R-38...how thin can you get

[xd2005]

I just received my permit from the city on a partial garage conversion to living space. The garage is not ideal for this type of thing, but I've been working through the various issues and had everything nailed down (thus the permit).

I have 2x4 rafters, which I will be sistering with 2x6 rafters to increase the support, as well as adding 2x6 collar ties. My main issue has been height, as the walls are only about 80" tall, so clearance has been a significant issue I've had to deal with. Because of the height issue, I have limited insulation space for about 1/2 of the roof space, until it gets to the collar tie level and opens up to allow more room for insulation.

Before submitting my application for the permit, I talked with the city and they told me that I should be able to fit foam insulation (sheathing) into the space (5.5") and be able to meet R-38 requirements. When I got my permit today, it was noted to use "Foam or Foil R-38." It was also stamped (on another page) "min 1" of air space; extend baffle 6" above batt insulation; 12" above blown WSEC 502.1.4.5."

Considering I have 2x6 rafters and need 1" ventilation space between the insulation and roof, I basically need to find a way to insulate 4.5" of space with R-38. Is this even remotely possible? When I was talking with the person from the city before submitting the application, he had indicated to use the foam because he said it had like r-8 or r-9 values per inch. In actuality, from what I can tell it's more like r-6 per inch (I have seen 0.5" with r-3.2, which at 4.5" would get me to a whopping 28.8 value, not really even close to R-38. So my questions are:

A.) Am I completely missing something with my calculations?
B.) If not, is there an insulation that would allow me to get an R-38 value utilizing 2x6s for rafters?
C.) Related, but different issue. Are soffits a requirement? Currently the garage has no ventilation at the eave level. I have 4 roof vents (2 will be above the "converted" side) and I had planned to put in a gable vent, but hadn't really thought of vents at the eaves.

Thanks.
Brian

Below is a ghetto image of what I am talking about:

[hj]

You have the roof on one side of the 2x6 and sheetrock on the other. HOW will air get into that 1" space the city specified above the insulation if there are NO eave vents in every rafter space? Without them, you will be creating an over above the sheet rock, or at least a very efficient solar heater.

[xd2005]

I guess I figured there would be at least some air echange going on with the gable vent and roof vents. With that, I figured if the rafter area got hot, that air would be pulled upwards, while cooler air coming in would settle down to replace it. I didn't say I was right, just what I was thinking (really, I hadn't even really thought about it).

[frenchie]

Where are you located? Look into unvented roof assemblies (ie, sprayed foam). That'll get you an inch...

There's still no way to get r-38 in a 2x6 rafter, though. Even closed-cell foam stabilizes at about r6... leaving you about an inch shy.

Is 2" really that big a deal for you, that you can't sister with 2x8's instead?

[xd2005]

Originally Posted by frenchie

Where are you located? Look into unvented roof assemblies (ie, sprayed foam). That'll get you an inch...

There's still no way to get r-38 in a 2x6 rafter, though. Even closed-cell foam stabilizes at about r6... leaving you about an inch shy.

Is 2" really that big a deal for you, that you can't sister with 2x8's instead?

Washington state.

Thanks, I'll look at unvented roof assemblies.

Unfortunately, 2" in this situation is a pretty big deal for me. The entrance door needs to be on the side and the sides, with 2x6, will only be approximately 80" tall. I think 78" would be pushing it for wall height.

[xd2005]

Well, talked with the plans inspector. They agreed to allow R-30 in the space, but still require eave ventilation.

[Scuba_Dave]

R30 in a 2x6 rafter will still only get you R21 since it will be compressed
R30C will get you R22

[geniescience]

Closed cell foam panels with a radiant barrier on both sides. Look into using spray foam above it in pillars.

[frenchie]

Uh, Dave? We're not talking about fiberglass, no way he could hit the target with Fiberglass. We're talking about closed-cell spray foam, at this point.

Another problem, Brian - look into it, depending on your area you might not be able/allowed to spray 5" thick. At any rate, it will limit your choices of products, they all have a maximum thickness allowed...

No matter how you proceed, that's going to be a pretty expensive 2 inches.

[xd2005]

That chart looks to be for fiberglass insulation.

Their assumptions are based on foam-type insulation, such as Thermasheath 3 (link to data below).

http://www.rmaxinc.com/downloads/Dat...20Pages%29.pdf

[Scuba_Dave]

Woops...thought you were trying to install fiberglass

[xd2005]

Originally Posted by Scuba_Dave

Woops...thought you were trying to install fiberglass

No problem. I will be in the walls, and probably in the upper ceiling area, but the roof area where I only have the width of the rafters will need to be more compressed.