The days of the good ole rafter baffle are over. What are your feelings on sealing the tops of exterior wall plates? Important to do with many choices. Two part foam ( hard to get to the plate ), use cellulose bags stuffed on top of the plate ( stops all venting from soffet)
We have had good success using rigid and one part foam to get those darn exterior wall top plates. Two part foam can be cost prohibitive. We use a fireblock foam with a dispensing gun that has a 3ft barrel. The long barrel gets out to those tight spots. Once the existing insulation is pulled back (garden rake), a generous bead of foam goes down, then slide in the thickest piece of rigid foam in place. Another bead of foam over the top, push the insulation back, and you’re done. Then just repeat, repeat, repeat.
I think this is an excellent topic especially with the rush to own a spray foam rig and not understanding building science.
I like the "hot roof" type also, but I also like the suggestion of having a firred-up roof. With the decking that has the foam applied to it essentially seperated from the decking that holds the roof covering which creates the venting opportunity. I have read about these roofs and heard about them being used in cold climates which helps prevent ice damming.
Sean, would you still consider this a "hot roof"?
I think the hardest part in creating a "hot roof" in a retrofit is the level of difficulty at not only those darn top plates and those darn top plate outside corners, but also in all of the other tricky roofing design crevices that are in the market today. I have only seen a couple of jobs that have done a great job retrofitting with spray foam. And even some of those jobs have cathedral/vaulted ceilings that have been totally ignored!!! What is a building envelope when the barriers are not continuous and contiguous? A little air leakage seems to be able to cause big problems.
Jamie, first welcome aboard and secondly in response to your question - technically that would not be a "hot roof" as it is ventilated
I do agree with you on the hardest parts, but not because of air leakage, but the thermal transfer in really cold climates - if the CC foam was installed properly, the amount of air that can get through should be pretty close to zero & the only problem as mentioned is the thermal breaks (rafters, trusses, studs, plates, corners, etc... in the house) but that does apply to all houses unless the entire house is wrapped on foam outside
I should also state that the air & water barriers should be on the exterior and one should not rely on the insulation to perform that job - as now you have brought the battle inside the house. Granted insulation can help in that fight, but relying on it to do the job is an issue - as for retrofits, if there are issues with water intrusion, that needs to be dealt with before any insulation goes in the wall, no matter what type it is