Spray Foam Insulation

[zzvyb6]

BTW: I saw on a foam manufacturers spec sheet that the use of closed cell foam is limited to 4 inches because of concerns about 'spontaneous combustion'. Thus you are limited an R rating of 37 based on the use of 4, 1" coats.

 

[3Ts]

Looking at the picture that was posted, I agree that spray foam is probably the best route to insulate the backside of it. I would probably buy a Froth Pak that is rated to cover about twice the square footage of the outside of the skylight and have at it. I would probably also pull the insulation back around the base so that you can foam around the whole sky light outside from top to bottom. The bottom part where the other insulation is I would just try to do a skim coat but the upper part you would probably want a couple of inches on.

Aaron Z

The above is probably the best solution.

However, If you have to use batts, you'll have a problem keeping them vertical unless you can staple them to the studs in that area. Another idea is to use chicken wire to tie the batts to for support before hanging them from the rafters.

 

[IXLR8]

BTW: I saw on a foam manufacturers spec sheet that the use of closed cell foam is limited to 4 inches because of concerns about 'spontaneous combustion'. Thus you are limited an R rating of 37 based on the use of 4, 1" coats.

I have read the same, I also talked with a Dow Corning spray foam applications engineer and he sent me some documentation that indicated that one should wait a MINIMUM of 20 minutes between each 1" layer. Apparently that little tidbit of info is not in most applications sheets, at least it isn't in any of the standard Dow Corning paperwork. This is to give time for each layer to cool down. Otherwise the chemical reaction of the foam chemicals can get very hot and the added layers of foam trap that heat in. If you put too much on too quick, it can get hot enough to ignite the wood. Based on what the applications engineer told me, if you wait for the foam to cool down between layers, there shouldn't be any issues or limitations on thickness.

 

[EddieWalker]

I attached a picture of one of the skylight areas. I am interested in the insulation of the areas marked in red on the attic side. Below the red horizon line they are insulated by ceiling bats and I am thinking about adding blown in insulation on top of that as Eddie recommended in a post above. Above the horizon like line they are insulated by vertical bats between studs loosely supported by strings wrapped around the column going up to the skylight. Not the best fastening method. The bats over the years have sagged a bit and some sheetrock is exposed to the attic temperatures. One advice I had was to resupport and add another layer of bats horizontal. Reading this thread spray on closed cell seems to make sense. Since heat rises, these skylight areas could lose more heat than my walls depending on attic temperatures.
View attachment 634218

Wind isn't a factor inside the attic area. The amount of air movement that happens from the vents isn't enough to create anything significant with fiberglass bats of insulation. I've only insulated a few attics with skylights and each time it was a little different. I put in fiberglass bats and staple them to the studs. Then I figure out where 2 feet of blown in insulation will come up the side of the skylight wall, and add a second layer of 24 inch wide R19 fiberglass insulation over what's inside the studs above where the blown in will go. Sometimes the R19 goes from the joists on up, it really depends on what it takes to get it into place. I've always been able to wedge this second layer in place so it remains fluffy, but tight with each other. Once that's all in place, I spray in the 2 foot thick layer of AttiCat. Years ago I used to use other types of spray in insulation and it required the mask and was very dusty. AttiCat is easy to work with.

I would not spend the money for spray foam for a roof with a skylight. They are such massive holes in the envelope that there is no advantage to spending top dollar for insulation when you have such a massive hole in your roof.

 

[ovrszd]

I would not spend the money for spray foam for a roof with a skylight. They are such massive holes in the envelope that there is no advantage to spending top dollar for insulation when you have such a massive hole in your roof.

Yep, that goes back to my earlier question of "what's the R value of the skylight".

What do they call it when they look for heat loss in a structure? Infrared or something like that? When you see red areas where heat is being lost? A picture of this house roof on a cold day would be very enlightening.

 

[EddieWalker]

Skylights are one of those things that make zero sense to me. Clients tell me how much they light the amount of light that they give, and I understand that to a small degree. But they are not even home during the day 5 out of 7 days to enjoy that light. They have also told me that they save energy by not having to turn a light on during the day with a skylight, but how much are they actually saving by not turning that light on the few days a week that they are home during the day when they get the extra light from the skylight? Comparing how much they save from not turning on that light a couple days a week, for a couple of hours during the day to what they lose in energy from having that hole in their roof doesn't even come close for me.

