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Best way to insulate between studs with rigid foam

Posted by gabb (My Page) on
Tue, Oct 27, 09 at 12:14

Hi everyone,

I have this shed outside that I want to insulate so I can work in there in the winter and so it will hopefully be less damp in the summer.

It's an old construction. Most of the studs are 2x3 and in some places I only have 2 inches to work with. Since I don't have a lot of depth for insulation (and I don't to start changing the structure) I was looking for the most efficient insulator. Polyurethane was my first choice but couldn't find it in rigid foam sheets so polyisocyanurate it is. (R-6.5 per inch)

Now I was wondering how to install the thing properly. First time with foam insulation so I have a few questions.

First, I need to install the stuff between the studs. I didn't find much info on the web for that. Only that (of course) it will need be tight between the studs. What's the best way to do it? Try to fit it as tight as possible between the studs? (also considering that the studs are old and not perfectly square)

Or I was thinking that I could also cut the foam 1/2" smaller all around and fill that space with spray foam? What do you think? Too much trouble of worth it?

2) Do I need to glue the foam to the wood structure of the tight fit will be enough? If I need to glue it, I assume the best thing for the job is that foam glue they sell?
3) Do I need to tape the joint between the stud and the foam with a tape like tyvek or that foil tape?
4) Would I gain a lot from using those rabbeted foam products that cover the studs with some insulation material or just filling the space between the studs would be fine?
5) Anything else I should do or I'm ready to drywall? The foam acts as a vapor barrier as I understand. So I'm all set or there are some things I could do to improve the insulation efficiency?

6) The roof. It has beautiful horizontal 10" long beams. I was planning to put 4" foam in there (R-26). I assume I should simply glue it to the roof. But how do I put the drywall over it? Driving screws in there would reduce the insulation efficiency. Glue it to the foam? A way to fasten it to the beams on the sides? Or (and even better) could I simply paint the foam with some sort of appropriate paint that would adhere well to the foam and look fine. (it is a shed after all)

Hope you can help,
Thanks,
Gabriel


Follow-Up Postings:

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RE: Best way to insulate between studs with rigid foam

Gabriel,

Do you have the foam insulation already?

If not, I would consider spray foam instead. It can do all of the things that you need to accomplish, has a high R value and will seal the air gaps.

Here is a link that might be useful: DIY spray foam


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RE: Best way to insulate between studs with rigid foam

Yes we thought about doing this and I read a lot on the products but we're doing everything ourselves and have a relatively low budget.

There are some DIY spray foam products but from what I gather the quality of these products is somewhat inferior to the products used by the bigger contractor offering the service.

So unless hiring a contractor to do the job or getting a DIY package would cost us about the same as doing the insulation with foam panels, I guess we'll have to do as planed.


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RE: Best way to insulate between studs with rigid foam

I'm no building contractor, but when I had a bedroom insulated the way you want, the contractor told me that it was necessary to leave a small gap between the insulating boards and the sheet rock. I think there needs to be a small air flow there. We used "K-board". The best version has aluminum reflective skin on one side. It is polyisocyanate I think. He just fit the boards tightly between studs without glue. Hope someone more knowledgable also responds. I have long felt that solid foams are the way to go for insulation upgrades where there is some poor insulation already taking up space. I had the wallboard removed and replaced.

nerdy


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RE: Best way to insulate between studs with rigid foam

First, I need to install the stuff between the studs. I didn't find much info on the web for that. Only that (of course) it will need be tight between the studs. What's the best way to do it? Try to fit it as tight as possible between the studs? (also considering that the studs are old and not perfectly square)

thats the way..cut to fit, install with button cap nails
and use liquid nail

Or I was thinking that I could also cut the foam 1/2" smaller all around and fill that space with spray foam? What do you think? Too much trouble of worth it?

too much time, cans of foam are costly..use above method

2) Do I need to glue the foam to the wood structure of the tight fit will be enough? If I need to glue it, I assume the best thing for the job is that foam glue they sell?

see above

3) Do I need to tape the joint between the stud and the foam with a tape like tyvek or that foil tape?

not if you caulk it. most tapes won't adhere to wood
the ones that will seal both to foam boards and wood are costly..caulk is the cheapest just buy long life caulk.

4) Would I gain a lot from using those rabbeted foam products that cover the studs with some insulation material or just filling the space between the studs would be fine?

between studs. with foam tight against outside of wall

5) Anything else I should do or I'm ready to drywall? The foam acts as a vapor barrier as I understand. So I'm all set or there are some things I could do to improve the insulation efficiency?

make it as much of an air tight install as possible.
if you want add layers of foam board..just keep in mind
that you'll need longer and longer button cap nails.

