I need to insulate a 2x6 constructed ceiling. I was going to use plain-old-batts (don't get me started) with vent holes (top and bottom) and a roof side air gap for ventilation.

However, I have realized (duh) that I could sure use some higher R insulation between the rafters, and have thought of using 4" of closed cell foam.   R-25, or something like that.

A less expensive alternative, though, is to use 2" of closed cell foam and then 3.5" of fiberglass.  R-21, or thereabouts. Is this really a viable option?

And then, is 5.5" of open cell foam a viable option, or is that just asking for trouble?

If it matters, I am in Monterey, CA.  No heating days, 3500 cooling degree days per year. 

Thanks for the advice in advance,

Jeff

Blown cellulose would be much better than fiberglass batts and much less expensive than spray foam.

Jeff:

What is the required ceiling insulation for your climate? A quick Google brought up this EPA web site. I'm sure you can find others. Even without knowing that answer I'd say that foam is definitely indicated in your instance based on the minimal depth of the rafter bays - but I think you knew that; that is why you asked the Q.   One thing about foam is that the performance of a ceiling that is insulated to, say R-20 with foam is significantly better than R-20 of fiberglass because the foam stops air movement much better.

How about this idea: The 4" of closed cell foam you referred to above only with a ventilation gap above it that would be achieved using insulation baffles. In this scenario you would still have ventilation at the bottom (soffit) and top (ridge).  

Open cell foam would be fine. Dense-packed cellulose MIGHT work, depending on your roofing material & structure ( sun beating on a rain-soaked or dewy roof deck can create high vapor drives, and you could end up with wet cellulose up against a wooden roof deck on a regular basis.) A 2" lift of closed cell foam with interior cellulose or fiberglass would be just fine in Monterrey. Cellulose under foam would likely outperform fiberglass due to fg's slight IR translucency.

As for the total R of the stackup, Title 24 2008 spells out your legal minimum insulation levels for upgrades, based on your CA climate zone and roofing material type, and existing insulation levels. Going somewhat better than Title 24 minimums is usually cost-effective if your AC is older/less efficient, or if your utility rates are higher than the state average.

Dana and Matt -

Thanks for the replies. Do you think that I can get away with not venting the all-open-cell options?  From a cost perspective, that would be my best option, but I certainly don't want a rotten roof deck...

Jeff

Posted By jbaron on 01 Apr 2010 03:12 PM
Dana and Matt -

Thanks for the replies. Do you think that I can get away with not venting the all-open-cell options?  From a cost perspective, that would be my best option, but I certainly don't want a rotten roof deck...

Jeff

High vapor drive conditions that occur during hot sun on a rainy roof may temporarily put moisture into the roofing deck, and even into the vapor permeable open cell foam, but during the heat of the day it'll bake out of the roof deck in both directions with open cell foam, and any condensation that occurs within the foam will migrate downward due to gravity, drying toward the low-humidity air-conditioned interior.  Unlike cellulose, open cell foam has little to no capacity for holding onto moisture (sometimes that's a good thing, sometimes not, but here it's good.) 

With closed cell foam under high vapor drive conditions from the exterior, some vapor still enters the foam,  but more stays in the roof deck, and the roof deck dries primarily to the exterior, taking longer to dry.  Open cell foam provides much higher inward-drying capacity, and it dries in both directions.  This inward vapor drive adds a small amount of latent load to the AC, but in CA the latent loads are already a fraction of what they are at similar temps on the sticky-humid right-coast- it's somewhat "in the noise". 

Having the drier roof deck trumps the small addition to the AC load that will occur from time to time. The all open-cell solution is the preferred solution here.  Open cell foam is still an excellent AIR barrier, which reduces both sensible & latent loads by controlling outdoor air infiltration.

With any insulated roof deck there is a small reduction in shingle-life if you're using composition shingles (your 25 year shingles might give up in 23 years.)  There's no evidence that it reduces slate/tile/cement roofing material lifespan. Even the shingle issue is more affected by roof pitch and solar orientation than insulated/uninsulated roof decking.  In CA under Title 24 you're required to use cool-roof materials for roof pitches below 2:12 anyway, and still have some (but much lower) restrictions on the ratio of solar absorption to emissivity for higher-pitch roofs as well, which will keep the shingle temps under control whether the roof deck is insulated or not (and shingle life is all about temperature, eh?)

Open cell foam applied under roof decks has more issues in cold/very-cold climates, where interior air vapor can reach the roof deck at condensing/freezing temps unless there are interior side vapor retarders in place.  In cooling-dominated climates like yours, it's much easier to design for.

Thanks Dana. That's very helpful.

Jeff