Do eave vents defeat insulation above?

[Yikes]

I have an existing apartment building with 2x8 roof rafters with 3.5" mineral wool (R-11) in the rafter bays. The bays have two 2"x2" holes in the eave blocking to provide cross-ventilation in the remaingin 3.75" rafter space.

We are remodeling the buildings, and the owner (and state energy code) requires a minimum R-19 roof insulation. The 2x8 bays are too narrow (and have too much blocking) to sucessfully blow-in the insulation without totally destroying the ceilings.

We'd like to put R-9 rigid insulation on top of the roof deck, but there is some concern that the eave block vents will allow outdoor air to come into the rafter bay and bypass our new R-9 insulation. But even with such rafter cross-ventilation, isn't the air static enough in a space this size to not defeat the benefit of our R-9 insulation?

 

[mtlogcabin]

Do eave vents defeat insulation above?

Yes

 

[Yikes]

Is it a "complete" defeat? In my mind, I'm comparing it to an insulated home where somebody left the front door open a crack. It's still functioning better than a home with no insulation, right?

 

[mtlogcabin]

Is it a "complete" defeat?

Yes the tempatures in the ventilated space and the top of the roof deck will more than likely be within a few degrees of each other.

 

[FredK]

Yes a little but then the DOE recognizes that fact. See page 2 of brochure.

http://www.ornl.gov/sci/roofs+walls/insulation/fact%20sheets/attic%20floors.pdf

 

[peach]

If you are going to insulate only the roof rafters, you need to condition the attic space rather than ventilate it. If you also insulate the ceiling joists, it doesn't matter.. the roof needs to stay cold during the winter... which means keeping the attic cold.

If you insulate the roof and not the ceiling (since you'll still need to ventilate the attic), you will have a cold house. Insulate the roof and turn the attic into conditioned space, the house will be toasty. It's all very hard to do with batt insulation.. moisture is a problem.. moisture leads to rot, etc.. and more etc.

 

[TimNY]

You should probably look at 806.4, Unvented Attic Assemblies. Se Table 806.4-- you would have to provide sufficient rigid insulation on the exterior. R-10 may or may not be enough depending on climate zone.

 

[Yikes]

Sorry, I need to be more specific: this is an existing flat-roofed apartment building. The ceiling is attached directly to the bottom of the 2x8 roof rafters. There is no "attic"; but the 2x8 joist-rafters have about 3 1/2" of mineral wool insulation, leaving an un vented 3 3/4" rafter space.

 

[JBI]

OK, I'm going out on a limb here... 'yikes' is from Southern California (or so the profile claims...). Now, I'm no expert on these things, and could very well be wrong, but I'm just not seeing much in the way of cold weather in SoCal when I watch the news. And I'm pretty sure the only way that roof is going to get cold is if you refrigerate it.

I know that answer doesn't really speak to the OP, just want to keep everybody focused.

 

[conarb]

Yikes:

California does not allow unventilated ceilings, under the new residential code they allow unventilated conditioned attics, but not cathedral ceilings, and Im sure a flat roof would be considered the same as a cathedral ceiling laid flat. That being said, you have an apartment building, so the residential code wouldn't apply, but the same theory would apply, what does your Title 24 consultant say? I bet a good one could add the R-Value of the existing ceiling retaining the existing mandatory ventilation and adding the additional insulation above the roof, but each Title 24 consultant does his own tradeoffs etc. don't try to do your own Title 24 calcs, those guys can perform miracles and charge very little, I pay less than 10¢ a square foot for a building and am in the most expensive part of the state.

 

[mtlogcabin]

An apartment can be residential under the 2009 energy code. Don't know Ca ammended the definition or not.

RESIDENTIAL BUILDING. For this code, includes R-3 buildings, as well as R-2 and R-4 buildings three stories or less in height above grade.

 

[conarb]

Mountain Man:

Interesting, but California doesn't use the International Energy Code, we have our own.

Yikes:

Which code now governs apartment occupancies, still the CBC not including the R sections?

 

[Alias]

Sorry, I need to be more specific: this is an existing flat-roofed apartment building. The ceiling is attached directly to the bottom of the 2x8 roof rafters. There is no "attic"; but the 2x8 joist-rafters have about 3 1/2" of mineral wool insulation, leaving an un vented 3 3/4" rafter space.

Yikes -

How about baffles for the eave vents and then use spray-in insulation on the underside of the roof deck and top of ceiling? I'm sure some posters from jurisdictions east of the left coast have had experience with these products. A couple of the local folks here are now using spray-in insulation as they seal well and they can get better R-value into a smaler space, especially on a remodel. The spray-ins being used here are very 'GREEN'.

When I was doing direct weatherization I became very familiar with DOE, DEO, and PG&E requirements for weatherizing. oh boy.....

 

[conarb]

Sue:

I wouldn't go near spray foams, even the so-called green soy-based foams are loaded with formaldehydes and isocyanates, this is a long discussion on The Green Building Adviser, but well worth reading all of before installing any of the foams.

 

[Yikes]

UPDATE: OK, thanks to all for your comments.

My original post here asked the question if a (1) VENTED 2x8 roof joist bay, that had both (2) R-11 batt insulation laid on the ceiling (partially filling the vented space), and (3) R-9 rigid insulation laying on the plywood roof deck could have the R-value of BOTH added together; or if the venting inside the joist bay would defeat the reason for having rigid insulation above. This was troublesome for many of you- -thanks for the honest feedback.

Meanwhile, on the "commercial building codes" forum, I approached the problem from a different perspective. If I could eliminate the existing joist bay venting, then I know I could add the R-11 and R-9 together = R-20. The only downside would be the potential for condensation. On the other thread, responders linked to enough information which, when coupled with the local climate data (DOE labels it ("hot+dry, showed that condensation would not be a problem.

In fact, the nearly identical building next door to this project is also being remodeled - - it had no joist bay ventilation, and it's had not moisture problems for 30 years.

The local building official has now approved an unvented installation, which means we can close of the vents and not worry about it defeating te R-9 rigid insulation.

Thanks again to all!

 

[mtlogcabin]

Usr Polyiso on the roof deck. I believe you can get r-15 with 3"