Spray foam insulation under roof decking

[INTMD8]

Along the same subject, I've noticed some spray over the roof perlins and some only between them.

Is there any concern of the insulation only being between the purlins in terms of heat loss through the wood or not a problem? (this would be more convenient for interior wall installation)

 

[TheBadDog]

I have 6" old school rafter construction. I specified 1"+ foam seal over all exterior framework to avoid it becoming a "thermal bridge". So it will be 6" minimum between, 1" minimum over rafter, edge to edge end to end with no breaks except where bracing pierces, and going over the rafters automatically collars the bracing to minimize thermal transfer there.

 

[volleyball]

Along the same subject, I've noticed some spray over the roof perlins and some only between them.

Is there any concern of the insulation only being between the purlins in terms of heat loss through the wood or not a problem? (this would be more convenient for interior wall installation)

Yes, to keep a moisture barrier, the entire area needs to be covered with the appropriate thickness of foam. You need to make sure there is no organic material to mold touching the outside.
If you contract with an installer, they need to know that they are to spray across the framing which is opposite of framing being insulated and wall boarded.

 

[Beemer533]

I've not seen it done like that for a finished space...

I assume you are referring to unfinished attic space?

This post edited by the NSA

 

[TheBadDog]

Not sure on the other, but mine is unfinished attic space.

And I just got another (likely the final) quote, and it's another outlier. My previous outlier was right at $10k. This one is $12.5k. The median are in the $6.5-7k range, with the low outlier at just over $5k. That low one is suspect though as it states "Base Quote". Considering that and the low outlier figure, I suspect they would likely find "surprises" to add to the cost, though I will call them tomorrow to see if they are willing to provide a binding quote. In any case, I'll be looking them over to try as much as possible to make sure it is an apples-apples comparison from the median ~$6.5k group, settle on a final few, then likely make the final call based on scheduling.

 

[volleyball]

Yes it is for unfinished space. For a sheetrock ceiling, spray will be too uneven. That is why I had suggested doing the rigid foam panels on the bottom of rafters and then spraying those in. That gives you a continuous foam envelope.
You can also use standoffs and run purlins and foam under those and a place to mount the sheetrock to.

 

[mark2457]

Hi Guys

Just budgeting my garage build (Chicago). Gonna be a truss roof (2x4 top chords and 24" OC). Will be 22'x24' 4/12 pitch roof (I make that about 556 sq ft). There will be no ceiling below trusses

Will get some guys out for quotes eventually. What would I be looking at (approx in Chicago) for closed cell under roof decking?

TIA

Mark

 

[LX-Markham]

Yes, to keep a moisture barrier, the entire area needs to be covered with the appropriate thickness of foam.

The foam layer does not need to be continuous, it just needs to seal between all the framing member.

For either type of foam, the wood framing provides sufficient inherent vapor resistance to maintain the moisture content within the safe range even in very cold exterior climates
...
Closed-cell (about 2 pounds per cubic foot density or more) spray foam applied in thicknesses of over 2” (50mm) will control vapor diffusion to safe levels in all climates up to
10000 HDD and interior winter-time relative humidities of up to and over 50%RH. As thickness increases the level of control increases. The diffusion control was equal or better
than walls with the traditional fiberglass batt and polyethylene

http://www.buildingscience.com/documents/reports/rr-0912-spray-polyurethane-foam-need-for-vapor-retarders-in-above-grade-walls

 

[volleyball]

The foam layer does not need to be continuous, it just needs to seal between all the framing member.

http://www.buildingscience.com/documents/reports/rr-0912-spray-polyurethane-foam-need-for-vapor-retarders-in-above-grade-walls

That is not how I read it. And that was for very cold climates. You need to not have a organic path for moisture and airflow to the cold.

 

[LX-Markham]

And that was for very cold climates.

Isn't that what we're talking about?

Suffice to say, there is still debate on it. Our building code however accepts the combination of wood framing with closed cell foam spray in between as sufficient. There is nothing that says the foam needs to be sprayed over top of the wood framing.

 

[volleyball]

The OP is in Phoenix, not that cold ever.
What is acceptable and what is optimal is not usually the same thing.