R value | Ask Mason Knowles - SPF Consultant | Spray Foam Technical Questions and Answers | SprayFoam Magazine Q&A Forums

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Harry S Posted: Jun 15, 2010 11:54 AM	R value Mason, I have to bid a job for cathedral ceilings in 7 townhomes. The general contractor wants to achieve the greatest amount of R value in these overheads. Rafters are 2x8....If I fill the full 8" cavity with 2# cc, would I get R 56 or close to it? If not, what R value would I be getting? Thanks in advance
mason Posted: Jun 15, 2010 05:45 PM	Different SPF systems have various R values as tested according to FTC and ASTM standards and requirements. You can have anywhere from 5.8 to 6.7 R values. So at 8 inches it could vary from 47 t0 54. In my opinion anything over 5 inches of closed cell foam is overkill unless you are in extremely cold temperatures such as northern Alaska or Canada.
jimcoler I have over 10 years of experience specifying and installing open and closed cell spray foam. I've sold my business but I'm still selling for the new owners and consulting on large and custom specific jobs. I've expanded my knowledge into t Posted: Jun 17, 2010 12:50 PM	So, this question leads us all to the issue at hand. R-values!! We're being held to them as if they're gospel and there is no way to refute it. But in fact, it's a true and false measurement -endorsed by the FTC. Who's going to tae this project on!!! SPFA????? It was our number 1 request from the contractor's meeting and I haven't heard anything since. My research shos that once you reach 3-4" of closed cel, you have surpassed the point of diminishing returns. That means that you gain very little from the additional foam applied over 3-4". My research also shows that 5-6"of open cell is also the point of diminishing returns which would do the same job as the closed cell. So, which is beter 3-4" of closed cell with studs only at an R-3-4 causing possible condensation on them inside the wall or 6 inches of open cell with the studs covered to prevent thermal bridging? I'd go with the 6" and call it done!
Harry S Posted: Jun 21, 2010 12:46 PM	Just received a call from the contractor....cost is too much for 7.5" (which I already informed him was too much foam anyway...I suggested 3-4"). He now wants only 1" & he'll blow in fg or cel to hold costs down. I really don't particularly want to do this because of possible moisture problems. Am I correct fearing this? It's in eastern Iowa where summers are hot & humid and winters are colder than a lady of the evnings' heart!
Dan Beecher Posted: Jun 21, 2010 08:31 PM	You say these are cathedral ceilings. That said are you figuring spraying them as a hot roof? If so, I wouldn't spray 1" and then fill with filterglass. My opinion you are asking for a diaster. I'm in ND. The minimum we would do would be 3 to 4" of closed cell. With just 1" you really aren't doing anything. If too expensive now, let em do them with filterglass, then in a couple years when they have to tear them apart you can come in and be the hero and charge them more for the retro for having to cover every wall and floor.Go in there, spray 4 or 5" of closed cell and be done with it.
Posted: Jun 22, 2010 08:34 AM	jimster is dead on... 6-8 oc really puts you away from the edge of the cliff,,, harris,,,same goes in the sidewalls... lots of "skim coats" as the filterglass newbee foamers call it goin on out there... 1/2" or so cc and then glass,,,and the builders market "we've got foam in this home"...and then they have their own filterglass peeps do the heers rating..fraud,,but a different topic,, there will still be condensation on the exterior sheathing,,which in this case is the foam layer,,in the dog days of winter,,or on the back of the poly vb in the dog days of summer,,,90+% rh right now in the cornfield,,,moisture drive from the outside in,,,and when hot meets cold...you know the drill,, you need at least 2"cc in our area,,and if you can sell 2" you should be able to sell 3" which will afford the end user increased thermal performance,,ie: lower energy use compared to similar structure with non foam insulation... ...ramblein after thought,,then there is the geothermal folks,,toutin "be green" green my booty,,all you are doing is reducing your cost to produce the btu to heat/cool the home...if it has crap for insulation,,,the btu loss is the same out the walls/ceilings no matter how you produce it,,boiler,,forced air,,geothermal,,burnin horse poop,,you have done nothing to reduce the energy use,,just the energy cost..green my booty,,,we have the means to reduce our energy use all the while useing traditional building techniques,,, frame-it,,,foam-it,,,f-it!!! ,,,

