1947 Cape Cod Attic Insulation Advice Needed

[Lordoftheflies]

Good luck just take your time. I skipped once wearing the full tyvek suit. Definitely recommend cutting the feet off and just duct taping it to your boots. Had zero traction with the feet. And then I had to patch from the bedroom which wasn't so bad but obviously would have been better if avoided in the first place.

However long you think it will take you double it for your wife. Things may snowball like me. Haha. Maybe even triple it....

 

[Romefolks12]

lordoftheflies you are the man for tackling this project and good job. Thanks for doing this and sharing your experience with everyone. I have a similar house built in 1949 in Chicago. I have an a mostly brick structure with siding for the two front window dormers. The back of the house faces east with slightly pitched roof. My knee walls are similar to yours with built in book shelves in them. The front of my house faces west and it's not as hot but in the summer the two rooms with the knees walls never really get below 78 on the hottest day and the air conditioner runs all day without a break almost. The furnace in the winter can keep the rooms at 70 but it will turn on often.

I just bought this house in 2018 and learned in 2019 that cape cods are terrible homes for insulating. I don't have gable vents, soffits vents or a ridge vent. I only have a powered solar vent. I has some leaks in my living room which prompted me to get a new roof but my roofers were incompetent and didn't do a good job. I still have the leak. That's a different thread.

I have an open knee wall space cavity for the area in between the two dormers which I have started to air seal and insulate. My biggest issue is that my houses doesn't end at the end of the roof rafters. I have a slightly less pitched roof/ overhang that is the ceiling for my living room below. I have no way of insulating that portion or air sealing. Does anyone have any idea what I should do?

 

[Sponsor]

 

[Romefolks12]

View attachment 68898 View attachment 68898 View attachment 68899

As you can see the 2x6 roof rafters end but the space continues. You can tell that at some point something was living back under there but I can't get to that space unless I year down the plaster ceilings in my living room below.

 

[Lordoftheflies]

What did you decide to do in the end? I am now faced with a similar problem and will be happy to receive your advice. The air in my house is quite stale and my friend advised me to install the solar whiz roof ventilator. As my friend explained to me, this is a new more efficient ventilator that can definitely help me deal with my problem. Also, it is solely solar-powered and controlled. I'm still confused, because I've read quite a lot of positive reviews about this thing on the Internet, but I'm still not sure if it's worth installing. What do you think about this?

The air in your attic.....should have nothing to do with the air in your house....if it is properly sealed. That's kind of the whole point of what I did - I filled ALL the gaps, be it too plates, waste stack chase, electrical wire holes, or any other gap - in order to keep the inside house air separate from the attic/roof air.

lordoftheflies you are the man for tackling this project and good job. Thanks for doing this and sharing your experience with everyone. I have a similar house built in 1949 in Chicago. I have an a mostly brick structure with siding for the two front window dormers. The back of the house faces east with slightly pitched roof. My knee walls are similar to yours with built in book shelves in them. The front of my house faces west and it's not as hot but in the summer the two rooms with the knees walls never really get below 78 on the hottest day and the air conditioner runs all day without a break almost. The furnace in the winter can keep the rooms at 70 but it will turn on often.

I just bought this house in 2018 and learned in 2019 that cape cods are terrible homes for insulating. I don't have gable vents, soffits vents or a ridge vent. I only have a powered solar vent. I has some leaks in my living room which prompted me to get a new roof but my roofers were incompetent and didn't do a good job. I still have the leak. That's a different thread.

I have an open knee wall space cavity for the area in between the two dormers which I have started to air seal and insulate. My biggest issue is that my houses doesn't end at the end of the roof rafters. I have a slightly less pitched roof/ overhang that is the ceiling for my living room below. I have no way of insulating that portion or air sealing. Does anyone have any idea what I should do?

If you want to do it right. You'll have to open up the ceiling from the inside of your house on that slightly less pitched section of your roof. I think the same idea applies - you want to insulate but you need an air gap for the roof deck. You can do your best and apply faced insulation from the inside....or you can possibly consider getting it spray foamed but that is more expensive. In this case it may be worth it. I'm sure @Dana can give you some great advice! He was advising me as I was up in the attic sitting on my butt replying back and forth to me. I can't thank him enough for all his help and patience.

The change has been amazing and coupled with me heating with wood (and splitting over 45 cord in the past 14 mo the) my electricity usge has gone down 33% compared to the same period last year......and this is with 3 kids now full remote o. Their computers, phones, and chromebooks all day long. I've been running my fireplace with no heat on during the day.