There are 3 things in life that nobody can avoid. Death, paying taxes, and leaking skylights.

When that skylight starts to leak, the cost to fix it is bad to horrible. If you are lucky, it happens when you need a new roof and you have the money set aside for that new roof, or insurance is paying for it from hail damage. I've been part of those jobs where I seal off the ceiling. I've also done a few jobs where the skylight leaked and stained the walls of the sheetrock like in the picture, and part of the ceiling. There is always something with having a skylight that causes more money to be put into repairs then you could ever possibly gain by not having to turn on a light in that room, during the day when you are home.

My favorite metaphor is rebuilding the engine in your car, and adding aftermarket parts to get another 100 HP, but leaving the bald factory tires on it.

 

[ovrszd]

Certainly nothing personal about this and not dogpiling on goeduck's situation. So hopefully it's not taken that way.

Just never heard of an insulation concern surrounding a skylight.

Some friends of ours have a couple. They are curtain people and barely allow light into their house thru the windows. They talk of the light allowed in thru the skylight. We don't have a single curtain or blind in our house, not one. If we had skylights I'm not sure anyone would notice.

 

[goeduck]

Certainly nothing personal about this and not dogpiling on goeduck's situation. So hopefully it's not taken that way.

Just never heard of an insulation concern surrounding a skylight.

Some friends of ours have a couple. They are curtain people and barely allow light into their house thru the windows. They talk of the light allowed in thru the skylight. We don't have a single curtain or blind in our house, not one. If we had skylights I'm not sure anyone would notice.

I agree that skylights do not make sense, but I bought the house, I did not build it. So they are there. The bats on the walls up to the skylights have partially pulled away from the studs and Sheetrock so I need to do something with it. The builder back in 1988 simply put up bats and wrapped string around the attic side of the walls to hold the bats in place. I bought the house in 2015 and there have been no leaks. We get a lot of wet days out here so the roofer must have done a good job. Interestingly the ceiling does have a vaulted area adjacent to two of the skylights and when we do get cold weather the snow stays on the vaulted sections of the roof longer than the area immediately surrounding the skylight area. The insulator did not do as good of a job as the roofer. This tells me I have enough loss through the side walls of the skylight column. Hence my questions. Thank you all for your input.

 

[ovrszd]

I agree that skylights do not make sense, but I bought the house, I did not build it. So they are there. The bats on the walls up to the skylights have partially pulled away from the studs and Sheetrock so I need to do something with it. The builder back in 1988 simply put up bats and wrapped string around the attic side of the walls to hold the bats in place. I bought the house in 2015 and there have been no leaks. We get a lot of wet days out here so the roofer must have done a good job. Interestingly the ceiling does have a vaulted area adjacent to two of the skylights and when we do get cold weather the snow stays on the vaulted sections of the roof longer than the area immediately surrounding the skylight area. The insulator did not do as good of a job as the roofer. This tells me I have enough loss through the side walls of the skylight column. Hence my questions. Thank you all for your input.

Looking forward to your solution and improvement. Thanks for posting the pics.

 

[quicksandfarmer]

I was referring to a standard 16"OC wall.

In a standard 2x wall, 8' high with 16" stud spacing, each stud bay is 16x96 or 1,536 square inches. The thermal breaks in that wall are:

1.5 inch stud 91.5" high, or 137.25 square inches.

A bottom plate and a double top plate, each 16" x 1.5, or 72 square inches.

A fire block, 1.5 x 13.5 or 20.25 square inches.

Total is 229.5 square inches, or 14.9%.

Wood studs have an R-value of about 1.25 per inch. Except for closed cell foams, most insulations are about R 3.5 per inch. Combined, the wall has an effective R-value of about 2.9, or about 80% of the value of the insulation alone. Almost always, the most cost-effective way to increase the effective R-value is to make the wall thicker, either go up a size in framing or add a layer of continuous foam.