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Does It Make Sense to Move the Building Envelope to the Roofline?

Spray Foam Insulation On Roofline For Building Envelope Relocation

spray foam insulation on roofline for building envelope relocationPutting spray foam insulation on the roofline of both existing homes and new homes has been growing in popularity over the past decade. But, is it always the best way to treat the building envelope? When does it make sense to spray foam the roofline, and when does it make more sense to keep the building envelope at the flat ceiling?

Putting spray foam insulation on the roofline of both existing homes and new homes has been growing in popularity over the past decade. But, is it always the best way to treat the building envelope? When does it make sense to spray foam the roofline, and when does it make more sense to keep the building envelope at the flat ceiling?

Reason #1 to move the building envelope upward and encapsulate the attic is to get HVAC equipment and ductwork out of the blast furnace. An attic, which can get up to 130° F or so on a hot summer day, is not a good environment for your heating and cooling system. By spraying foam insulation at the roofline, the air handler and ducts in the attic now come inside the conditioned space boundary.

Reason #2 would be to spray foam at the roofline because the air leaks at the flat ceiling level are plentiful and difficult to seal that it’s practically impossible to do a good air-sealing job there without a lot of time and labor. Some problems that might send you looking upward for a new building envelope location could be:

  • A lot of can lights
  • Tongue-and-groove ceiling boards
  • Low roof slope that makes sealing the top plates at the eaves near impossible
  • Decking on top of the ceiling joists that would have to be pulled up

Reason #3 would be that the attic has a lot of headroom and is used for storage. Also, if the plan is eventually to finish out some space in the attic, insulating the roofline can make sense even without the issues described above.

Having spent some time doing a lot of air-sealing at the flat ceiling level and doing the before and after Blower Door testing to measure air leakage, I can tell you that I’d much rather have the spray foam crew come in and insulate the roofline. It’s not easy work to pull insulation back, seal the top plates and wiring penetrations, make covers for can lights, and do all the other work associated with improving the existing building envelope, all while balancing on your knees on top of the ceiling joists. So, overall, I’m generally in favor of insulating the roofline.

Sometimes, though, the best option is to keep the building envelope where it is in existing homes. If it’s a newer house with relatively low air-leakage and no air handler or ductwork in the attic, it’s probably more cost-effective to do the air sealing that you can do at the flat ceiling and then look for better opportunities for improvement. You may not end up with quite as good a Blower Door number, but the money you didn’t spend on spray foam may allow you to retrofit that horrible ductopus in your crawl space.

In new construction, my preference is not to have an unconditioned attic at all, from the beginning. Buffer spaces like attics and crawl spaces are where most of the problems arise in existing homes, so let’s just design them away. If you’re going that route, spray foam isn’t the only game in town. I like structural insulated panels, myself, having built a house out of SIPs once upon a time.

Bottom line: If you get a home energy audit with a recommendation to have spray foam insulation installed on your roofline, make sure you ask them to justify their recommendation. If you get a new home designed, and it includes unconditioned attic, make sure they aren’t intending the HVAC system to go there.

This Post Has 24 Comments

  1. How do shingle manufacturers
    How do shingle manufacturers feel about spraying the underside of the attic?

  2. Allison, I aplaud your
    Allison, I aplaud your succinct description of why one would encapsulate an attic. I also appreciate you pointing out doing the roofline isn’t the only option, that one can also seal the attic floor. I differ a little in your characterization of the attic floor job’s benefits and ease, however. We’ve done a few case studies to determine how effective performing a critical seal on an attic floor can be, and we’ve shown some impressive results, esp. when combining the job with sealing the rims in the basements. We reduced ACH50 by 20-40% in these 3 cases. Using low-pressure spray foam is the key to making an attic floor job easy, though. Also, doing just a critical seal on the floor can be much more economical than encapsulating the entire attic. Ultimately, though, it’s the homeowner’s choice, and I again applaud you describing the principal options available and doing so with a dispassionate and objective tone. A lot of folks in the marketplace either avoid this topic (presumably because of its complexity), or they discuss the options in slanted, biased terms.

  3. I own a 1930s cape cod,
    I own a 1930s cape cod, notorious for air leaks in knee walls, etc. My blower door test proved this to be true. In general, safe to assume a good candidate for roofline air sealing? I would like to hear back from anyone with experience applying roofline to a cape cod. How does this affect air flow/roof deck and asphalt shingle life?

  4. Roger in short it may affect
    Roger in short it may affect the shingles life by 2 weeks. While there are still ways to ventilate a “hot roof” in many cases it is not only not needed – but can cause further problems.  
    Jack – you would have to check with the manufacturer, some will say no, while most accept it.  
    With an unabashed plug for more info on shingles, sips & hot roofs: 
    I also saw Martin Holliday posted a blog article (GBA) this morning on vented hot roof assemblies and the issues associated with them

  5. Good article, Allison.
    Good article, Allison. Important to emphasize in this discussion is the value of testing with the blower door. Most people assume that all spray foam jobs are the same and have no clue that the quality of the job is 50% mix and 50% quality of application. Even when mixed and apllied properly, ALL foam jobs have leaks. Using a blower door with the foam guy right behind you with his touch up kit, can reduce ACH50 by 40% or so.

