3/4" foam board insulation help
 

So I just picked up 17k sqft of foam board insulation. It is 200 sheets of 4x21' by 3/4" thick. If I remember right, it is R4.

I'm building a two story and right now I'm framing. I've got 2x6 construction and OSB sheething with house wrap on the outside of that. I was thinking of hanging the foam insulation on the outside of the building vertically. This would allow very few cuts. I was thinking 3 layers and offset the seams so they don't line up.

My question is, what tape do I use on the seams?

What fasteners do I use to fix to the house?

With 2.25" of insulation on the outside, what do I need to do about putting on horizontal siding?

thanks
Austin

There is a website called Building Science that has some wonderful information on what you are doing.

Regarding attachment, long stainless steel screws with the plastic equivalent to fender washers would be just great.

Regarding tape, duct tape will not hold up. You will need a tape that is designed for the job and it will not be inexpensive.There are a lot of emerging products that will help. Fine Homebuilding magazine did a great comparison a few issues back, look at their site, go to your public library.

The structure that you intend to put over the insulation layer is called a "Rain Screen". If you Google that you will see much good info. The idea is to build it so that you will have abundant bug-proof air flow behind, so that if rain comes in gusts, there is a pressure equivalence and the rain itself is not driven through the screen, and any minor droplets can run down the water-resistant layer that is behind the siding, behind the air gap.

It's a fairly new concept, but it has been well tested.

By the way, I have used loads of the insulation board, and 3/4" would have the following R-value, depending on the type you bought:

White EPS = R-3.2
Pink or Blue XPS = R-3.75
Yellow Polyiso = R-4.9

The big benefits from what you are doing will be primarily a big reduction in air infiltration, secondarily reducing 'thermal bridging', and lastly increasing the overall R-value of the walls.

Be sure to prime the back side of your siding material.

Good luck on your project!!

Best,

-AC

I will for sure check more on Building Science. I was also just reading on GreenBuildingAdviser.

Thanks for the info on the work Fine Homebuilding did on the tape. That is the kind of info I'm looking for.

I'm very familiar w the rain screen. I'm thinking I will go with some 1x4s 24"OC to set the siding off.

I think I've got the Blue XPS. It is being delivered tomorrow. I'm really pleased it is 21' long. Much less taping and less chance for air. On the front and back of the house, it will be one long piece from top to bottom. On the gable end, the peak is 25' up so there will be some horizontal taping there, but at least it will be under the deep eave (16").

We are in east texas so stopping the hot moist air infiltration is a big deal. You should see how much water our ACs produce!

Since I have your attention:
I was thinking going three layers over the whole house, but I could go four layers on the roof and then two layers on the house. I do plan to come back and do spray foam under the roof deck in a couple of years and that is why I thought it would be good to get three layers on the walls.

Thoughts?

This sounds like a really good idea. I don't know that the insulation codes are in TX, but in OR the current insulation requirements are:

• Roof - R49
• Walls - R20
• Floor - R30

I know that the Oregon Department of Energy does really good work. I have been personal friends with a couple of people employed there, and they are very smart and very interested in moving the state forward in anticipation of eventual global energy decline.

If you are confidant that the TX energy department is trustworthy, you might check them.

But I mention this stuff to give you an idea of the ratio of insulation distribution. So, I would say that your idea of more in the roof and less on the walls looks good.

I'm sure that you are well aware of the need for vent channels under the roof layer.

Another idea that you may not have considered is aluminum shingles. I have them on my house and they are a total miracle. They don't wear out... that's right, they don't wear out.

I made the mistake of going with green painted aluminum, I really should have gone with unpainted aluminum, my house would have been much cooler in the summer.


The photo above is one that I took about three minutes ago. The roof in the photo is on a house that is right across the street from mine. The roof you see in the photo is unpainted aluminum, and has been there since the early 1950's. That's about 60 years ago. The roof looks absolutely new. No failures, no leaks, no blown-off shingles (they all interlock together).

Aluminum roofs are more expensive, but I did the calculations that I could have a crew do my roof in comp, or I could do my roof in aluminum myself (it took a really long time) for the same price. So, now I'm good for as long as I am in this house. Roof issues are completely over.

Not enough info to make a decision on this...

Best,

-AC

For the county I'm in, we have:
Roof - R30
Walls - R13
Floor - R13

So, yes, I think I will put four layers on the roof and two on the walls.

"vent channels under the roof layer" Not sure if you are talking about a vented attic or above sheathing ventilation. I'm going to have a non-vented attic so no venting there but I will space the metal roof 1/2" above the foam for an air channel there.

Aluminum shingles would be great but way out of the budget. I will be going with a galvanized metal roof. That will give me the bright silver finish to reflect heat. With the air gap under it, I don't think I can beat it for the cost.

House Build: http://i297.photobucket.com/albums/m...ps45b2aacb.jpg

thanks for the info
Austin

One final note...


The above graphic was based in the work done in a study by Penn State, on the various modes of thermal transfer.

In the case of your roof, radiant heat will be the dominant mode, the left most graphic in the triptych.

So..........

The 'bright silver finish' of your metal roofing will loose much of its silvery brightness, and also reflectivity as time goes by.

On another thread I have been investigating Multi Layer Insulation that was developed by NASA.

The idea is that a reflective surface will reflect back a large percentage of the radiated heat that hits it.

The heat that gets through radiates on to the next layer, which will in turn reflect a percentage back, etc. etc. ... in NASA's case for up to 50 layers.

Now I know that your house will not be in orbit, but it will be subject to radiated heat energy just the same... so other reflective layers will do their job.


The graph above suggests the efficiency of heat radiated back vs. the numbers of layers.

As you can see, there is a very large improvement in the first layer, but not so much when you get to the 100th layer.

I think that when you get right down to it, aluminum foil, like you might use in the kitchen, is an extremely effective reflector, available at a commodity price.

Could make a pretty cheap 2nd reflective layer to go under under your metal outer roof.

-AC

Thanks for the graph. Ten years in, I'm glad I put the radiant foil under my stone coated Decra tile (metal) roof.

Kitchen foil! I also put some 4' radiant barrier double bubble across the collar ties looking for more improvement, not sure yet, might have messed up my convection airflow.
http://i1204.photobucket.com/albums/...d/CAM00492.jpg

Not sure if your foam board has foil on it but, if you are setting the siding off with 1x4s it would be a good opportunity to radiant barrier the inside surface. Some one is making a 8' radiant house wrap.

AC hacker and GTO,

Yes, I agree that most of the roof heat will transfer via radiation. I didn't realize the % was so high.

A little back ground. If you search my old posts, you will see that we were building a house a year ago. Right before we moved in, that house was lost to fire. We were not insured since I was building the house. This is the rebuild so money is VERY tight.

I would love to put a radiant barrier on top of the foam, leave an air gap and then put on the metal but the radiant barrier is out of budget. The foam is used so it does have some nail holes in it. So for my build, it will be OSB decking on top of the rafters. Then Grace Triflex as the roof underlayment. Set the foam on top of that. After foam will be verticle 1x4 boards with horizontal 1x4s on top of those. Then the metal roof. I'm hoping that will let any water that gets past the roof out and also let air enter low and take the heat out at the top. Yes, my silver roof will pick up some heat but with the air gap and the color and the foam, I'm 100% better than most of the roofs in TX. Most are vented attics with black asphalt shingles, along with the AC system up there!

With all that said, I do have some old rolls of radiant barrier from the first build that was in the fire. They are pretty bad off but I think if I unroll some of it, I might get to clean material. If I do, then I will use some on the south side of the roof! My house faces solar south so it will do the most good there!

thanks
Austin