6) The roof. It has beautiful horizontal 10" long beams. I was planning to put 4" foam in there (R-26). I assume I should simply glue it to the roof.

if you install on the botton of the rafters then you could just use luaun, paneling or plywood. are you trying to
show the beams? are these beams roof line or ceiling
hard to say without seeing.(and its a shed)

best of luck


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RE: Best way to insulate between studs with rigid foam

Hi guys. Thanks so much for your help! Very much appreciated.

I have some little question left
1) If I use cap nails, won't that making the insulation less effective because of the holes?
2) energy rater, can you confirm what nerdyshopper said about leaving space between the foam and the drywall?
3) Any technique for an efficient way us cutting the foam to size so it's gonna be as airtight as possible on irregular shaped dimensions? -->The dimensions are never rectangular. Sides are like: 27" 3/8, 44" 1/4, 28" 1/8 and 43" 3/4 with odd angles.

"not if you caulk it. most tapes won't adhere to wood. the ones that will seal both to foam boards and wood are costly..caulk is the cheapest just buy long life caulk."

So you mean caulk every single place where the foam meets the studs? Is it really gonna improve the insulation?

"are you trying to show the beams? are these beams roof line or ceiling"

Yes I want to show the beams. (roof line)


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RE: Best way to insulate between studs with rigid foam

1) If I use cap nails, won't that making the insulation less effective because of the holes?

the nails have a plastic cap that will give you a decent seal and will cover the hole left by the nail. as you are nailing into a solid surface (wood) and foam coveres gaps in siding its a non issue as long as you caulk the sides to the studs.

2) energy rater, can you confirm what nerdyshopper said about leaving space between the foam and the drywall?

his was a conditioned space application. in homes foamboard can't be left exposed, it has to be behind a fire rated surface like drywall. in his application they used a foil sided foamboard and I guess the radiant barrier (foil)
is facing the inside of the wall, putting anything in contact with a radaint barrier is a no no as it doesn't allow the rb to reflect the heat. we put it on exterior walls withthe foil facing the brick, but not to the interior of the wall. yours is a shed.if you plan on finishing the wall out..use a dow blue board and insulate the walls with batts and install sheetrock over them..if its a fancier shed..are you planning on heating and cooling the shed? spending a lot of time in there?
what is your location?
there is a vapor barrier location criteria.
oh and radiant barriers are for hot climates.

3) Any technique for an efficient way us cutting the foam to size so it's gonna be as airtight as possible on irregular shaped dimensions? --> The dimensions are never rectangular. Sides are like: 27" 3/8, 44" 1/4, 28" 1/8 and 43" 3/4 with odd angles.

Yep, time and straight edge, and extra razors for the razor knife. cut each to fit tightly and if its too small
cut a slice of foam board make it tight fit and nail in place with button cap nails seal with caulk.

So you mean caulk every single place where the foam meets the studs? Is it really gonna improve the insulation?

exactly. what you are trying to do is make the space comfortable enough to work in. foam sheathing will stop the air flow. if you install it with gaps you will have lots of air infiltration.
this is the tradeoff...you do the install of foam sheathing
material cost and time or hire a foam company for higher cost and less time. a shop is a diy imo.
caulk is cheap. foamboard is easy to work with.
insulation doesn't perform because of the air that moves thru it. stop the air movement and it makes a huge difference.

how do you plan to finish the ceiling?


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RE: Best way to insulate between studs with rigid foam

Hi energy rater! Wow thanks for your help again.

"are you planning on heating and cooling the shed? spending a lot of time in there?
what is your location?
there is a vapor barrier location criteria.
oh and radiant barriers are for hot climates."

Yes I will spend a good amount of time in there. The insulation was mostly for the winter but was hoping it might help the place to be less damp in the summer. (I live in Canada)

I thought the vapor barrier was done with the foam itself?
Good to know about radiant barriers. I thought they could also reflect heat in the winter like it was the case for me.

I have 2 more questions. I have from 2" 1/2 to 3" to work with with the insulation. I am getting 2" thick foam sheets. What do you suggest I do with the space left? Try to put more foam in there if possible (get 1/2" thick foam?) Put fiberglass? Use the foil facing foam and leave the space empty?

And second. Heat escapes more through the ceiling than trough the walls right? So it was a good idea to put two sheets of foam in there to get to 4"?

"how do you plan to finish the ceiling? "
I wanted to finish it without drywall and put a simple primer directly on the foam. Is that OK?


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btw

BTW sorry for the delay I was away for the weekend...


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RE: Best way to insulate between studs with rigid foam

I have 2 more questions. I have from 2" 1/2 to 3" to work with with the insulation. I am getting 2" thick foam sheets. What do you suggest I do with the space left? Try to put more foam in there if possible (get 1/2" thick foam?) Put fiberglass? Use the foil facing foam and leave the space empty?