  6. Jack: I
    Jack: I don’t know what all shingle manufacturers think about this, but I’ve heard there are some who are OK with spray foam and don’t void the warranty because of it. The truth is that shingle temperature doesn’t change a lot when you have an unventilated attic. I wrote an article about an FSEC study shingle temperature, which showed that unventilated attics have shingles that are about 9° F hotter at the most. What kills shingles isn’t temperature, though, it’s UV radiation. 
    Matt: I certainly didn’t mean to give the impression that air-sealing the attic floor was easy or achieved great results. Having spent a lot of time doing that and then measuring the results, I know it’s not going to give you the tightest house. What I was trying to say is that in some cases, it makes more sense to do that because you have to look at all the opportunities for improvement, the conditions of the attic, and the homeowners’ budget. 
    Roger: Yes, it could be a good candidate. Kneewalls can create a lot of liabilities when they’re part of the building envelope, though I think you’re using a different definition of that term than home energy pros use, which is a wall that separates conditioned space from unconditioned attic space. I think the first indication that an insulated roofline may be a good way to go is if you have an HVAC system or ductwork in the attic. See my response to Jack for the answer to your shingle question.

  7. Allison, you and I are
    Allison, you and I are essentially in total agreement. There are certainly those instances where doing the roofline is easier. There are also those instances where doing the attic floor is easier. Each hose is different, and each homeowner’s desired endstate and set of needs is different, too. Best approach is for the installer to be competent in a waide array of approaches. Your discussion does a good job of pushing that very point. Many thanks. 
    The Building Research Council out of Champaign IL @ the Univ of IL performed an extensive study on the effects spray foam on roof decking may have on shingle temperature/life. Their conclusion was that there is no noteworthy or significant impact whatsoever. NOW, getting the shingle manufacturers to agree to that might be a different story, but the BRC’s data was pretty tough to argue with. Jeff Gordon @ BRC is a contact for more info.

  8. Where I live in Houston,
    Where I live in Houston, there are a few builders who seem to understand and discuss all aspects of building science. One in particular says he has seen any number of cases where roof material failed due to moisture problems. This is in and around the Houston TX area, which has a hot-humid climate much like yours. They say there a number of pending lawsuits they know of. 
    It would be most positive to understand what they are talking about, why problems arise and exactly what one must do to ensure they do not. 
    This builder argues they do a very good job of sealing ductwork (removing Lstiburek’s original argument for sealed attics right there), and making the ceiling plane airtight. From that the argument is they have captured the benefits at a lower cost, and the resulting product is more robust in a hot-humid climate. 
    Any high level debate or argument would be most enlightening. Thank you.

  9. Good points, all. Regarding
    Good points, all. Regarding shingle temperature, most, if not all, fiberglass shingle manufacturers have removed the requirement for ventilation from their warranties, and besides, who has ever collected on a roof warranty anyway? I have been a fan and promoter of SPF roofline insulation for a long time but I have recently become concerned about the negative health effects of the chemicals in SPF. Check out this recent articles on Green Building Advisor on the subject: The EPA is looking very closely at SPF and we need to think about the potential downside of exposing workers and clients to these chemicals as well as their potential negative effects on the environment. As with most things, the correct answer always depends on the situation, and there is never a free lunch.

  10. Great comments, questions,
    Great comments, questions, and links, all of you. I won’t be able to address each one individually till this evening at the earliest because I’m on a little vacation and getting ready to pack up and head home. Please keep the discussion going in my absence.

  11. I’ll add an additional and
    I’ll add an additional and the most important reason of all. This comes after a conversation with Tony Woods about how Can Am was performing a second generation of weatherization work on houses in Toronto. Literally homes they had wetaherized 30 years ago were being re-weatherized again. 
    The reason – is that during renovations and remodeling the air sealing work they had done on the attic plane had been undone by not-so-knowledgeable workers. 
    I like the idea of putting the envelope at the roofline because if nothing else, the roof probably will never get remodelled. And once the envelope is at the roofline it will acoomodate a multiple of sins created by soffits, furred walls, pocket doors, etc. 

  12. It might be worth mentioning
    It might be worth mentioning to watch out for atmospherically vented furnaces and water heaters in the attic before encapsulation. We don’t want “carbon peroxide” to spill into the house and “sophisticate” the owners.  

  13. The only thing I might add to
    The only thing I might add to this excellent discussion is the diminishing returns issue in heating dominated climates. I’m not suggesting encapsulated attics are never cost justified in cold climates, but rather that the benefits may be significantly reduced. 
    * The colder the climate, the higher the R-value to keep surfaces above the dew point. Given high price of foam, this has a large impact on cost-benefit 
    * The colder the climate, the shorter and less severe the cooling season, thus reducing cooling benefits. 
    * The colder the climate, the less likely ducts and furnaces will be located in the attic 
    * Insulating the roof deck eliminates much of the the beneficial radiant gain, offsetting some of the heating season benefits. 
    Although these effects cannot be modeled with any degree of precision, it’s not that hard to bracket the problem in round numbers to see if it’s even in the ballpark. 
    At least in new construction, the cost-benefit of sealing the ceiling and plates, raised-heel trusses, and lots of blown-in insulation is hard to beat in any climate, especially with HVAC properly located below the ceiling plane.