 

[Lordoftheflies]

As you can see the 2x6 roof rafters end but the space continues. You can tell that at some point something was living back under there but I can't get to that space unless I year down the plaster ceilings in my living room below.

I had senty earlier reply without seeing your post with the pictures. That's definitely strange - my best guess is they added that weird roof addition to accommodate the extension of the living room....but I don't know why someone would go through all that trouble for such a small extension. Doesn't seem worthwhile and now it's more of a problem than anything else.

Thanks for the kind words btw. Much appreciated.

 

[Dana]

I have a slightly less pitched roof/ overhang that is the ceiling for my living room below. I have no way of insulating that portion or air sealing. Does anyone have any idea what I should do?

Yes.

Re-roofing is an opportunity-moment to put R20 rigid foam (3.5" of foil faced rigid polyisocyanurate foam board seams taped, or 4" of fiber faced foam board, seams taped) on the exterior of the roof deck, clamped in place with a 5/8" nailer deck through-screwed to the structural roof deck with pancake head (not pan-head) timber screws. In Chicago's IECC climate zone 5 that provides provides sufficient dew point control at the roof deck for up to R30 between the rafters, with NO interior side vapor barrier tighter than interior latex paint on the ceiling sheet rock. In the kneewall ceilings where it's impossible to sheet-rock adequately, use Certainteed MemBrain (a "smart" vapor retarder) detailed as an air barrier.

Use a 2-part foam kit (Tiger Foam, FrothPak, Fomo-Foam,etc.) to air seal over the top plate of the exterior walls. Before foaming it in with an inch or two of foam, install R5 rigid foam board (3/4" polyiso) air dams on the exterior side of the exterior wall top plates to stop air infiltration, sealing the edges to the framing with the spray foam.

If 3.5" of rigid foam up top is too pricy it's fine to drop back, but make sure that any insulation UNDER the roof deck is no more than 1.5x the foam's rated-R. That keeps the roof deck warm enough in winter to not accumulate excessive moisture from the indoor air over the winter.

My usual preference for this type of project is to install reclaimed roofing polyiso taken from commercial building re-roofing & demolition projects. In my area there are multiple reclaimers, and it's usually under $25/sheet for a 4x8 sheet of 3", under $15/sheet for 2". There are a handful of reclaimers operating in Chicagoland- run this search every few days and you'll find some of the bigger operators, or maybe even a great one-off deal.


If laying it up in two layers, put the first layer down with cap nails to keep it in place, tape the seams then install the second layer using foam board construction adhesive and longer cap nails (tape those seams too) before installing the nailer deck.

Ideally the plank sheathed structural roof deck would get a fully adhered membrane (Grace Ice & Water or similar) to both air seal and water proof it from leaks before the foam goes up. The nailer deck can get the usual #30 felt + shingle layup. Be sure to install new kick-out flashing above the nailer deck, lapped correctly with the felt.

 

[Romefolks12]

Thanks Dana for your reply. I wish I would have found you and this thread two years ago. I got this roof done in 2018 and because of the lack of knowledge I am paying for it now. I take it the only way to insulate this area is a new roof or doing it from the inside?

What about if I removed some shingles and roof decking and blow in loose cellulose from above?

 

[Dana]

Thanks Dana for your reply. I wish I would have found you and this thread two years ago. I got this roof done in 2018 and because of the lack of knowledge I am paying for it now. I take it the only way to insulate this area is a new roof or doing it from the inside?

Pretty much.

What about if I removed some shingles and roof decking and blow in loose cellulose from above?

That would be a code violation, and increases the risk of rotting out the roof deck. Cellulose is somewhat protective of the wood by wicking away moisture, but it doesn't have an infinite storage capacity. Once the cellulose becomes truly wet (it can happen, even from vapor diffusion, particularly on north facing or shaded roof areas) it's doing more harm than good.

Where you have access to the roof deck from the inside you can install closed cell foam directly to the underside of the roof deck. Most closed cell foam is a Class-II vapor retarder at 1" of depth (~R6) , which would be sufficient to protect the roof deck, but not enough to keep more than (R6 x 1.5=) R9 of fiber insulation dry. As long as the foam is at least 40% of the total R the fiber stays dry. If those are 2x8 (?) rafters you have 7.25" to work with. Installing 2" of closed cell foam (~R12-R14) and filling the remaining 5.25" with cellulose (R19.4) blown in netting (or compressed R20 cellulose batts), or a compressed R20 fiberglass batt (~R19 @ 5.25" ) or compressed R19 batt (~R17 @ 5.25") using Certainteed MemBrain as an interior side vapor retarder would be pretty safe, even if a bit marginal on R-ratio. Any of those solutions would put it in the R30 range, which isn't terrible.

