# Spray foam sections of an attic?



## mjhaston (Jul 15, 2012)

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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## JayDoesIt (Jan 25, 2020)

mjhaston said:


> 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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## Nealtw (Jun 22, 2017)

mjhaston said:


> 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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## Windows on Wash (Aug 30, 2011)

https://www.finehomebuilding.com/pdf/021221068.pdf


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## mjhaston (Jul 15, 2012)

Nealtw said:


> Spray foam would be my last choice.



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.


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## Nealtw (Jun 22, 2017)

mjhaston said:


> 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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## Windows on Wash (Aug 30, 2011)

Are you talking about enough R-Value for code compliance or just in general? 4" of foam is more than effective enough if you are leaving a 1.5" ventilation pathway. More than enough.


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## Mystriss (Dec 26, 2018)

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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## mjhaston (Jul 15, 2012)

Nealtw said:


> 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.
> 
> ...



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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## Nealtw (Jun 22, 2017)

mjhaston said:


> 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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## APA (Jul 13, 2018)

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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## APA (Jul 13, 2018)

mjhaston said:


> 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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## Calson (Jan 23, 2019)

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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## Windows on Wash (Aug 30, 2011)

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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## APA (Jul 13, 2018)

Calson said:


> 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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## Nealtw (Jun 22, 2017)

APA said:


> 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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## APA (Jul 13, 2018)

Nealtw said:


> 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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## Nealtw (Jun 22, 2017)

APA said:


> 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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## APA (Jul 13, 2018)

Nealtw said:


> 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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## Nealtw (Jun 22, 2017)

APA said:


> 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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## APA (Jul 13, 2018)

Nealtw said:


> 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.



I had 2x8 rafters. I put in the vents and compressed r-30 batts because I had read that remodels met code at R-22 in the cathedral run. The drywallers hung the drywall and we were up there working running electric space heaters later that week. I insulated the attic with r-30 faced batts and about 3-4 inches of blown fiber on top of that. The first night it got down to 18 degrees, I went to the attic just to look and there was frost everywhere. The styrofoam chutes had water on them from top to bottom in the upper attic and lower attic. We are only talking about a 5-foot cathedral run here. By this time it was November and I did not want to have the drywall finished out with this going on and cold weather coming. The condensation _could_ have been caused by the warm air getting into the attic through the gaps in the unfinished drywall seams, but I just decided that instead of chasing it and figuring it out on my own that I would just pay a company to come in and use foam. Yes, the quote seemed high for the foam, but it will be one and done and this 3-year project can be finished. I am done with DIY. This project I took on seemed easy, but Cape Cod homes are a nightmare. Foam is the choice. I am having it done this spring. Sometimes peace of mind is worth it. There are too many problem-free foam installations to be derailed by a few articles where the people only had problems because they did not follow the directions of the installer. Also, using a company, they are on the hook if they screw it up. I had a friend do their home with a froth-pak. They had zero issues with fumes. So I think a company that does it hundreds of times a year knows what they are doing.


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## Nealtw (Jun 22, 2017)

APA said:


> I had 2x8 rafters. I put in the vents and compressed r-30 batts because I had read that remodels met code at R-22 in the cathedral run. The drywallers hung the drywall and we were up there working running electric space heaters later that week. I insulated the attic with r-30 faced batts and about 3-4 inches of blown fiber on top of that. The first night it got down to 18 degrees, I went to the attic just to look and there was frost everywhere. The styrofoam chutes had water on them from top to bottom in the upper attic and lower attic. We are only talking about a 5-foot cathedral run here. By this time it was November and I did not want to have the drywall finished out with this going on and cold weather coming. The condensation _could_ have been caused by the warm air getting into the attic through the gaps in the unfinished drywall seams, but I just decided that instead of chasing it and figuring it out on my own that I would just pay a company to come in and use foam. Yes, the quote seemed high for the foam, but it will be one and done and this 3-year project can be finished. I am done with DIY. This project I took on seemed easy, but Cape Cod homes are a nightmare. Foam is the choice. I am having it done this spring. Sometimes peace of mind is worth it. There are too many problem-free foam installations to be derailed by a few articles where the people only had problems because they did not follow the directions of the installer. Also, using a company, they are on the hook if they screw it up. I had a friend do their home with a froth-pak. They had zero issues with fumes. So I think a company that does it hundreds of times a year knows what they are doing.


So how did moisture get in that area. You had sealed around outlets and lights? and and a temp seal on the doors to the nee areas until you could get sealed doors installed?


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## APA (Jul 13, 2018)

Nealtw said:


> So how did moisture get in that area. You had sealed around outlets and lights? and and a temp seal on the doors to the nee areas until you could get sealed doors installed?



Why do all that when some guys with a foam truck can take care of it in an afternoon?


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## Nealtw (Jun 22, 2017)

APA said:


> Why do all that when some guys with a foam truck can take care of it in an afternoon?


I am trying to figure what you did not what you are going to do. I guess we have the answer.


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## mjhaston (Jul 15, 2012)

Nealtw said:


> 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.
> 
> ...


I watched a video of a guy doing just want you said. It's a lot of work. How is rocking over foam board? Not sure I've ever done it in 12 years of hanging and finishing drywall. It seems like the screws would put through pretty easily?

I'm still up in the air. I could pack out just my slanted ceiling section, although it would be quite a bit, like 6". And then fiberglass the whole thing.

