Intello vs. Majrex in Zone 5B — Both are Fine, But What’s Best and Why?
Alan_Woods
| Posted in General Questions on
I’m a frequent reader preparing to detail a wall in my Southern NH renovation project, and trying to decide between Intello and Majrex (I realize both are great products.)
Our wall detail (which could not be energy optimized due to historic / aesthetic reasons) from the outside in:
Weathered and unpainted clapboard siding
3/4″ rainscreen furring
Our wall detail (which could not be energy optimized due to historic / aesthetic reasons) from the outside in:
Weathered and unpainted clapboard siding
3/4″ rainscreen furring
Liquid applied STPE (wrb, vapor permeable)
3/4″ plywood sheathing
2×6 studs
R23 Rockwool batt insulation
3/4″ plywood sheathing
2×6 studs
R23 Rockwool batt insulation
[Intello OR Majrex]
5/8 Blueboard
Plaster
Probably a Class I paint, 0.010 (Class I, limit <0.14)
The wall detail is mostly finished. While I know it could be more insulated, as I said, it wasn’t in the cards for this renovation.
1) We are trying to be attentive to the fact that, in general, our temps are going to rise, and in general, weather is going to be extreme and sometimes just downright weird.
2) I realize that both Intello and Majrex would probably do a fantastic job, and I can’t go wrong with either (correct me if I’m wrong).
3) That said, they are not identical products, and they perform a little bit differently. Can someone explain the specific benefits of each, and how I might consider these differences in the context of my renovation?
We have a high efficiency wood stove in the house. (We crack windows and have makup air.)
We like to keep our RH at 40% (or a touch more) even in the winter.
The house is fairly exposed to the elements — south side gets direct sun, and the north side gets blasted by winter storms. It’s a cape and we have minimal overhangs.
Kind thanks and happy building.
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Replies
If you're using a vapor barrier paint, you don't need the membrane at all unless you want to use it for your air control layer.
Both products use a polyamide film. The difference is that Intello's permeance varies with humidity levels, from a low of about 0.13 perms when dry to 13 perms when "wet." Salespeople sometimes attempt to simplify what that means by saying it dries one way in summer and the other way in winter, but that's not accurate; it prevents most moisture from getting into the assembly from the interior, but if the framing cavity gets elevated moisture levels for any reason, the permeance increases so the assembly can dry to the interior. All other variable permeance membranes I'm aware of work similarly, except Majrex.
Majrex is a uni-directional membrane. For moisture trying to get from the building interior into the framing cavities, it's 0.23 perms, so it blocks most moisture but not quite as effectively as Intello at low humidity levels, but better than Intello at high humidity levels. From inside the framing cavity toward the building interior, it's 0.68 perms--higher than in the other direction but not particularly high.
Both are very high quality products, cost roughly the same and are readily available online. I tend to spec Majrex more often because I design in a heating-dominated climate and use vapor-open exteriors, so I'm not too worried about the walls being able to dry to the interior--the weakest aspect to Majrex. But I'm perfectly fine with Intello as an alternative, and if inward drying were more important to me, I would prefer it over Majrex.
Thank you, Michael.
Leaving aside the air barrier benefits for a moment, do you think Intello or Majrex offer an extra layer of "insurance" over relying on painted plaster for stopping vapor, or are they simply redundant?
Our plywood sheathing is very well sealed and I am pretty confident in it as an air barrier, but I always assumed we would have a second air barrier on the interior.
I think, ideally, we would not want to rely on the plaster work as an interior air barrier as we have some fairly unusual details with our framing and built-ins.
I'm all for belt and suspenders, and sometimes an extra pair of suspenders is not a bad thing. If the cost doesn't deter you, there's no harm in adding a membrane, and it will likely do at least a little good.