Replies

1. 
   Jon_R | Sep 29, 2019 01:15pm | #1

   You are considering making the attic semi-conditioned. Note that IRC 806.5.1 requires " The unvented attic space is completely within the building thermal envelope." This is violated with a sealed, semi-conditioned attic below an unvented roof (if that is what you have). My guess as to the reasoning: the potential for moisture problems with anything that makes the attic cooler (Winter) or doesn't have interior airflow to dehumidify (Summer).

   Edit: see https://www.energyvanguard.com/blog/3-reasons-to-remove-attic-floor-insulation-in-a-spray-foam-attic

2. Expert Member
   BILL WICHERS | Sep 29, 2019 06:29pm | #2

   Code generally sets minimum allowable standards. In this case, I’d interpret that to disallow semi-conditioned attics UNLESS either one of the two insulated surfaces (roof or floor) met the ENTIRE required R value for insulation in that part of the structure. Basically you can’t split R38 into R23 in the roof and another R15 in the floor to make up the R38 total.

   I think if you put R38 in the roof, or R38 in the floor, any insulation you put in the “other” location would just be extra over the code-minimum required amount and you’d be OK. Note that I’m just using “R38” here, but you’d need to use whatever the minimum is for your climate zone. Any extra insulation will slow down heat flow, so you’ll gain a little bit of efficiency. Whether or not that “little bit” is enough to be worthwhile is up to you.

   Bill

3. 
   mfleck | Sep 29, 2019 08:01pm | #3

   Thanks, Bill

   I believe R49 is code for me here at the northern edge of NYC. I don’t think I’m going to be able to get to that R value, but it’s going to be a huge improvement over what was there when I bought the home (a thin, 3-inch batt of porous, dirty rock wool).

   I recognize that I can’t get to code by splitting—honestly that’s not my intent, just trying to do the best I can with the inches of insulating space I have.

   I guess my question is more: can I do any damage in insulating the attic floor—are there any unexpected risks that I should be aware of?

   Thanks again for the advice here.

   Matt

4. 
   BrianPontolilo | Sep 30, 2019 07:20am | #4

   Hi Matt.

   Maybe I'll learn something today, but I don't think there are any risk in insulating the floor other than spending money on an improvement that doesn't actually help with efficiency.

   I assume that you have a plan for insulating the roofline, but you may consider reading this article, because it is a tricky project: How to Build an Insulated Cathedral Ceiling

   And keep in mind that air sealing is as important as insulating, particularly at that attic ceiling.

5. 
   Trevor_Lambert | Sep 30, 2019 08:44am | #5

   I think you'll find that a small amount of insulation in the attic floor will make a difference, given the relatively low Delta T. I'm presuming you'll want the attic to be something reasonable, like above 40F.

6. 
   mfleck | Sep 30, 2019 09:19am | #6

   Thanks for responses.

   Brian, thanks for the advice. I’ve definitely read the cathedral ceiling article a few times. Very helpful. My plan is 1 inch air gap under sheathing created by cobbling in 1-inch foam with spacer. Then 3.5” of rock wool unfaced, followed by 2 inches of polyiso under the rafters and drywall. I think that’s the best I can do.

   Sounds like there are mixed opinions on insulating the attic floor as well. In my mind it seems like it would help keep the heat we generate in the space we live in most, but I guess in reality that may not play out.

   Any advice on air sealing attic? Is there a good article on this?

1. 
   BrianPontolilo | Sep 30, 2019 09:54am | #7

   Hi Matthew,

   The good news is that your drywall ceiling makes a great air barrier, with some caveats, like, don't poke holes through it for recessed can lights, air seal electrical boxes in the walls, and seal the perimeter with caulk or foam. Here's another article for you about airtight drywall: Airtight Drywall

7. Expert Member
   Peter Engle | Sep 30, 2019 11:01am | #8

   You can also tape the seams of the polyiso insulation. Foil tapes stick like crazy and make it relatively easy to air-seal the underside of the roof.

   There is some risk in insulating the attic floor and maintaining the attic as a semi-conditioned space. Many people find that attics run a bit more humid than the rest of the house once the roof is insulated. if you insulate the floor, the attic space will run both cooler and more humid and in winter, that can be a recipe for mold growth.

   Use some easy numbers and take a look: Your roof stackup is about R-25. If you fill the floor cavities with cellulose, that might also get you close to R-25. So potentially, the attic will run halfway between inside and outside temperatures. When it is 10F outside and 70F inside, the attic will drop to near 40 degrees. Not in danger of freezing, so good right?

   However, if your interior air is 70F and 35% RH (not unusual conditions), the dewpoint is 41 degrees. Uh-oh. You are right at the cusp of getting condensation on the inside of your vapor barrier. Just a little bit more interior moisture, and you might be in trouble.

   So, keep you indoor air a little bit dryer, your attic a little bit warmer, and/or your floor insulation a little bit thinner.

   Of course, unless you've got pretty good separation between the attic and the rest of the house, convection will keep the attic warm enough by itself. You only really get into trouble when the attic is truly closed off and unconditioned.