Foam estimates have been coming in around $6,600. Batts $2,800. It's probably about $1,000 worth of batts if I do it myself. I checked into a green foam DIY kit and it only saved me $1,000. Not worth it my opinion.


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## Nealtw (Jun 22, 2017)

mjhaston said:


> I watched a video of a guy doing just want you said. It's a lot of work. How is rocking over foam board? Not sure I've ever done it in 12 years of hanging and finishing drywall. It seems like the screws would put through pretty easily?
> 
> I'm still up in the air. I could pack out just my slanted ceiling section, although it would be quite a bit, like 6". And then fiberglass the whole thing.
> 
> Foam estimates have been coming in around $6,600. Batts $2,800. It's probably about $1,000 worth of batts if I do it myself. I checked into a green foam DIY kit and it only saved me $1,000. Not worth it my opinion.


 Spray foam is mixed chemicals and things can go really bad. 

You can get drywall screws up to 3" long but my first choice would be to add inches to the bottom of the rafters. 

I think I would go for a strip of foam board on the rafter and then a 2x2 or 2x3 with an air chute and batt insulation.


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## mjhaston (Jul 15, 2012)

I may need to level out some rafters anyway, so adding some wood to the rafters to get 3 layers of foam board in-between could be a good way to go.


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## mjhaston (Jul 15, 2012)

APA said:


> Why do all that when some guys with a foam truck can take care of it in an afternoon?



I get it. I'm just having hard time thinking of paying 3 or 4 times what I could do it for.


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## mjhaston (Jul 15, 2012)

I think I'm pretty settled on my approach. 

On the slanted portion of my walls/ceilings my rafters are 5.5" so I'm going to leave a 1.5 air gap. Going to put spacers in. In between the rafters 2" of foam board. Another 2" of foam board. That should bring me flush with the rafters. Then I'll lay full 4' x 8' x 2" sheets. I don't like the idea of drywall right on top of the foam board and screws pulling through, so I'll lay some 1" strips horizontally and rock over top of that.

I'll batt the top flat ceiling and the knee walls. To spray everything was on average $6,800. This approach should cost me $3,000 in materials tops.


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## APA (Jul 13, 2018)

mjhaston said:


> I get it. I'm just having hard time thinking of paying 3 or 4 times what I could do it for.



You know there are thousands of homes with 2X6 rafters crammed full of fiberglass that have survived for years with no issues. I got advice from a builder and he said that we just go up the knee wall with fiberglass and continue up the cathedral run after stapling the baffles in. I have been in 1/2 story homes they have finished. They are neither too hot or cold...


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## mjhaston (Jul 15, 2012)

I'm not sure I'd feel comfortable (pun possibly intended) with only an R value of 16 or 17 in the roof of a Northeast home. I'm not looking forward to dealing with all of the foam board, but even 2 layers of 2" will get me to an R value of 26.


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## mjhaston (Jul 15, 2012)

So just to be clear. I have a roof ridge. My soffits are open now anyway, so I can knock the blocking out of each bay that is there now. 

1- R21 in my knee walls. 

Slopped section:
2- 1.5 inches of air gap. 
3- 2" foam board with silver coat out between rafters. Either fit it super tight or leave some space and foam spray the edges. 
4- Second layer of 2" of foam board between rafters with silver coat in.
5- Possibly 1" foam board over everything?

6- R38 in ceiling rafters.

That should allow proper venting correct?


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## APA (Jul 13, 2018)

mjhaston said:


> So just to be clear. I have a roof ridge. My soffits are open now anyway, so I can knock the blocking out of each bay that is there now.
> 
> 1- R21 in my knee walls.
> 
> ...



I see that very solution posted quite often. The only problem is screwing drywall through all that thickness. Not really a problem, I did read one article where a guy had condensation problems inside his upstairs because it was so tight.


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## mjhaston (Jul 15, 2012)

My 48 boards of 2" foam board arrived yesterday and I got started with that portion of the project. My son and I tacked in our spacers. I cut, he used the nail gun.

Yesterday I found that my brand new $90 foam board cutting blade doesn't extend far enough out of my table saw to cut the entire 2" of foam, so I reversed my wood blade as someone suggested. Less mess, but still mess. The foam blade had virtually no dust.

https://marshalltown.com/pro-3403-centerfire-circular-foam-blade

Now to the installation of the foam between the rafters ..... what a huge pain in the ass and mess! I'm finding a friction fit to be handy after I gut the board to the approximate width on the table saw. I'm using a small hand saw with fine teeth to trace the rafter and push the board in. There's just a lot of extra work since I'm having to go around a knee wall stud at the bottom of each section of foam and a ceiling rafter when I get up to the top.

This is going to take a while! But it should work. The 8' sheets run almost perfectly from the knee wall to just above the R38 insulation I'm putting in the ceiling. So the venting should work.


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## APA (Jul 13, 2018)

mjhaston said:


> My 48 boards of 2" foam board arrived yesterday and I got started with that portion of the project. My son and I tacked in our spacers. I cut, he used the nail gun.
> 
> Yesterday I found that my brand new $90 foam board cutting blade doesn't extend far enough out of my table saw to cut the entire 2" of foam, so I reversed my wood blade as someone suggested. Less mess, but still mess. The foam blade had virtually no dust.
> 
> ...


That is my issue with this certain DIY method. It is so labor intensive and if there are issues it was all for naught. There is nobody to call and have come fix it, and you are out all that time and money. I really hope it works out.


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