Dow Super Tuff-R

[Dana]

When there's no insulation the furring & lath stayed pretty much at room temp, and was inherently always above the dew point of the interior air. Stripping the old furring and setting it up just as dimmi's wall using rigid foam would be fine. How thick you go depends on your budget for foam & Tapcons. (The fasteners start to add up when you're looking at 7" screw or something. ) If you can find a local source for recycled roofing foam it gets to be pretty reasonable to go with 3"-3.5" of iso (R18-R22), and 5" screws. (I did my basement this way using scavenged 3" fiber-faced iso @ $20/sheet for 4x8s.) With a solid concrete wall there's no getting around using a hammer-drill/roto-hammer.

You also need to figure out how you're going to extend the window framing and electrical drops, etc in when going thicker. Be sure to make it air tight using low-expansion foam too- you don't want room air getting between the foam & wall and condensing there if you don't have to. Even though the concrete can handle the moisture, the subfloors & joists won't necessarily. The embedded studs in the concrete will be colder and more susceptible too, but it's primarily air-leaks not vapor diffusion that would cause a problem. In fact, using only relatively vapor-open foam is probably the right way to go- no more than 2" if XPS, 4" of EPS or fiber-faced iso, and definitely not foil-faced goods. (As a sanity check, fiber faced iso is usually somewhat more vapor permeable than the kraft facers on batts.) Air tight with sealed seams & edges is key. In the summer there could be fairly high moisture drives from the exterior from solar-heated damp concrete- you HAVE to let that moisture come through the finish wall, at least a little bit, to help keep those concrete-clad studs dry & happy.

Dimmi- as long as you seal the joints with foam or tape it's not a big deal to use squared edged goods.

 

[Tuff-R user]

Hi, I found this old thread and hopping getting some help with my question. I am insulating an attic in my 80-year old house. The idea to put either one or too layers of 2" foil faced Tuff-R 4x8 boards, cut them to fit between 2x6 rafters leaving at least 1.5 inch gap between board and inside part of the roof which will allow air traveling from soffit vents to ridge vents.

The space between Tuff-R boards; and boards and rafters will be fill in with Great Stuff foam and aluminum duct tape creating air tied envelope.

1. what is the difference b/w two sides of foil faced Tuff-R where one of the silver color and the other one is blue color?
2. Is the anything I am missing and need to pay attention.

Thanks you,

 

[Sponsor]

 

[Dana]

Ideally you would have clear soffit-to-ridge vent panels and vent openings for cut'n'cobble like that.

The blue facer side of Super Tuff-R is more rugged, harder to scar up than the bare foil side. That's basically it, and in your application it hardly matters.

For slightly higher performance, have the silvery foil facing the vent channel. But there's an even better, cheaper option:

The rafters are about R1.2/inch to the foams ~R6.5/inch, so even though it's relatively safe to do in a vented roof, it's not buying you nearly what you think it is, and it's a lot of work. If these are standard 5.5" deep 2x6s set 24" on center (or close to that)if you have the room you'll be better off installing a cardboard or other thin cheap baffle on the outside of 3.5" deep high density "cathedral ceiling" fiberglass or rock wool batts (rock wool preferred), with a continuous 2" layer of Tuff-R on the under side of the rafters. The ~R13 thermal break over R4 of thermally bridging rafter adds far more performance than the only marginally higher center cavity R, and the fiber is cheaper per square foot than 2" Tuff-R. Cap nailing full sheets of Tuff-R to the bottom of the rafters is also a heluva lot less work than a cut'n' cobbled and foamed in place (2 layers, no less) approach.

More here.

 

[Tuff-R user]

Thank you Dana.
1. One of my problems is that old house has irregular distance between rafters (17.5", 18", 20" and 22") which makes more complicated using Stone Wool Insulation by only way of installing by cutting it perpendicular and fitting each rafter space by custom cut.
2. The other problem that I am facing is the only way to get into the attic is by using pull out stairs to attic which limits the size of any material can pushed there.

The only advantage as you mentioned is the cost per sq foot which is for
- 3.5" stone wool bats is ~$072/sq ft;
- 5.5" stone wool bats is ~$1.08/sq ft;
~2" Super Tuff-R rigid is ~1.24/sq ft;

Do you think it would make more sense for 2x6 rafters to use 5.5" stone wool bats with R23 rather than 3.5" with R15 ?

 

[Dana]

Don't forget to factor in the cost of the can-foam & tapes needed to seal an all cut'n'cobbled approach.

Can the hatch manage a 2' x 8' foam board?

You can't use 5.5" deep batts and still have sufficient ventilation gap under the roof deck. A code minimum gap is 1.0", but 1.5" or 2.0" is measurably better (especially on roof pitches lower than 4 : 12.)

For a batt solution of any size that required trimming for width or length, buy a purpose-made insulation knife for trimming them- it'll make your life a heluva lot easier. A still reasonable alternative tool would be an 8-10" bread knife. Doing it with a utility knife compressing the batt with a 2x4 is almost OK for low density batts, and it can be done with 3.5" rock wool, but it takes more time than with better tools and makes of for a third rate fit.

And fit matters! Give the batts ~3/8" of extra width for a compression fit to the rafters. At a half inch or higher it'll be more prone to buckling & gaps, but if it's too loose allows more performance-robbing air to flow along the rafters between the batt & rafter.

For cutting 2" Tuff-R, take a 4" steel taping knife and sharpen the side, and use a straight edge for a guide.

 

[Tuff-R user]

I think 2'x8' should be ok.

If i use 3.5" batts, should I still use, just in case, those baffles that ensure of making air channels b/w rafters?
Also, is there a way to figure out if I have enough air flow inside rafters after I insulate them?

Thank you for taping knife advice - very good idea.

 

[Dana]

Yes, install baffles, and locate them at a depth where they will be just touching the batts. (If 3.5" batts, install them 3.5" from the bottom side rafter edges.) That ensures maximal vent area, but also reduces convection through or around any side-fit imperfection of the batts.

 

[Tuff-R user]

Dana, should I use any plastic as vapor barrier to cover Stone Wool Insulation on the rafter edge side and before Tuff-R foam boalds?

 

[Dana]

The plastic is not necessary, since the sheets of Tuff-R are easy to air seal with aluminum tape on the seams. The foil facers are true vapor barriers.