose insulation.

For what you are trying to accomplish, this will be the most economical way to keep heat from escaping from through the roof and between the roof rafters. Let me know if you have any other questions.

wheatkingq on December 10, 2010:

I have a flat roof in a very old building approx 10 feet high. I want/need the roof insulated b/c its freezing in the winter. I have reason to believe that if I pop out vent holes on both ends of the joist span and leave a 4 inch gap between the roof and insulation I can then have a great passage way for air flow and still have the roof insulated with Batt style just under the gap.


I can cut out rectangular shapes to fit a register cover to allow air flow from the inside of the building to flow thru and still insulate with a batt style with a gap between the roof and the insulation and a vapor barrier underneath the insulation?

Please let m e know what you think...

Energy Guild (author) from Ripon, WI on September 30, 2010:

I would install a minimum of 4 inches of foam board insulation. Keep in mind you are not just insulating from the cold, but on a flat roof, you are especially insulating from the heat. The last time I was in DC was in the summer time and it was hot. It may be costly, but you will be upset if you go through the trouble of re-roofing and do not perform the insulation the correct way.

Here is a link from the Department of Energy so you make sure that you get the right type of foam board.

Good luck with your project. Let me know if you have any other questions.

John in DC on September 29, 2010:

Another super cool site that gives you historical data on Heating Degree Days and Cooling Degree Days in different zip codes...

John in DC on September 29, 2010:

Thanks for the input. I have pretty much decided to go with the rigid insulation on the roof rather than spray, and I will also make sure to insulate at the front and rear wall perimeter above the ceiling (it's a row house so the sides are bordered by other houses).

Where I am still a bit torn is on how much R value to make sure the rigid insulation has. Check out these two calculator sites I found.

They seem to indicate that in my situation (being in DC which isn't as cold as say upstate new york), anything more than a 15 R value on the roof (since its only 700 sf of area) will not be too cost effective and wont save that much additional fuel costs.

I know these calculators may not be very accurate (and their creator admits the error they may have), but they are the best I could find online for helping you get an idea of the bigger picture.

I'd be interested to hear your thoughts on these. Thanks again for all the help!!!

Energy Guild (author) from Ripon, WI on September 29, 2010:

Hey John in DC. It sounds to me like a combination of an exterior foam board insulation on the roof surface would be a good idea. However, the easiest way to insulate your roof would be from the underside to keep the ductwork serviceable. The best product for this is closed cell spray foam insulation, which is costly, but you do it once and it is done forever. Closed cell spray foam has an R-value of R-7 per inch with the blends that they are producing today.

Keep in mind that this type of roof system is a hot roof system that should not have any ventilation to it. You will have to make sure that the insulation contractor you hire diligently seals around the perimiterof the roof.

Even if you do the foam board on the surface of the roof, which is a perfectly good insulation technique, you will want to have an interior perimiter sealed and insulated with closed cell spray foam insulation to avoid heat loss at the edges.

In either case, you should be shooting for a minimum of R-35 or 5 inches of spray foam from underneath or on the surface of the roof.

Hope this helps.

John in DC on September 27, 2010:

I have a situation similar to scenario 3. I live in DC in an old house built in the 1930's with limited to no insulation in the roof cavity. I also have existing Air Conditioning ductwork (added in after the house was built) up in the cavity since my AC unit is on the roof. I need a new roof and I am looking at getting a Coolstar white reflective membrane, but I am trying to figure out how to best insulate. I'm trying to figure out how thick and how much R value the rigid insulation should have if I put it on the top of the roof (scenario 3A). But I am also worried since I read that having the rigid insulation on the "cool side" of the roof in the winter could trap moisture in the cavity and lead to mold issues. It seems like the best thing to do would be to go with the reflective membrane, but do blow-in insulation above the drywall ceiling, but I am worried that system would be harder to install given all the ductwork up there and the fact that I have old plaster ceilings. It doesn't seem like any solution is a good one!!! Do you have any advice? Thanks so much!!!

EnergyImprovement from Texas on September 20, 2010:

Article was very informative. Thanks

Albama Roofing Company on September 17, 2010:

Really something to think about.Insulating the ceiling was never easy.

Energy Guild (author) from Ripon, WI on September 16, 2010:

You can insulate between the joists, but it has to be right against the roof sheeting. If you are going to do this, I suggest using closed cell spray foam insulation. You could also cut out sections of foam board insulation and stick them between the ceiling joists, but they have to be against the roof surface. This is not an easy task and will be very time consuming. You can buy closed cel spray foam kits for around $600 to $750 for every 600 board feet. To figure out how man y board feet you need to spray, multiple to square footage by how many inches you want to install. In your case, I think that you have less of a roof insulation issue and more of an issue around the perimeter of the roof that is really part of the wall. See what kind of insulation you have between the ceiling joists. If there is fiberglass or no insulation, spray foam this area with 3 inches of foam. This may be more affective than insulating the underside of the roof. Hope this helps.

tim on September 16, 2010:

i have aflat roof with 4" of rigid foam under membrane and tar and rock. I was wondering if i could insulate from the inside between joists. it has a drop ceiling .