Read and understand this document before making any big changes. The cheat sheet is Table 3, the zone 5A Chicago row. You can see over in the 2" ccSPF + spray fiberglass column that at R38 total the roof deck is completely safe with 2" of closed cell foam, but with R12 foam at the exterior side and R26 on the interior the fiber insulation could get damp enough to support mold on the rafters. MemBrain detailed as an air barrier would fix that.

If hiring a pro to do the foam part DO go with the newer foams blown with HFO1234ze, which is quite a bit less environmentally damaging than the cheaper stuff blown with HFC245fa (an extreme greenhouse gas.) The HFO foam runs about R7/inch rather than R6/inch, giving it a bit more dew point margin too, but it still takes 2" to be able to fill out the rest of the R with much cheaper (and DIY-able) fiber. If using a DIY foam kit you're pretty much stuck with the R6/inch HFC245fa goods for now. The HFC foams run about $1.00-$1.25 per square foot per inch of depth, the HFO foams about $1.25-$1.40 per square foot per inch of depth. It's not cheap, but it does air seal well at 2", protects the wood from taking on excessive moisture via diffusion, yet still allows the roof deck to dry toward the interior. Asphalt shingles + #30 felt layups severely inhibit drying toward the exterior, as does rain/dew wetting, snow cover, etc. Roofs are not just tilted walls, and the assembly MUST be allowed to dry toward the interior to have any resilience (even if that drying is only very slowly through 2" of spray foam.)

The CO2e footprint of closed cell foam (even HFO blown foam) is considerable- its' expensive environmentally as well as financially:

If you care about this stuff, cellulose (blown or batts) is by far the greener way to fatten out the R-value. It becomes sequestered carbon, and thus has a negative carbon footprint. Denim batts have similar hygric capacity- it too is protective of the wood, but growing cotton is more carbon intensive than growing trees, even when the pulp & papermaking aspects are accounted for.

 

[Fitter30]

Pretty much.



That would be a code violation, and increases the risk of rotting out the roof deck. Cellulose is somewhat protective of the wood by wicking away moisture, but it doesn't have an infinite storage capacity. Once the cellulose becomes truly wet (it can happen, even from vapor diffusion, particularly on north facing or shaded roof areas) it's doing more harm than good.

Where you have access to the roof deck from the inside you can install closed cell foam directly to the underside of the roof deck. Most closed cell foam is a Class-II vapor retarder at 1" of depth (~R6) , which would be sufficient to protect the roof deck, but not enough to keep more than (R6 x 1.5=) R9 of fiber insulation dry. As long as the foam is at least 40% of the total R the fiber stays dry. If those are 2x8 (?) rafters you have 7.25" to work with. Installing 2" of closed cell foam (~R12-R14) and filling the remaining 5.25" with cellulose (R19.4) blown in netting (or compressed R20 cellulose batts), or a compressed R20 fiberglass batt (~R19 @ 5.25" ) or compressed R19 batt (~R17 @ 5.25") using Certainteed MemBrain as an interior side vapor retarder would be pretty safe, even if a bit marginal on R-ratio. Any of those solutions would put it in the R30 range, which isn't terrible.

Read and understand this document before making any big changes. The cheat sheet is Table 3, the zone 5A Chicago row. You can see over in the 2" ccSPF + spray fiberglass column that at R38 total the roof deck is completely safe with 2" of closed cell foam, but with R12 foam at the exterior side and R26 on the interior the fiber insulation could get damp enough to support mold on the rafters. MemBrain detailed as an air barrier would fix that.

If hiring a pro to do the foam part DO go with the newer foams blown with HFO1234ze, which is quite a bit less environmentally damaging than the cheaper stuff blown with HFC245fa (an extreme greenhouse gas.) The HFO foam runs about R7/inch rather than R6/inch, giving it a bit more dew point margin too, but it still takes 2" to be able to fill out the rest of the R with much cheaper (and DIY-able) fiber. If using a DIY foam kit you're pretty much stuck with the R6/inch HFC245fa goods for now. The HFC foams run about $1.00-$1.25 per square foot per inch of depth, the HFO foams about $1.25-$1.40 per square foot per inch of depth. It's not cheap, but it does air seal well at 2", protects the wood from taking on excessive moisture via diffusion, yet still allows the roof deck to dry toward the interior. Asphalt shingles + #30 felt layups severely inhibit drying toward the exterior, as does rain/dew wetting, snow cover, etc. Roofs are not just tilted walls, and the assembly MUST be allowed to dry toward the interior to have any resilience (even if that drying is only very slowly through 2" of spray foam.)

