Replies

1. GBA Editor
   Martin Holladay | May 07, 2017 12:06pm | #1

   Matthew,
   No, the rules are not the same. One of the reasons that roof assemblies need thicker rigid foam than walls is that the stack effect puts roof sheathing at particular risk. When it comes to floor assemblies, the stack effect works in your favor, lowering rather than increasing the risk of moisture accumulation.

   You can use rigid foam of any thickness on the underside of a cantilevered floor in your climate zone. Of course, you need to protect the rigid foam from abuse with a layer of plywood, OSB, or soffit material.

   Here is a link to a relevant article: How to Insulate a Cold Floor.

2. 
   hickhead00 | May 07, 2017 01:15pm | #2

   Thanks Martin!!

3. 
   JDuchek24 | May 07, 2017 09:13pm | #3

   Martin,

   Within the linked article (which is very helpful), the "smaller room over larger garage" picture specifically highlights using rigid foam in the sloped ceiling section of the assembly. Would that layer of rigid foam be placed between the drywall and other cavity insulation? Why is rigid foam particularly helpful in that spot?

   I have been considering using Roxul (mineral wool) as the cavity insulation in the sloped section (between drywall and site built rigid foam baffles). Therefore curious about the additional layer of rigid foam against the drywall.

   Regards,
   Joe

4. GBA Editor
   Martin Holladay | May 08, 2017 07:01am | #4

   Joe,
   Q. "Would that layer of rigid foam be placed between the drywall and other cavity insulation?"

   A. In the illustration that you are talking about (reproduced below), the rigid foam is sandwiched between the drywall and the rafters. On the exterior side of the rigid foam, the rafters are insulated with some type of fluffy insulation (fiberglass, cellulose, or mineral wool). This approach works well, as long as there are ventilated air gaps between the top of the fluffy insulation and the underside of the roof sheathing.

   The method under discussion is one of many methods mentioned in my article, How to Build an Insulated Cathedral Ceiling.

   Q. "Why is rigid foam particularly helpful in that spot?"

   A. The rigid foam performs at least three functions: (a) It reduces thermal bridging through the rafters, (b) It raises the R-value of the assembly to meet minimum code requirements, and (c) If properly installed, it improves the airtightness of the assembly.

   .

   
5. 
   JDuchek24 | May 09, 2017 07:01am | #5

   Thanks Martin - I appreciate the clarification (and apologies, I should have realized this upfront).