Replies

1. 
   JC72 | May 03, 2017 10:39am | #1

   Roxul requires vertical furring strips for "light-weight" cladding. The furring strips also help keep the mineral wool in place.

   http://static.rockwool.com/globalassets/rockwool-na/downloads/technical-guides/residential/technical-guides/fasteners_guide_techguide_en.pdf

   http://static.rockwool.com/globalassets/rockwool-na/downloads/technical-guides/residential/comfortboard80_installation_techguide_en.pdf

2. GBA Editor
   Martin Holladay | May 03, 2017 10:42am | #2

   Jeremy,
   You don't need a rainscreen gap.

   Your biggest problem is that most vinyl siding manufacturers probably don't allow their products to be installed on a squishy substrate like mineral wool. (For that matter, most vinyl siding manufacturers don't allow their products to be installed on furring strips, either.)

   You can always "go outlaw" -- install the siding in a way that is contrary to the manufacturer's installation instructions -- but then you won't have any warranty if something goes wrong.

   For more information, see Can Vinyl Siding be Applied Over Furring Strips?

3. 
   JeremyArch | May 03, 2017 01:19pm | #3

   Not one to break the rules when it comes to a warranty. I will look at using a more rigid foam instead.

   Thank you both for the links and information

4. 
   JeremyArch | May 12, 2017 03:56pm | #4

   Martin,

   I am still trying to develop a solution by adding some insulation on my exterior while using vinyl siding. Would a solution as follows work on the exterior of my existing wall in climate zone 6. I am limited on the amount of material I can put on and want to allow my wall to dry outwards with the existing interior poly in place.

   drywall
   poly
   2x6 16"oc fiberglass batt
   plywood
   WRB
   1" mineral wool
   0.5" rigid foam
   vinyl siding

   Thanks for your thoughts on this!

5. GBA Editor
   Martin Holladay | May 12, 2017 04:20pm | #5

   Jeremy,
   You're going to have fewer problems with rigid insulation than mineral wool, and it's best if this type of assembly dries to the interior.

   You can install vinyl siding on top of rigid foam that is up to 2 inches thick without any furring strips. If you use polyiso, you'll be fine for 2x4 walls -- and a little marginal for 2x6 walls. If you have already framed your house with 2x6s, you could skimp a little on the insulation between the studs to stay out of trouble.

6. 
   JeremyArch | May 12, 2017 05:22pm | #6

   Sorry, this is a retrofit with existing poly so im looking at a way to add 1.5" allowing to dry to the exterior using vinyl siding

7. GBA Editor
   Martin Holladay | May 13, 2017 06:00am | #7

   Jeremy,
   Adding enough continuous exterior insulation -- in your case, continuous insulation with a minimum R-value of R-11.25 -- is far more important than worrying about the interior poly. If I were you, I would go ahead with a solution that supplies continuous insulation rated at R-11.25 or more, and I wouldn't worry about the polyethylene.

   For more information on why I think you can stop worrying about the interior polyethylene, see this article: The Exterior Rigid Foam is Too Thin!

8. 
   JeremyArch | May 13, 2017 02:58pm | #8

   Thanks for your replies Martin, they are appreciated.

   I would love to add a r-12 to my exterior but I guess thats my problem, I am unable to add more then 1.5 inches. My gable ends dont currently have more then a 2 inch overhang so to add R-12 i would need to build this out. Its a new roof (done by previous owners) and i dont want to get into that sort of work with my current budget. Given these constraints i am trying to come up with a way which to use a vapour permeable insulation in the event my sheathing does get wet as well as use a rigid backing for my vinyl siding.
   I just received results from my first blower test and the original condition 1983 build has an ACH of 2.6. With air sealing and upgrades this will lower, ideally under 2ACH. perhaps i dont need to worry about much vapour diffusion through the wall resulting in condensation and i can just use a vapour semi-permeable exterior insulation.
   Hope our enjoying the weekend!