The CO2e footprint of closed cell foam (even HFO blown foam) is considerable- its' expensive environmentally as well as financially:

If you care about this stuff, cellulose (blown or batts) is by far the greener way to fatten out the R-value. It becomes sequestered carbon, and thus has a negative carbon footprint. Denim batts have similar hygric capacity- it too is protective of the wood, but growing cotton is more carbon intensive than growing trees, even when the pulp & papermaking aspects are accounted for.

Like to have your take on my house purchased 15 years ago modified a frame cathedral ceiling wood R19 shingled roof. When purchased dark red shingles were worn out, turning hard and black. Osb was used for decking and the decking was waive. Engineer from a shingle manufacturer told me the roof got cooked and wouldn't recommend his or anybody elses. Had a ladder style 1x4s screwed over the shingles and metal roof over them ( dropped my cooling load at least a ton). Do you think lordoftheflies could run into the failure?

 

[Romefolks12]

Pretty much.



That would be a code violation, and increases the risk of rotting out the roof deck. Cellulose is somewhat protective of the wood by wicking away moisture, but it doesn't have an infinite storage capacity. Once the cellulose becomes truly wet (it can happen, even from vapor diffusion, particularly on north facing or shaded roof areas) it's doing more harm than good.

Where you have access to the roof deck from the inside you can install closed cell foam directly to the underside of the roof deck. Most closed cell foam is a Class-II vapor retarder at 1" of depth (~R6) , which would be sufficient to protect the roof deck, but not enough to keep more than (R6 x 1.5=) R9 of fiber insulation dry. As long as the foam is at least 40% of the total R the fiber stays dry. If those are 2x8 (?) rafters you have 7.25" to work with. Installing 2" of closed cell foam (~R12-R14) and filling the remaining 5.25" with cellulose (R19.4) blown in netting (or compressed R20 cellulose batts), or a compressed R20 fiberglass batt (~R19 @ 5.25" ) or compressed R19 batt (~R17 @ 5.25") using Certainteed MemBrain as an interior side vapor retarder would be pretty safe, even if a bit marginal on R-ratio. Any of those solutions would put it in the R30 range, which isn't terrible.

Read and understand this document before making any big changes. The cheat sheet is Table 3, the zone 5A Chicago row. You can see over in the 2" ccSPF + spray fiberglass column that at R38 total the roof deck is completely safe with 2" of closed cell foam, but with R12 foam at the exterior side and R26 on the interior the fiber insulation could get damp enough to support mold on the rafters. MemBrain detailed as an air barrier would fix that.

If hiring a pro to do the foam part DO go with the newer foams blown with HFO1234ze, which is quite a bit less environmentally damaging than the cheaper stuff blown with HFC245fa (an extreme greenhouse gas.) The HFO foam runs about R7/inch rather than R6/inch, giving it a bit more dew point margin too, but it still takes 2" to be able to fill out the rest of the R with much cheaper (and DIY-able) fiber. If using a DIY foam kit you're pretty much stuck with the R6/inch HFC245fa goods for now. The HFC foams run about $1.00-$1.25 per square foot per inch of depth, the HFO foams about $1.25-$1.40 per square foot per inch of depth. It's not cheap, but it does air seal well at 2", protects the wood from taking on excessive moisture via diffusion, yet still allows the roof deck to dry toward the interior. Asphalt shingles + #30 felt layups severely inhibit drying toward the exterior, as does rain/dew wetting, snow cover, etc. Roofs are not just tilted walls, and the assembly MUST be allowed to dry toward the interior to have any resilience (even if that drying is only very slowly through 2" of spray foam.)

The CO2e footprint of closed cell foam (even HFO blown foam) is considerable- its' expensive environmentally as well as financially:

If you care about this stuff, cellulose (blown or batts) is by far the greener way to fatten out the R-value. It becomes sequestered carbon, and thus has a negative carbon footprint. Denim batts have similar hygric capacity- it too is protective of the wood, but growing cotton is more carbon intensive than growing trees, even when the pulp & papermaking aspects are accounted for.

Well that escalated quickly. Thank you for the friendly hand slap. I feel like a kid getting ready to stick a fork in the toaster.