Author

Comments

Harry S

Posted: Jun 15, 2010 11:54 AM

R value

Mason, I have to bid a job for cathedral ceilings in 7 townhomes. The general contractor wants to achieve the greatest amount of R value in these overheads. Rafters are 2x8....If I fill the full 8" cavity with 2# cc, would I get R 56 or close to it? If not, what R value would I be getting? Thanks in advance

mason

Posted: Jun 15, 2010 05:45 PM

Different SPF systems have various R values as tested according to FTC and ASTM standards and requirements. You can have anywhere from 5.8 to 6.7 R values. So at 8 inches it could vary from 47 t0 54. In my opinion anything over 5 inches of closed cell foam is overkill unless you are in extremely cold temperatures such as northern Alaska or Canada.

jimcoler

I have over 10 years of experience specifying and installing open and closed cell spray foam. I've sold my business but I'm still selling for the new owners and consulting on large and custom specific jobs.

I've expanded my knowledge into t

Posted: Jun 17, 2010 12:50 PM

So, this question leads us all to the issue at hand. R-values!! We're being held to them as if they're gospel and there is no way to refute it. But in fact, it's a true and false measurement -endorsed by the FTC. Who's going to tae this project on!!! SPFA????? It was our number 1 request from the contractor's meeting and I haven't heard anything since.

My research shos that once you reach 3-4" of closed cel, you have surpassed the point of diminishing returns. That means that you gain very little from the additional foam applied over 3-4". My research also shows that 5-6"of open cell is also the point of diminishing returns which would do the same job as the closed cell. So, which is beter 3-4" of closed cell with studs only at an R-3-4 causing possible condensation on them inside the wall or 6 inches of open cell with the studs covered to prevent thermal bridging? I'd go with the 6" and call it done!

Harry S

Posted: Jun 21, 2010 12:46 PM

Just received a call from the contractor....cost is too much for 7.5" (which I already informed him was too much foam anyway...I suggested 3-4"). He now wants only 1" & he'll blow in fg or cel to hold costs down. I really don't particularly want to do this because of possible moisture problems. Am I correct fearing this? It's in eastern Iowa where summers are hot & humid and winters are colder than a lady of the evnings' heart!

Dan Beecher

Posted: Jun 21, 2010 08:31 PM

You say these are cathedral ceilings. That said are you figuring spraying them as a hot roof? If so, I wouldn't spray 1" and then fill with filterglass. My opinion you are asking for a diaster. I'm in ND. The minimum we would do would be 3 to 4" of closed cell. With just 1" you really aren't doing anything. If too expensive now, let em do them with filterglass, then in a couple years when they have to tear them apart you can come in and be the hero and charge them more for the retro for having to cover every wall and floor.Go in there, spray 4 or 5" of closed cell and be done with it.

Posted: Jun 22, 2010 08:34 AM

jimster is dead on...
6-8 oc really puts you away from the edge of the cliff,,,
harris,,,same goes in the sidewalls...
lots of "skim coats" as the filterglass newbee foamers call it goin on out there...
1/2" or so cc and then glass,,,and the builders market "we've got foam in this home"...and then they have their own filterglass peeps do the heers rating..fraud,,but a different topic,,
there will still be condensation on the exterior sheathing,,which in this case is the foam layer,,in the dog days of winter,,or on the back of the poly vb in the dog days of summer,,,90+% rh right now in the cornfield,,,moisture drive from the outside in,,,and when hot meets cold...you know the drill,,
you need at least 2"cc in our area,,and if you can sell 2" you should be able to sell 3" which will afford the end user increased thermal performance,,ie: lower energy use compared to similar structure with non foam insulation...
...ramblein after thought,,then there is the geothermal folks,,toutin "be green" green my booty,,all you are doing is reducing your cost to produce the btu to heat/cool the home...if it has crap for insulation,,,the btu loss is the same out the walls/ceilings no matter how you produce it,,boiler,,forced air,,geothermal,,burnin horse poop,,you have done nothing to reduce the energy use,,just the energy cost..green my booty,,,we have the means to reduce our energy use all the while useing traditional building techniques,,,
frame-it,,,foam-it,,,f-it!!!
,,,

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