Is an energy/high heel truss of any value if the rafters are being spray foamed?  My understanding of the high heel design is to allow adequate ceiling insulation at the edges of the structure.  If the attic is now foamed, then there is no issue of insulation at those edges?

superduty-

I am no expert on raised heel trusses, but they are included in my house. (The purpose of raised heel trusses, for those not familiar with them, is to essentially raise the roof relative to the rest of the house to allow full height of insulation out to the edge of the roof and over the top of the walls.) In most cases, you need baffles running between the roof rafters near the edge of the roof so that air from the soffit vents is not blocked by the insulation. Without raised heel trusses, the space between the rafters and the top of the wall, and between the baffles and the top of the wall approaches zero at the outside of the walls. Even if you are using spray foam insulation, the insulation height will approach zero at the outside edge of the walls. This could result in snow melting at that point, and then refreezing as an ice dam further out on the roof. This, of course, would lead to water damage.

I think that it is not a big extra expense to include the raised heel trusses in the roof design. I have about 18" to 19" of cellulose in my attic, and am glad to have the raised heel trusses to assure good ventillation and full insulation out in those corner areas. Why not include them?

I've seen a few people mention that the raised heel doesnt add too muh expense.  However, in my case the truss company is talking about 20-25% increase in cost.  Works out to aconsiderable amount of money.

If the R-value between the top plate of the wall and the roof deck is dramatically less (say half) than in the middle of your sprayed roof, you've kinda wasted the R value in the middle of the roof due to the high losses at the edges. Whether you need an energy heel depends a bit on just how thick your foam is going to be. R38 in open cell foam is going to be 10-12", and if your truss chords & rafter elements are 2x4s and there's little or no overhang you could be looking at some pretty thin stuff at the edge-wedges.

"a considerable amount of money" is also a description of an all-foam insulation solution. (From a cost/benefit analysis even an all open-cell solution code-min won't always be NPV+ even in a 50 year analysis, and rarely is with closed cell.) If cost is an issue, consider trading off the up-charge on the trusses with a lower cost insulation. With 2" of closed cell on the roof deck and the rest of the R wet-sprayed "blown in blanket" cellulose or Spider might be somewhat thicker than an all-foam solution, but a heluva lot cheaper. The minimum amount of closed cell it takes varies with climate though. In US zones 3 or warmer you can go with a 1" flash-foam +fiber. Another lower-cost possibility is rigid foam above the roof deck, fiber below.

Posted By Dana1 on 09 Sep 2011 05:38 PM 

"a considerable amount of money" is also a description of an all-foam insulation solution. (From a cost/benefit analysis even an all open-cell solution code-min won't always be NPV+ even in a 50 year analysis, and rarely is with closed cell.) If cost is an issue, consider trading off the up-charge on the trusses with a lower cost insulation. With 2" of closed cell on the roof deck and the rest of the R wet-sprayed "blown in blanket" cellulose or Spider might be somewhat thicker than an all-foam solution, but a heluva lot cheaper. The minimum amount of closed cell it takes varies with climate though. In US zones 3 or warmer you can go with a 1" flash-foam +fiber. Another lower-cost possibility is rigid foam above the roof deck, fiber below.

Dana1, just thinking out loud - it seems that setting up the spider or cellulose scrim or support system below the spray foam would be a very difficult job and likely pretty expensive.  Obviously the spider or cellulose product itself is considerably cheaper [thank spray foam], but it would seem the labor to install the scrim would be exorbitant?  I ned to find a company that does the suspended spider or cellulose.


Dana1 (or anyone else  ), in your opinion will the insulating and performance properties be the SAME?  In other words will 8.5" of Open Cell foam (R30) or the equivilent in closed cell perform the same as 2" of closed cell followed by the spider or cellulose below it.  I am guessing the 2" will accomplish the air tightness and sealed attic.  


Just found this about Spider.  So, is no netting or anything needed over the spider insulation?


The trusses will be 2x4.  If I do 2" of spray foam, then the Spider the trusses will completely dissapear.  That means the scrim will have nothing to staple to?????

Superduty,
The simple answer is no, energy heel trusses serve no purpose if you are spraying the rafters.

Using an energy heel in roof trusses is a good way to keep the soffit above windows when the roof is steep. 

Other than that I do not see much of a reason for an energy heel if the area will be spray foamed with closed cell.

By the way, stops are available to go between each set of trusses to keep the spray foam from entering the soffit area.  Accu Block - stops spray foam from entering soffit for non-vented attics - made from recycled flame-retardant plastic that staples in place.  See   www.brentwoodindustries.com

Posted By Alton on 10 Sep 2011 07:58 AM
Using an energy heel in roof trusses is a good way to keep the soffit above windows when the roof is steep. 

Other than that I do not see much of a reason for an energy heel if the area will be spray foamed with closed cell.

By the way, stops are available to go between each set of trusses to keep the spray foam from entering the soffit area.  Accu Block - stops spray foam from entering soffit for non-vented attics - made from recycled flame-retardant plastic that staples in place.  See   www.brentwoodindustries.com

Not a problem.  Here in Los Angeles (and probably all of CA) we are required to block between every truss with 2x.  Commonly called a frieze block.

Not necessarily. You must have a check over attic installed.

Posted By blossom2347 on 12 Sep 2011 01:37 AM
Not necessarily. You must have a check over attic installed.