I didnt anticipate tearing down that large of a portion of plaster ceiling to get access to the roof deck. Believe it or not they used 2x4s for that roof section. If I had drywall I would be less hesitant. Either way since I don't want a rotten roof deck I might have to live with cold living room.

I also have reservations about spray foam. I'm sure I don't have all the information but I'm concerned about the off gasing of foam and any health effects it could cause.

 

[Dana]

Well that escalated quickly. Thank you for the friendly hand slap. I feel like a kid getting ready to stick a fork in the toaster.

I didnt anticipate tearing down that large of a portion of plaster ceiling to get access to the roof deck. Believe it or not they used 2x4s for that roof section. If I had drywall I would be less hesitant. Either way since I don't want a rotten roof deck I might have to live with cold living room.

I also have reservations about spray foam. I'm sure I don't have all the information but I'm concerned about the off gasing of foam and any health effects it could cause.

There are "slow rise" versions of DIY foam kits, where you can drill one easy to repair hole, squirt some in (not too much at a time or you risk blowing out/cracking the plaster) and let it expand to fill the space. That effectively glue the lath holding the plaster as well as the rafters to the roof deck. At 3.5" (the depth of a 2x4 cavity) you'd be at ~R20 (center cavity-R), and it would be pretty air tight. Also at 3.5" closed cell foam is itself structural, the roof deck won't come off without taking the rafters with it, and if the storm is that bad you probably don't have a house anyway. (In FL they use 3" of closed cell foam to hurricane-proof the roof.) Even if the roof deck rotted away the 3.5" of foam bonded to the rafters & lath would provide nearly as much structure as a 1x ship-lap roof deck.

Offgassing issues only arise when not following the manufacturers instructions regarding the temperature of the cannisters and surfaces being foamed during installation. With pro equipment that can fine-tune the part-A and part-B chemical mix a bad mix can also result in offgassing. DIY kits are pretty idiot-proof if you pay close attention to the temperatures, but of course the more idiot-proof they make something the more creative the idiots become. Floating the chemical cannisters in a tub of warm water near the upper end of the recommended temps gets around the cooling that occurs from falling pressure in the cans as the chemicals get dispensed.

With the roof deck you also have to pay attention to solar heating of the roof, which can be several 10s of degrees warmer than the outdoor air temperatures on a sunny day. You pick your weather-window of opportunity and go while the getting is good.

 

[Dana]

Like to have your take on my house purchased 15 years ago modified a frame cathedral ceiling wood R19 shingled roof. When purchased dark red shingles were worn out, turning hard and black. Osb was used for decking and the decking was waive. Engineer from a shingle manufacturer told me the roof got cooked and wouldn't recommend his or anybody elses. Had a ladder style 1x4s screwed over the shingles and metal roof over them ( dropped my cooling load at least a ton). Do you think lordoftheflies could run into the failure?

Third party testing has shown the color (or more precisely, the SRI- "solar reflective index") of an asphalt roofing shingle has a bigger effect on peak shingle temp and longevity than an un-vented insulated roof deck. Traditional reds take up solar heat almost as much as black or dark gray shingles, but there are now "cool roof" reds using different pigments that can knock 10s of degrees off the peak. The answer from an engineer working for a shingle company is always the same- they'll make up just about anything to get out of paying a warranty. (I've never heard of anyone collecting on a shingle warranty anyway- have you?)

The pitch of the roof matters too- anything under 4:12 provides less convection cooling to the outdoors, with a measurable effect on shingle temps.

An R19 batt stuffed in a 5.5" deep 2x6 rafter bay only performs at R18, and isn't very air-retardent- it's more of an air filter than an air barrier, allowing infiltration to pass remarkably freely. Low density fiberglass is also translucent to IR radiation- with batts laid flat on an attic floor the temperature inside the batt about an inch from the top is often hotter than the attic air temperature when the roof deck is hot. So before the metal roof went up, when the roof deck was north of 130F a lot of heat was radiating right through the batt. If it were my house I'd be inclined to retrofit dense packed cellulose into those rafter bays (which you can usually get away with in zone 4A or 4B, which is most of Missouri), which can be done from the interior side without pulling down the ceiling. The low density of R19 & R11 batts makes it dead-easy to insert a dense packing hose thorough the batt itself. (Recently I personally dense packed the walls of a 1960s house that had a mix of R11s, R13s, and empty cavity bays. Post-insulation IR imaging showed that it worked pretty well- better than I had expected!) Dense packing improves the air leakage a lot (more than 90%) of walls & roofs insulated with low density fiberglass.)