  14. Do not forget the simple
    Do not forget the simple things about energy efficiency here:  
    1) The conditioned volume is increased substantially. 
    2) The roof has significantly more area to be insulated, than the ceiling plane. 
    Remember Lstiburek has gone on record stating a gross efficiency disadvantage (was it 5% or 10%?) which would be surpassed by getting leaky ducts inside conditioned space.

  15. In cases where its been
    In cases where its been impossible (in other words, the builder refused) to install the system in the conditioned space, we have had success in convincing the players to at least install the air handler in the conditioned space. This usually goes in a centrally located closet where the return can stay in the conditioned space. The supply duct goes up into the attic with the outlets located on the inside wall, minimizing exposure. Then we go to the extreme on the duct insulation. Its probable you could spend a fraction of the cost to insulate the roof deck to aggressively insulate the duct instead. We have sprayed closed cell R-19 on trunk ducts in the attics and double insulated with duct wrap. I even saw one contractor wrap the trunk with 2 inch XPS under duct wrap. (no the inspector didn’t see it) But the flex is still the weak link. R-8 is usual and customary but also pitiful. At least they are short runs most of the time.

  16. @M.Johnson said &quot
    @M.Johnson said “conditioned volume is increased significantly”, and “roof has significantly more area to be insulated” 
    Spot on. In fact, I would argue that encapsulating the attic may in some cases actually increase overall loads (even though attic is not directly conditioned). For example, if you assume a nominal 10F delta-t across the uninsulated ceiling plane on a design day, it’s not that hard to calculate conducted heat transfer using the parallel path layers) method, compared to, say, a 50F delta-t in the case of a well insulated ceiling. (You’d also need to account for infiltration in both cases.) What’s difficult to analyze is the annual performance for an encapsulated attic, since I’m not aware of any way to estimate delta-T’s in an hourly sim. This is an area that deserves further research. 
    I recently worked on a large new home with a 14/12 roof that was specified to be encapsulated. Not surprisingly, the insulation bid was just ridiculous. As usual, there was no possibility of moving HVAC inside. By roughly bracketing potential savings in duct losses, I couldn’t see any way for this turkey to fly. Fortunately, I convinced the builder to vent the roof and spend a small part of the savings on atomic air sealing the ceiling plane. I specified a horizontal AHU (note that upflow in a vented attic is MUCH worse) with maximum verified leakage at 2% of fan flow. 
    In any case, the real key is to get the ceiling sealed up. To Allison’s original point, the cost of achieving this in retrofit plays heavily in the cost-benefit analysis.

  17. Folks, re: safety with spray
    Folks, re: safety with spray foam, please consult the official site established by the Center for the Polyurethanes Industry (CPI),; it is intended for SPF contractors, weatherization professionals, do-it-yourselfers (DIYers), and homeowners, and provides guidance on related health and safety issues.

  18. Allison, 

    Long time no see (charleston 2009). I have a small truss roof brick ranch in North Texas. Local foam contractors keep recommending open cell foam, or spray-in radiant barriers. They want to shy away from closed cell foam on the attic roof line.  
    1. Can the open cell foam provide the same envelope encapsulation on the roofline as closed cell? 
    Do you know anything about these spray-in radiant barriers (metal paint) ?

  19. Insulation should be removed
    Insulation should be removed when spraying foam in an existing attic. Existing insulation can hold in the odors generated when spraying the foam. In the near future, you will not see foam manufacturers standing behind their product when there are odor issues if the insulation is not removed.

  20. My husband and I have been
    My husband and I have been thinking about the different options that we have for insulation. We are building a new home to live in, and we have been leaning towards the spray foam insulation. We have read and heard that spray foam insulation is much more efficient, and much safer than the fluffy insulation. Another plus to this, is that bugs and other critters cant nest in the spray foam insulation , and so we would never have to worry about any little surprises. Has anyone used the spray foam insulation ? What are the pros and cons to this product? Is it worth the investment to purchase the spray foam insulation ?

  21. In all of your experience,
    In all of your experience, what is the best way to encapsulate to the ceiling line for new construction?

  22. @George, I don’t normally
    @George, I don’t normally specify SPF on flat ceilings under a vented attic. There are less expensive ways to seal the ceiling, and then pile on blown-in fiberglass for cellulose. Applying foam to the ceiling provides very little extra benefit.  
    When encapsulating the attic (e.g., insulating roof deck rather than ceiling), it is necessary to use an air impermeable insulation on the roof to prevent condensation on the cold underside of roof deck (not an issue in a properly vented attic, if ceiling is tight). Code requires a minimum amount of air impermeable insulation when envelope follows the roof line. See Section R806.4.5, and Table R806.4 in particular:

  23. Does anyone ever use foam on
    Does anyone ever use foam on the attic side of the ceiling? Eliminates the thermal/ignition barrier issue as it is against the drywall ceiling. Less square footage than the roof deck.

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