What is a "check over attic installed"?

Posted By superduty on 09 Sep 2011 06:13 PM

Posted By Dana1 on 09 Sep 2011 05:38 PM 

"a considerable amount of money" is also a description of an all-foam insulation solution. (From a cost/benefit analysis even an all open-cell solution code-min won't always be NPV+ even in a 50 year analysis, and rarely is with closed cell.) If cost is an issue, consider trading off the up-charge on the trusses with a lower cost insulation. With 2" of closed cell on the roof deck and the rest of the R wet-sprayed "blown in blanket" cellulose or Spider might be somewhat thicker than an all-foam solution, but a heluva lot cheaper. The minimum amount of closed cell it takes varies with climate though. In US zones 3 or warmer you can go with a 1" flash-foam +fiber. Another lower-cost possibility is rigid foam above the roof deck, fiber below.

Dana1, just thinking out loud - it seems that setting up the spider or cellulose scrim or support system below the spray foam would be a very difficult job and likely pretty expensive.  Obviously the spider or cellulose product itself is considerably cheaper [thank spray foam], but it would seem the labor to install the scrim would be exorbitant?  I ned to find a company that does the suspended spider or cellulose.


Dana1 (or anyone else  ), in your opinion will the insulating and performance properties be the SAME?  In other words will 8.5" of Open Cell foam (R30) or the equivilent in closed cell perform the same as 2" of closed cell followed by the spider or cellulose below it.  I am guessing the 2" will accomplish the air tightness and sealed attic.  


Just found this about Spider.  So, is no netting or anything needed over the spider insulation?


The trusses will be 2x4.  If I do 2" of spray foam, then the Spider the trusses will completely dissapear.  That means the scrim will have nothing to staple to?????

I'm not convince that the adhesives in Spider will have enough adhesion to closed cell foam during application to use it without netting in the sloped roof application, but it you can find a contractor who has experience doing it way, by all means, go for it!

Cellulose is sufficiently air-retardent against convection within the fiber layer that it won't have convective losses cutting into performance, and it would in fact OUTPERFORM open cell foam at the temperature extremes (assuming equivalent tested-R).  I'm not sure if 1.0lb density Spider won't have some performance-loss issues at the temperature extremes but  confidence is high that when applied at 1.8lbs density or higher (which may require netting) it will be thermally as-good as open cell foam.

There may also be local fire codes disallowing getting the full code-min R out of open cell foam too (the local foam installers would be all over that one- call and ask if there's a maximum allowed thickness in L.A.)  If 10" is OK and no interior ignition barrier is required, that might be the easiest, but don't take that route without prior approval (hacking it all back out would be expensive indeed!)

I spoke to the rep from Gaco.  Assuming I understood everything correctly.  He indicated with the firestop product no additional ignition barrier is needed assuming you dont use the attic for storage.  If you use the attic for storage then you need an additional barrier such as drywall.  He also talked about HVAC furnace in the attic.  He said you need an ignition barrier if you have HVAC in the attic, but if the HVAC furnace is 90% efficiency or above then it is not needed.  Does this make sense?

Posted By superduty on 12 Sep 2011 01:00 PM
I spoke to the rep from Gaco.  Assuming I understood everything correctly.  He indicated with the firestop product no additional ignition barrier is needed assuming you dont use the attic for storage.  If you use the attic for storage then you need an additional barrier such as drywall.  He also talked about HVAC furnace in the attic.  He said you need an ignition barrier if you have HVAC in the attic, but if the HVAC furnace is 90% efficiency or above then it is not needed.  Does this make sense?

With the furnace in the attic you'll need an ignition barrier on the foam.  No matter what the efficiency of the burner, it's still a burner, but for sealed-combustion units with combustion air provided in a compliant manner there are exceptions.  Codes for attics with no ignition barrier typically have the following constraints per the IRC:

-------------------begin clipped text

a. Entry to the attic is limited to the service of
utilities, mechanical and electrical systems. No
storage is permitted.

b.  Air in the attic is not circulated to other parts of
the building.

c.  There are no interconnected attic areas.

d. Attic ventilation is provided when required by
IBC Section 1203.2 or IRC section R806.

e. Combustion air is provided in accordance with
Section 701 and Section 703 of the
International Mechanical Code

--------------------end clipped text

I haven't read the referenced IMC sections, but I suspect some or most sealed-combustion heating appliances would qualify, and that no atmospheric drafted or units using the attic air as combustion air would.

Dana1, i am still a little unclear on the ignition barrier situation.  I want to use the attic for storage.  Addtionally, the furnace will indeed be located in the attic.  So, I guess that means I need an ignition barrier?  What constitutes an ignition barrier?  Obviously drywall.  What about JM Spider or cellulose (still contemplating if it will be possible to hang the scrim and use the spider or cellulose in addition to the spray foam?

If the furnace's sealed combustion/direct vent meets the IMC 701 & 703 criteria you wouldn't be required to install an ignition barrier on those grounds.

But my understanding of the IRC is that if the attic is used for storage an ignition barrier is required.

Wet-sprayed Spider and most wet-sprayed (but not dry blown) cellulose meet the ignition barrier requirements @ 3" or more thickness. Half inch gypsum, and most half inch plywood or OSB will also meet the requirements.

http://www.specjm.com/files/pdf/T07-020JMSpiderIgnitionBarrierV2.pdf