Metal roof SRIs are typically lower than asphalt shingles (often by a lot), and with any amount of convection cooling through the 1x purlins the peak roof deck temps are lower. The SRI of the metal roof finishes have quite a range too, but it doesn't have to be titanium white to have fairly low SRI.

 

[Romefolks12]

There are "slow rise" versions of DIY foam kits, where you can drill one easy to repair hole, squirt some in (not too much at a time or you risk blowing out/cracking the plaster) and let it expand to fill the space. That effectively glue the lath holding the plaster as well as the rafters to the roof deck. At 3.5" (the depth of a 2x4 cavity) you'd be at ~R20 (center cavity-R), and it would be pretty air tight. Also at 3.5" closed cell foam is itself structural, the roof deck won't come off without taking the rafters with it, and if the storm is that bad you probably don't have a house anyway. (In FL they use 3" of closed cell foam to hurricane-proof the roof.) Even if the roof deck rotted away the 3.5" of foam bonded to the rafters & lath would provide nearly as much structure as a 1x ship-lap roof deck.

Offgassing issues only arise when not following the manufacturers instructions regarding the temperature of the cannisters and surfaces being foamed during installation. With pro equipment that can fine-tune the part-A and part-B chemical mix a bad mix can also result in offgassing. DIY kits are pretty idiot-proof if you pay close attention to the temperatures, but of course the more idiot-proof they make something the more creative the idiots become. Floating the chemical cannisters in a tub of warm water near the upper end of the recommended temps gets around the cooling that occurs from falling pressure in the cans as the chemicals get dispensed.

With the roof deck you also have to pay attention to solar heating of the roof, which can be several 10s of degrees warmer than the outdoor air temperatures on a sunny day. You pick your weather-window of opportunity and go while the getting is good.

Thanks Dana! That was helpful. Do you know where I can find that type of injection foam? Or better yet brands. I have seen the DIY froth packs at home depot but I'm not sure if that is the slow rise you are explains with small hole meghod. I'll like do something like this in the early summer or late spring. But sounds like it would be worth it either way.

Also, you mentioned to lordoftheflies that she should install baffles in his slopes before filling them with cellulose. My roofers used really long nails like maybe 5 inches because I can see at least 2 or 3 inches of nails hanging down in the rafter bays. How do I slide the baffles down past them then push them up to get the cellulose hose in there?

 

[Dana]

helpful. Do you know where I can find that type of injection foam? Or better yet brands. I have seen the DIY froth packs at home depot but I'm not sure if that is the slow rise you are explains with small hole meghod. I'll like do something like this in the early summer or late spring. But sounds like it would be worth it either way.

Many most polyurethane spray foam installers will have "2 pound pour" options" (most of which are actually between 1.5-1.8lbs per cubic foot density, a bit lower density than the sprayed versions.)

For DIY slow rise kits, Tiger Foam, Handi-Foam, Touch n Seal, Versi-Foam, and others have similar kits.

To avoid blow-out potential it can be useful to have an infra-red camera to monitor how full the cavity is. Most of the expansion is in the first minute (as opposed to the first 3 seconds with sprayed-on close cell polyurethane) so don't fill the cavities completely on the first pass. The curing process is exothermic- and where the foam has already filled out becomes easy to see in the IR image as it expands. That allows you to better guesstimate how much is needed to top it off.

Be sure to drop-cloth everything below/around where your installation holes are drilled. Polyurethane insulation is chemically similar to Gorilla Glue, and a real PITA to clean up where it drips (or over-sprays, with sprayed polyurethane foam.)

At more than a buck a board-foot (that's 1" thick x 1 square foot) even in the 600 board-foot kits it's not cheap stuff, so estimate carefully. It's often more than $2 per board foot if bought as 200 board-foot kits.

There are even lower expansion "injection foam" products using other chemistry out there (TriPolymer, CoreFill, etc.) with effectively zero blow-out capacity and much lower adhesion characteristics, but those have a tendency to shrink leaving potentially large gaps at the framing & roof deck for a lousy air seal, and they have higher water vapor permeability than closed cell polyurethane. Avoid using them a wood framed wood clad applications like stud walls or wood roofs. They are more appropriately targeted for use in masonry cavity retrofit applications where even low expansion polyurethane pours could create enough pressure to crack brick mortar seams or concrete block. Wood & plaster have a bit more flexibility than masonry.