CZ 7 Flash and Batt
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Hi Everyone,
The information on this site has been invaluable as I complete my small cabin high atop the Rocky Mountains in CZ 7. The structure has a cathedral ceiling with 11 7/8 TJI’s. To get the a R-49 the only permissible option by the plan approvers was F&B, which was a new concept to me. I did a lot of research, and on the surface and initially assumed 2″ SPF and 10″ of fluff would do it. Then I found the IRC table R806.5, which led me to correct my plans to have at least 61% of the R-value from SPF the the rest fluffy stuff. The SFP I choose has an R-value of 6.7 so I am planning to go with 4.5″ of SPF and then an R30 batt at 9″ thick the fill the cavity (compressing the batt slightly). A have a few thoughts that I cant get off my mind, and would appreciate any input.
-Does this approach seem accurate for CZ7, compressing batts goes against everything I was taught, but I understand with this approach it is actually a good thing to prevent air movement between the SFP and fluff.
– There is a 50sqft entry area framed with 8″ LVL’s. I obviously wont meet the R value requirements in this area, but are there condensation concerns being thin on the fluff if I still use 4.5″ of SPF?
-Do I need to search out wider batts for 16″ OC TJI’s? I currently have batts of R-30 15.25″wide, but as I write this realize that the width between the TJI webbing is slightly larger then if framing with dimensional lumber at 16″OC. I could also fill the flange void with a thin layer of fluff/foam before inserting the batt.
-I am doing 3/4 T&G on the ceiling, and planned on using it directly on the rafters, again after reading information on this site I modified.
-I had planned on using MemBrain as an air barrier under the T&G, but now based on the extreme cold in my CZ and how quickly warm air would condense on the cold foam (specifically the house is kept at 50deg when not in use I fear this could allow for a higher probability of the SPF reaching dew point). Should I also be considering a taped drywall barrier (1/4″ since it will be supported by T&G), or is this overkill?
Thanks in advance, lots of great info on here.
-Kyle
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Replies
Because the concern is long-term moisture accumulation, instead of using the brand-new value for the foam R-value, I recommend using the long-term aged R-value, which will be around R-5.6/in. (Air eventually displaces the blowing agent/refrigerant that provides higher R-value.)
61% of R-49 is about R-30; at R-5.6/in, that's 5.3". Call it 5.5" to be safe.
That leaves 6 3/8" of fluffy insulation to make up the remaining R-19. You could just use regular 5.5" R-19 batts, as long as they are pushed tightly against the foam. You can use metal wire rods meant for the purpose to hold them in place.
If you want to fill the entire cavity, you could use R-30 batts. Compressed to 6 3/8" they will perform at about R-24. But that throws off the ratio by dropping the foam to only 55% of the total R-value.
If you bump the foam up to 6", at R-33.6, that leaves 5 7/8" of space to fill. You may be able to special-order 6 3/4" batts which will perform at about R-20 when compressed into that space, leaving a safe ratio from a moisture accumulation (condensation) point of view. Or you could go with 6.5" of foam and regular R-19 batts.
There are no condensation concerns if you use a higher ratio of foam, or all foam, even in a shallow cavity. The reason flash-and-batt presents a condensation risk is that the fluffy insulation keeps indoor heat from reaching the interior surface of the foam, so it is cool. Without fluffy insulation, the interior surface of the foam will be close to room temperature.
A variable permeance membrane is all you need under the T+G. Membrain is ok but somewhat flimsy and more vapor-open than the competition. I specify products made by Siga, Pro Clima or Rothoblaas, which are more durable and allow less moisture through, but they are twice the price of Membrain. They can be ordered online.
More info on flash-and-batt here: https://www.greenbuildingadvisor.com/app/uploads/sites/default/files/Why%20Flash%20and%20Batt%20Makes%20Sense.pdf.
A chart by Owens Corning showing what happens when you compress fiberglass insulation is attached.
Thanks alot Michael! I had not considered/been aware of R-value loss over time in SPF.
I do have 700sqft of R30 that is left over and needs to be used, but I don't want to throw off the foam to fluff ratio. I assume (while not scientific) if I sprayed 5.5" of SPF and then stripped the R30 batts to ~6.5 that would be around R20 of fluff and allow for the cavity to be filled w/o compressing?
Is there an issue with 15.25" wide batts being used in 16"oc TJI's and the (what I perceive) as lack of tightness against webbing?
Thank you again, very helpful!
The variable perm membrane buys you a fair bit of margin with a hybrid insulated roof. You can safely push the SPF/fluffy ratio a bit without problems. Spray foam does a good job of air sealing the assembly which is the most important part. Make sure the spray foam is also carried from the roof deck all the way down to your exterior wall plates to ensure air barrier continuity.
You really want 16" wide batts for TJIs. Compressing the R30 might be enough to get a tight fit though.
I wouldn't risk pushing the insulation ratio; as it is, the calculations are based on keeping the interior at or below 45% relative humidity, which can be on the low side in a tight house. It's true that variable permeance membranes provide a degree of safety but the risks far outweight the potential cost savings, in my opinion. I do agree that with TJIs you should use full 16" batts, which are available but may be a special order. Or you can cut the batts to 16" lengths and install them sideways, being careful to keep them tight together.