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

So sorry about the fire...

-AC

http://ecorenovator.org/forum/solar-...bottom-up.html: See this thread. I'm not a big fan of using multiple layers of radiant barrier. A least here on Earth. Air can easily short circuit multiple layers though the process of conduction and convection. Here on Earth I would limit it two a maximum of two layers with a generous airspace between them. Btw, I'm still working on my project, which is coming along. And for anyone who doesn't know it, if you live in a mild climate then you can use a radiant barrier for both cooling and heating. That's providing your house is quite tight and well insulated. Again, see that thread. I too am sorry to hear of the unfortunate happenings with your previous house.

NOTE: I have moved this post over to THIS_THREAD as it is a better fit there.

-AC

AC, I'm not sure who you are accusing of being smug about insulation? I am, as far as I know, the first person here to promote a project using radiant barriers in a form other than for cooling. Have you even read and understood the concept behind that thread I just mentioned? All I'm saying is that while the concept of using multiple layers of radiant barrier may be good in principle in a vacuum it won't work as you are advertising. But hey, if bringing physics to the table is being smug on this forum then I plead guilty. However, if you really believe that the stuff sold in the UK has over an over 20 R value per inch then I have this bridge I'm selling that you'll find is a real bargain.:thumbup:

AC Hacker,

It was a year ago in July, so we are pretty much over it now (emotionally), just not finacially. Once the new house is done, we will be well on our way to finacial health.



Exeric,

Thanks for the info on your project. I like the idea of using hot attic air to heat a house (if you need heat). Here in East TX, we don't really need much heat. My family and I like it cold and don't really turn on the heat except late at night probably two months out of the year when an attic would have already cooled down. Also, I will have a sealled attic. For those two reasons, I can't really use anything from your project but I do find it interesting. I had friends that would put garbage bags over their turbines in the winter to retain heat in the attic. It worked well for the Dec/Jan/Feb heating period.

About the multi layer radiant barrier....well, who knows. I worked at NASA for 10 years and sometimes things that work in space don't work on earth. Sometimes they do. I would need to look into it much more before I said yes or no.

Austin

Removed because of double post

dablack, it might work for you or it might not. I haven't finished the project yet so it's premature to claim a proof of concept for a radiant barrier heating a home. However, your situation might not be that different from mine. Most of the heating needs come in the middle of the night for both of us. It really depends on one's home being very airtight and well insulated. I think of it like capacitor filtering for an AC power supply. As long as you don't draw too much current then a smallish capacitor should be able to supply the current to the load until the next replenishment cycle. So, as long as your house can retain heat, and its in a mild climate, then one should be golden. In theory anyway. (Theory has a way of going awry when theory meets world.) I should add that the same radiant barrier used to concentrate heat will work in the usual way to cool the house in summer by reflecting heat away from the attic. So it's a 2fer for places like Texas and California which both have hot periods in summer.

Exeric,

One of us isn't understanding. Right now I'm not sure who. Maybe I don't understand what you are building. From what I saw of the thread, it looks like you are capturing and using heated air from the attic. This heated air is heated because it is from above your radiant barrier and below your roof deck. Correct? In the summer you will vent it. In the winter you will use it. Right?

I will have a sealled attic. No venting. No heat to capture. My foam and radiant barrier (if I use one) will be external to the roof deck. I'm using attic trusses that give me a 8' wide room in the attic. I have a gable roof with no dormers and my house is 52' long so that gives me a 8' wide room, 52' long. In that room you will see the roof deck and rafters. That is it. Everything else will be external. I don't want any heat to get into my attic. Since there will be very little heat there, there is nothing for me to capture. \

My layers from the bottom up will be attic truss, OSB roof deck, grace triflex underlayment, three or four layers of 3/4" dow foam board (taped seams offset between layers), maybe radiant barrier, verticle 1x4 boards set at 24" OC, horizontal 1x4s set at 24" OC, and then a metal roof. The area between the radiant barrier and roof will be vented at the peak. That is a metal roof vent and not an attic vent.

thanks
Austin

Yes, you are right. I was thinking generically about climate. I don't think it will work with your particular construction details. It really does require very specific house construction details to have the potential to work.

I have moved this post from this thread (3/4" Insulation), over to Space Suit = Multi-Layer Insulation (MLI), where it is really a better fit.

-AC

Ok, so I'm late to the party, but I read an article from a guy in Houston, TX that may interest you.

Cool Roof Design for Hot Texas Climate | Houston Cool Metal Roofs

You might want to give this a few minutes of your time. I found it very interesting and will try to do this if/when I replace the galvalume roof on my 12 year old house.

I am a huge fan of foam insulation, both my home and my garage / workshop project were foamed. I just did the foam the lazy way. Traditional framing and foam the entire inside envelope. Sealed up nicely and did a pretty good job on a blower door test. My remaining air exchanges were from a leaky French Door install in the MBR, the vent for the gas HWH and the through the floor drain for said HWH. Still, a very energy efficient home and it's very comfortable too.

gasstingy,

That is exactly what I'm building. I read about ASV a couple of years ago and knew that was the way to go. I lived in Houston for 10 years. The "vented" attics with the black asphalt roofs got crazy hot. I tried better venting but it was never enough. The attic was never anywhere near the outdoor temp. It would be 90 outside and deadly up in the attic.

If you read what I wrote on page two, you can see it is the same thing.

"My layers from the bottom up will be attic truss, OSB roof deck, grace triflex underlayment, three or four layers of 3/4" dow foam board (taped seams offset between layers), maybe radiant barrier, verticle 1x4 boards set at 24" OC, horizontal 1x4s set at 24" OC, and then a metal roof. The area between the radiant barrier and roof will be vented at the peak. That is a metal roof vent and not an attic vent."

Austin