FHB #217 Why Flash and Batt
What do you think of that article? Found myself confused as the math didn’t seem to work, and much of the article accurately identified the potential risks of this approach. Costs less to do, but you get less performance. Present value, and long-term operational savings costs should be a concern. Looks like only initial cost is considered here. Even that appears to be unclear, as the closed cell system in an installed cost. The others (batts, and flash-and-batt) should have a labor to install added cost. Also, shouldn’t the batt system be derated for the design temperature of the climate zone of the house? I divided the cost by the R value of each system, and the flash-and-batt system is by far the most expense per delivered performance value, and it has several potential risks. What would be the cost of a R23 closed cell system? Proportional math would make it cheaper than the flash-and-bat system!
Replies
Dancingroofer,
I thought the article was ok but then again I wrote it.... Flash and batt has a lot of potential risks, and is not the best insulation for many projects. At our design/build firm we use it maybe 10% of the time. Sometimes it makes sense. Often it makes the most sense when clients are pre-sold on the value of foam insulation but their budget does not allow for full closed cell spray foam.
There was a mistake; the comparative costs should not have said "materials." They are average installed costs. The way I arrived at those figures was that I designed a hypothetical small house and had half a dozen insulation contractors price it out different ways. There are a lot of variables, so sometimes flash and batt will not save much money over full foam. It's often pretty close to dense-packed cellulose for similar cost.
One big advantage of flash and batt is that the fiberglass stays relatively warm, so no need to de-rate it. There was not space in the article to go into all the details but I am happy to try to answer any questions here.
Well, looky here
Mike,
Long time, no hear. Hope all is well up there in the north corner.
Hi Calvin,
Things are fine, good to see you here. How are you doing?
Mike
We're all fine here in NW Ohio. Weather sucks but golf league has started.
Baseball thus far is good.
Deer overpopulation and the dope fiends feeding them make it all the worse and the deer all more tame.
Work?
Hopefully.
Remember to get in touch if you and your sweetheart venture this way.
Calvin, DW and I are coming to Cleveland on Mother's Day weekend for her grandmother's 90th birthday. We'll have a car full of people but I'll try to swing out for a visit!
Mothers Day weekend?
While I'm not going to miss the only mother left here in our family on that day, I'd sure like to meet up with you. What side of Cleve are you going to......................I'm not quite remembering.
Maybe I could head east, you west and we meet up somewhere in between? Whatever works for your trip-I live here.
When the heck is the day of Mothers anyway?
Calvin, Mother's Day (or is it Mothers' Day?) is May 8 this year. I think we're driving out on the Thursday before, coming back the Tuesday after. The old folks used to be in Shaker Hts but are downtown now, and the family is scattered around in Shaker Hts, Lakewood and the west side. Maybe we can meet up halfway for lunch or something--good idea.
Meeting sounds good.
See it you can narrow down the day according to the wife and "others" plan. I've been to both sides of Cleveland b/4 on Mothers Day weekend and it is not too damn easy. With the Tribe in LA for the weekend, at least gametime downtown won't be bad..............unless there's something at the Q..
See what they say, get to me on your location and we'll find something worth going to.
flash&batt after the fact..?
mr maine, i am owner/builder of new cottage.heating systems are:propane-fired radiant floor and wood-fired masonry heater with a little passive solarbump...my walls are roughcut hemlock 2x6's,1' boards,heavy felt paper; insulated with +1" closed cell foam,6.5" kraft-backed fiberglass with a taped layer of heavy poly and painted 3/8"finished plywood.i dismissed potential moisture problems with smart alec remarks:either promoting the 'dry' heat provided by both systems or "i'll open a couple upstairs windows"...famous last words or am i good to go?..and how would you retrofit?
waiting with crowbar in hand,
ATILAN
Atilan, I hope your hand hasn't cramped waiting for my reply! I'm not clear on your wall assembly; can you describe the components in order from outside in? What it your climate zone? Is the ±1" foam sprayed on or rigid sheets?
At first glance I would say your system is questionable. How long have you been living with this system? Do you monitor indoor humidity levels? Have you had a blower door test?
Which climate zones risky/not?
I thought it was an interesting article, but somewhat unclear on which climate zones are or are not appropriate for this method. For example, would Seattle be appropriate (Zone 4 )?
BillA, the easiest way to tell if your project is a candidate for flash-and-batt is to check IRC2009, Section R601 Wall Construction. Table R601.3.1 shows in what situations you can get away with a Class 3 vapor retarder.
Table R601.3.1 shows that in Seattle's zone (Marine 4) you can use a Class 3 vapor retarder if you have insulated sheathing (aka exterior foam) with an R-value of at least 2.5 and 2x4 wall framing. That translates to a minimum of 1/2" of closed-cell foam and R-11 or R-13 fiberglass.
Because the table is meant for exterior foam, just imagine sliding that foam and fiberglass toward the inside of the house, until you end up with 1/2" of closed-cell spray foam against the inside of your sheathing and R-11 or R-13 fiberglass in the remainder of the stud bay. Compressing the fiberglass does reduce its R-value (though the R per inch goes up) so, following the table, you would need a full 4" or more in your stud bay.
The downside to moving the foam inside on any project is that you lose the advantages of a thermal break, and in a heating climate, the sheathing stays cold (and potentially wet).
I lived in Seattle one year so I know how damp it is there most of the time. Because of the foam, the sheathing will not be able to dry to the interior very well (though at 1/2" it will pass some water vapor) so you should make sure the sheathing has every chance possible to dry to the outside. That means using a rain screen behind your siding. It may be required in your location anyway. Rain screens are always a good idea, but when it's cold and/or damp for long periods of time it's even more important, not only to help the siding dry out but to help the framing dry out.