insulation

[ced257]

 Morning folks! I am finally about to start on my cabin. I got a question about some insulation. My funds are limited so I can't do solid foam like I would like to. I am using 1 1/2" foam insulboard, fiberglass batt and  maybe some  reflectix radiant bubble  insulation.
  My cabin is  going to  be a  16x32, with a gambrel roof and upper loft. I am adding a 8x32 lean to off the sides for my bath and 2 other beds. I am not sure if I am going to have exposed reafters or not, let's just say we are. I want to get the best r-value for the material I have on hand. The bottom of my cabin will be underpenned, so I am just worrying about the walls and roof right now. I got a deal on the 1 1/2 foam board and can get quiet a bit of it pretty cheap. I was wondering what you all's ideas are on the best way to insulate with what I have, especially the roof. That is my main concern right now. I am just trying to get my ducks in a row so I can get this lil cabin on the road! Thank you!

[poppy]

  This is a good place to get practical answers.

First it's a good idea to let us know where and under what climatic conditions you are building.

Second, John Raabe has posted several very good links in the Referral Links section of this forum.  They are good reads.

In any case, I would avoid using batts, and use as much of the foam board as you can (you said it was cheap).  Also, the radiant bubble insulation has limited applications, as I understand it.

[MountainDon]

1 1/2" foam insulboard

There are many types of foam board. Some can absorb water between the beads. Others resist water.

Question: how are you thinking of using the foam board? Between studs or over the studs?

Reflective "insulation" is oversold by the people who manufacture it. Have a look in the Referral Links board. There are a few topics on insulation there.

[ced257]

 Thank you for the replies. First off I live in northwest Arkansas. So the temps aren't too extreme.I forgot to mention that the roof is going to be metal.
The foam I am getting is blue poly foam board. I say poly, I'm not sure that's what it is. It's pretty dense. I planned on mostly using it on the roof. Put my decking down then 2x4 purlins vertically with foam in between, then another row of 2x4's horizontally with bubble insulation and sheet metal on that. If I don't leave my rafters exposed, I figure I would run batt in betweent the rafters with a layer of foam on the roof and metal on that.
  As far as the walls are concerned. I'm not sure there. I'm not a real big fan of putting foam board on the outside of the studs. Seems to be too much give when you nail anything over it. Oh yeah, the rafters and studs are 2x6.
  I'm just looking for the best way to insulate my cabin with the materials I have on hand. I am more concerned about my roof insulation than anything. I have done so much looking around and different sites, you kinda get overwhelmed with info and get confused. At least I do. I'll go take a look around the referall section. Thank you all once again!

[MountainDon]

You might find some interesting/useful information on building there at the Building Science website.
http://www.buildingscience.com/index_html

They have developed designs that are climate specific when looking at how to best insulate. It looks like you'd be in what they call a mixed humid climate.
http://www.buildingscience.com/documents/primers/plonearticlemultipage.2006-11-16.8755545932/how-to-use-this-package
You will note they make use of a lot of foam board insulation as well as blown in cellulose.

[poppy]

So am I understanding this correctly?  2x6 rafters, decking on top of those, 2x4 stringers on top of the decking, and then 2x4 stringers on top of the other 2x4's.

The blue board is good stuff, but cutting it up to put between purlins or stringers is not a good idea in my opinion.

I'm not familiar with the radiant bubble stuff, but I assume it is not very thick, maybe 1/2" to 3/4"?

Here's what I would recommend above the decking.  Put one or two layers of the blue board directly on the decking using 1x3 or 1x4 horizontal purlins for the roofing on top of that and nail down through the whole thickness to the decking and longer nails into the rafters.  If you don't like stacking 2 layers of board directly together then you could put 1x vertical stringers over the first layer and 1x purlins over the second layer.  This plan leaves out the bubble stuff.

If you want to use the bubble stuff, I would put one layer of blue board directly on the decking, the bubble stuff directly on top of that, and do 2x3 or 2x4 horizontal purlins for the metal roofing.  That should give you plenty of air gap for the radiant stuff.

Any dense foam board is designed to span the inside or outside of rafters/studs.  The fewer joints you have in any insulation, the better.

Using even one layer of blue board in complete 4x8 sheets (assuming that's what you're getting) is much better than even 5 1/2" of fiberglass batts between the rafters.

[ced257]

Thank you once again! My thinking was to put a row of foam vertically up the roof with 2x4s every 4 foot at the edges of the foam. That would give me something solid to nail another 2x4 purlin too. This would give me and 1 1/2" air gap for the radiant bubble to work between it and the foam, with the metal on top of the bubble. Am I backwards in this? Should the gap be bewteen the foam and the bubble or the metal and the bubble?
  I agree that it is better to butt the foam to foam and use no wood in between. I just never cared for nailing anything to foam. Always too much give for my liking. I like something solid to nail or screw to. But if it makes that much difference I will do it. I've done it before.
  The reason I'm using 2x4s is cost and quality. It is cheaper for me to buy 16' 2x4s than it is 1x4s, at least it was a month or so ago. Generally the 2x4s are alot better quality than the 1xs we get here. Plus it's stouter.
  The bubble insulation is like a bubble wrap with a relfective side. It's like packing material. I think one of the name brands is Reflectix. It has different R-values for different applications. I've used it before but i'm not real sure if it's worth the extra expense. I see the values of radiant barrier. I'm just not sure how to use it all the time.
I didn't realize that one solid layer of foam, with the joints taped i imagine, would be as good as 5 1/2" of fiberglass. I always thought each sheet only had an r-value of 4 or 5. I guess that stuff is pretty dense though. It would be pretty hard for air to move through it. I wonder what it would do if I put a run between each rafter on the inside, with a layer on the outside as well.

I've been building for about 10 years now. I've mostly worked for someone else. We are a small crew and we're all very knowledgable about different things. But this seems to be one area that kind of escapes us all. Most of what we have done is fast paced residential building. We have done quiet a few customs. Doing a dandy 56x36 eastern red cedar log home now. We've done the roofs a hundred different ways, I just want to figure out the best way to do mine. I've built all these other houses, you'd think it would be easy to build your own....lol! I've never thought and rethought so much in my entire life. My friends laugh at me cause I am constantly designing and redisigning. I feel like I've turned into one of our customers, dam people can't make up their mind  . I apologize for the long post. Thank you!

[MountainDon]

I've read that reflective radiant barrier effectiveness decreases over time as dust settles on the reflective surfaces, as in the typical roofing installation. The money spent for reflective barriers may be better spent on more foam insulation.