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I'm finishing an unfinished second floor. It'll be a lot like most attics. I'll have a 4' knee wall, then a 5' slanted section and 6' wide ceiling. My rafters are 5'6" deep, so I'm assuming I need to spray foam that area. In an effort to keep cost down. Is it possible/advisable to staple in fiberglass insulation in the knee walls and ceiling where I have plenty of room. And only spray foam the slanted ceiling areas?

Thanks.
 

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I'm finishing an unfinished second floor. It'll be a lot like most attics. I'll have a 4' knee wall, then a 5' slanted section and 6' wide ceiling. My rafters are 5'6" deep, so I'm assuming I need to spray foam that area. In an effort to keep cost down. Is it possible/advisable to staple in fiberglass insulation in the knee walls and ceiling where I have plenty of room. And only spray foam the slanted ceiling areas?

Thanks.
Yeah you can use fiberglass for the knee walls, Do not spray foam directly against the roof decking, Install durovents or likewise product from the peak to the soffit. Then spray foam against the durovent. This will allow airflow behind the insulation and allow the roof decking to breath.

I guess if your looking to save money, you could possibly run the durovent from the peak down to the knee wall and foam it all in and fill in the knee wall with fiberglass and leave behind the knee wall un insulated.
 

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retired framer
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I'm finishing an unfinished second floor. It'll be a lot like most attics. I'll have a 4' knee wall, then a 5' slanted section and 6' wide ceiling. My rafters are 5'6" deep, so I'm assuming I need to spray foam that area. In an effort to keep cost down. Is it possible/advisable to staple in fiberglass insulation in the knee walls and ceiling where I have plenty of room. And only spray foam the slanted ceiling areas?

Thanks.
Spray foam would be my last choice. You want to maintain venting
Air tight electric boxes




1. solid blocking under edge of floor and sealed, no air from house to attic.
2. 2x4 or 2x6 added to slope to allow for more insulation.
3. air chutes to allow air above insulation.
4 soffet vents
5. batt insulation to stop loose insulation out of the soffet
6. High vent boxes or ridge
7 . loose fill insulation
8. Just enough strapping to hold the insulation in the wall.
 

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retired framer
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Isn't spray foam my only choice if my rafters are only 5.5"? I don't know how to get enough R value otherwise.
Did you see my item #2 above.

You could just squeeze in some 4" foam board and leave 1 1/2" above that for venting and then cover the whole ceiling with 1" foam behind the drywall.


See how the have added 2x4s to the bottom of the 2x8 rafter member in this truss system.
 

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Related:


Where you want to vent moisture (to the inside or outside) depends on your climate.

Here's a really good primer on control layers (venting, vapor, insulating, etc.) -

I'd check out all of this guy's videos - just search his channel for closed cell foam and you'll get a huge list of videos with a ton of information about the science of all of this stuff. He usually works in warm climate, but he's done a bit of cold climate.
 

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Discussion Starter · #9 ·
Did you see my item #2 above.

You could just squeeze in some 4" foam board and leave 1 1/2" above that for venting and then cover the whole ceiling with 1" foam behind the drywall.


See how the have added 2x4s to the bottom of the 2x8 rafter member in this truss system.

From what I can tell 4" of foam only gets me to R24 or so, but it may be enough. I did get the recommendation to sister all the rafters, but I'm fighting for space as it is.

I'll probably have to do one or the other. Thank you.
 

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retired framer
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From what I can tell 4" of foam only gets me to R24 or so, but it may be enough. I did get the recommendation to sister all the rafters, but I'm fighting for space as it is.

I'll probably have to do one or the other. Thank you.
That is always a problem, If you do the 4" of foam flush with the inside of the rafters add anther 1" over that and you get the thermal break, better sealing and closer to 30 R.
 

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You can cobble around and try and do it yourself and probably screw it up, or just do what everyone else it doing and have it sprayed and done right the first time. Spray foam is unbeatable. Just do it right the first time. I DIY-ed my attic finish and screwed up the air sealing, got moisture, and am now paying to have it done right.
 

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Isn't spray foam my only choice if my rafters are only 5.5"? I don't know how to get enough R value otherwise.

Just pay for the foam. I am telling you from experience. It will be done in a day and you will have peace of mind. Get a reputable company and let them go at it. They know what they are doing. Reading this board and Greenbuildingadvisor.com will run you crazy. Just go with the foam. It is the best. I decided that my family deserves the best and not some hackneyed solution to save a few thousand.
 