I'd just leave it. The thinnest fg batt would be compressed so much that it would be very mininal R-value
and imo..not worth it.

And second. Heat escapes more through the ceiling than trough the walls right? So it was a good idea to put two sheets of foam in there to get to 4"?
Yes.
Unless you want to put some fg batts and use your 2" foam
nailed to the face of the rafters. Just thinking about all that overhead work makes my neck ache. Of course then you
would be covering your rafters you want to show off.

"how do you plan to finish the ceiling? "
I wanted to finish it without drywall and put a simple primer directly on the foam. Is that OK?

ok by me. there is an issue with leaving foam exposed in
living spaces in homes, but this is a shop.
the reason that it is covered in a home is that it burns and releases a nasty gas. thus it has to have a flame spread rated surface like sheetrock to contain and seperate it. (thats just fyi)

now I have questions.
are you going to add heat to this shop?

What type of foam are you using?
closed cell has more rigidity
open cell is white soft foam
closed cell has the higher R-value
dow blue board and corning pink are both
closed cell, although not limited to these
colors and brands.
I find open cell is harder to work with as it doesn't cut
as cleanly, but thats just my experience.

and last...what are you paying for your foam boards?
I never see 2" here, although I know you can special order it. I have a friend in Minnesota and he says 1 1/2" is common there.

Hope you had a nice weekend away.
And you are welcome for the advice, not a problem.

Best of luck and stay warm up there!


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RE: Best way to insulate between studs with rigid foam

Thanks again for your help!

>now I have questions.
>are you going to add heat to this shop?

yes

>What type of foam are you using?
I am using polyisocyanurate foam boards from a company named roofmart. It's a closed cell and is r-6 per inch.

My goal was to find the material that had the best r value because I didn't have a lot of inches to work with. So closed cell it had to be... At first I wanted to get polyurethane rigid foam sheets but this was impossible to find. polyiso is supposedly r-6 r-6.5

>and last...what are you paying for your foam boards?
> I never see 2" here, although I know you can specialorder it. I have a friend in Minnesota and he says 11/2" is common there.
The company I'm buying from has different thickness ranging from 1" to 3 or 4 I think. In fact I'm gonna get the 2" 1/2 after all. I'm gonna be paying about 80cents per square foot. Here in Canada it is reputedly more expensive than in the US.

Could we talk a little bit about the topic of radiant barriers and vapor barriers?
First: Would using a foil faced foam improve the insulation efficiency in my case?

I read a bit about radiant barriers and stumbled on a product called Prodex (www.insulation4less.com)
They claim it works in hot and cold climates and has a r-value of 15,67 for one layer of 5mm. Now that sounds too good to be true. What do you think?

Also radiant barriers are said to work only in hot climates but they state cold climate as well. Any thoughts?

Concerning vapor barriers, I was just wondering if the foam itself acted as the vapor barrier or if I needed to add something. And if I used a foil faced foam, would that act as the vapor barrier?


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RE: Best way to insulate between studs with rigid foam

Sorry for the delay...

Could we talk a little bit about the topic of radiant barriers and vapor barriers? Sure
First: Would using a foil faced foam improve the insulation efficiency in my case?

if left exposed you might
have minimal efficiency, but if you are planning on covering it with paint or anything then the answer is no.
RB need a minimium of a 3/4" air space to perform.
check out www.buildingscience.com for you climate
Joe will even answer emails and is an excellent building scientist..and from Canada eh!

I read a bit about radiant barriers and stumbled on a product called Prodex (www.insulation4less.com)
They claim it works in hot and cold climates and has a r-value of 15,67 for one layer of 5mm. Now that sounds too good to be true. What do you think?

it is sales hype. at best rb are R-10
florida solar energy center has good info on RB.
foil bubble foil works no better than a single sided foil
and note...fsec is a hot climate..where RB's work.
and is info only ..not selling products. gotta watch out
for those sales pitches.

the foam will be your vapor barrier..with or without the foil. persoanlly I don't think you'll see a benfit with the foil.

Also radiant barriers are said to work only in hot climates but they state cold climate as well. Any thoughts?

lots of thoughts..none of them good. companies sell
and move on to the next customer, while the customer
is left with a product they over paid for that doesn't
perform. this is why I only recommend websites that give info, not sell products.

Concerning vapor barriers, I was just wondering if the foam itself acted as the vapor barrier or if I needed to add something. And if I used a foil faced foam, would that act as the vapor barrier?
foam will be vapor barrier when installed as we discussed earlier..see above for foil vs foam in my answer.

do be careful with exposed foam and heating this shed
foam burns quickly and releases a toxic gas.
this is why it has to be covered with sheetrock in a
house to allow for flame spread time.

best of luck, and again...sorry for the delay in getting
back online.


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