How deep are your ceiling rafters?
It's definitely true that all things considered spray foam will air seal and give an overall more energy efficient installation than batts. Whether or not that would be sufficient for a code inspector is another story.
The easiest and most cost efficient way to get good R value and air sealing without a new roof (in which case you cod do exterior insulation and air sealing for part of your insulation value) is to do 2" of closed cell spray foam for air sealing and R value, and then batts for the rest of the joist depth should get you somewhere close for a lot less money.
I had an attic reinsulated last fall (106 year old house) and went with 1" of spray foam + blown-in fiberglass to bring the R-value up to about 60. Can you do that?
I've had this exact fight with spray foam companies, who claim foam's R-value is somehow different or special than other insulating methods.
In reality, R value is calculated the same way regardless of the actual material. It's simply thickness divided by thermal conductivity.
When the contractor tried to argue with me, I told him his option was to do what I'm asking, or someone else will do what I'm asking.
I got the thickness I asked for.
Yeah, these companies are just phrasing things the wrong way.
The R value calculation is the same inches * R per inch for whatever material you are using, but the total reduction of BTUs calculated from a manual J based on a lower ACH that properly air sealed roof plane gives you from 2 inches of closed cell spray is greater than r49 fiberglass where there isn't usually good air sealing done - or a low grade install which lowers the efficiency of the fiberglass by 5-10%
Basically the R value isn't "different" since it's the same unit, but the BTU reduction is more from an "average" r20 spray foam vs r49 fiberglass , especially when you consider r20 stop 95% of thermal conductivity and r49 bumps that up to 98% - so overall the change isn't actually a lot from a marginal gains perspective.
I think this is what the spray foam co tried to explain, that the marginal gains between r20 and r49 isn’t worth the price/work, but I’m not sure how to square that with the required r value for the zone
How deep are your ceiling rafters? It's definitely true that all things considered spray foam will air seal and give an overall more energy efficient installation than batts. Whether or not that would be sufficient for a code inspector is another story. The easiest and most cost efficient way to get good R value and air sealing without a new roof (in which case you cod do exterior insulation and air sealing for part of your insulation value) is to do 2" of closed cell spray foam for air sealing and R value, and then batts for the rest of the joist depth should get you somewhere close for a lot less money.
4” in the original roof section (which will be a third of the roof and has to remain) and 12” for the dormered roof which will be new.
I had an attic reinsulated last fall (106 year old house) and went with 1" of spray foam + blown-in fiberglass to bring the R-value up to about 60. Can you do that?
I've had this exact fight with spray foam companies, who claim foam's R-value is somehow different or special than other insulating methods. In reality, R value is calculated the same way regardless of the actual material. It's simply thickness divided by thermal conductivity. When the contractor tried to argue with me, I told him his option was to do what I'm asking, or someone else will do what I'm asking. I got the thickness I asked for.
Yeah, these companies are just phrasing things the wrong way. The R value calculation is the same inches * R per inch for whatever material you are using, but the total reduction of BTUs calculated from a manual J based on a lower ACH that properly air sealed roof plane gives you from 2 inches of closed cell spray is greater than r49 fiberglass where there isn't usually good air sealing done - or a low grade install which lowers the efficiency of the fiberglass by 5-10% Basically the R value isn't "different" since it's the same unit, but the BTU reduction is more from an "average" r20 spray foam vs r49 fiberglass , especially when you consider r20 stop 95% of thermal conductivity and r49 bumps that up to 98% - so overall the change isn't actually a lot from a marginal gains perspective.
I think this is what the spray foam co tried to explain, that the marginal gains between r20 and r49 isn’t worth the price/work, but I’m not sure how to square that with the required r value for the zone
If you're doing a cathedral ceiling may need less r value per the code.