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Spray foam is highly toxic and if it is done wrong it can require removal of the entire roof to fix. Many places to find this information on the internet as the homeowners have sued the contractors and the foam companies. Also in a closed attic that does not have outside air coming in, the outgassing can go on for months and make the inhabitants very ill. The foam companies installation guides specify the number of air exchanges needed over the first 24 hours to minimize problems but that requires running an 8" or larger hose to a special industrial vacuum and having an equal size hose proving make-up air. None of the SPF contractors in my area do this.

I would spray foam a barn or a garage but not a house.
 

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Foam is plastic. If mixed properly and by an approved installer, it is safe for use inside the home and is a great application in some cases. I fully agree that it is over used, but there are some applications where it is just the best option. That plastic shower curtain is likely more toxic to the inhabitants of the home than a properly installed foam job.
 

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Spray foam is highly toxic and if it is done wrong it can require removal of the entire roof to fix. Many places to find this information on the internet as the homeowners have sued the contractors and the foam companies. Also in a closed attic that does not have outside air coming in, the outgassing can go on for months and make the inhabitants very ill. The foam companies installation guides specify the number of air exchanges needed over the first 24 hours to minimize problems but that requires running an 8" or larger hose to a special industrial vacuum and having an equal size hose proving make-up air. None of the SPF contractors in my area do this.

I would spray foam a barn or a garage but not a house.

If it is done wrong, the contractor will be on the hook. Go with a gigantic company and you will be OK. i have changed my view on foam. It is the only way to insulate a cathedral.
 

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retired framer
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You can cobble around and try and do it yourself and probably screw it up, or just do what everyone else it doing and have it sprayed and done right the first time. Spray foam is unbeatable. Just do it right the first time. I DIY-ed my attic finish and screwed up the air sealing, got moisture, and am now paying to have it done right.

So you haven' told us what was done wrong with the first job that was done and why you think doing it right is impossible?


As you are just now paying to have it done, you are not talking about years of satisfaction, just a firm belief.?
 

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So you haven' told us what was done wrong with the first job that was done and why you think doing it right is impossible?


As you are just now paying to have it done, you are not talking about years of satisfaction, just a firm belief.?



The job he is talking about doing has been done wrong so many times and every forum on the Internet points toward foam as the best and easiest solution for his situation. My job is a retrofit now because we thought I had put enough fiberglass and sealed everything right and ended up with an attic full of frost an moisture. The problems with finishing these attics are written about extensively on the Internet. Mostly because of 2X6 or 2X8 framing and condensation. My foam guys said that the benefits of foam are really showcased in Cape Cod attics. It's as expensive as hell, but worth it. With all the talk of air sealing on here, why would people not use this stuff?
 

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retired framer
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The job he is talking about doing has been done wrong so many times and every forum on the Internet points toward foam as the best and easiest solution for his situation. My job is a retrofit now because we thought I had put enough fiberglass and sealed everything right and ended up with an attic full of frost an moisture. The problems with finishing these attics are written about extensively on the Internet. Mostly because of 2X6 or 2X8 framing and condensation. My foam guys said that the benefits of foam are really showcased in Cape Cod attics. It's as expensive as hell, but worth it. With all the talk of air sealing on here, why would people not use this stuff?
So, all the suggestions here that said not to do t like you did are just ignored and now you believe everything about foam like it was words from God.



If you don't add heat to the room, cold will make it to the inside. Adding more R value only gives a few more days before you have the same condition unless you add the heat.
 

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So, all the suggestions here that said not to do t like you did are just ignored and now you believe everything about foam like it was words from God.



If you don't add heat to the room, cold will make it to the inside. Adding more R value only gives a few more days before you have the same condition unless you add the heat.

I don't understand the hate here. Spray foam is recommended for Cape attics framed with 2 x 6 and 2 x 8 because it is so much better. Yes, it is expensive. It would have saved me a headache had I done it in the first place. I read these posts where guys try these alternative methods and then they worry about them being right.
 

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I don't understand the hate here. Spray foam is recommended for Cape attics framed with 2 x 6 and 2 x 8 because it is so much better. Yes, it is expensive. It would have saved me a headache had I done it in the first place. I read these posts where guys try these alternative methods and then they worry about them being right.
I don't hate it, there are times foam is the answer. I guess you missed the part about adding to the rafters so they are the size of 2x10s or 2x12. Each house can be judged on there own. I would sooner figure out what happened that caused yours to fail.
 
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