Insulation board itself is not going to reduce infiltration.\

You need to find the gaps and close them.

Canned foam is used to fill larger gaps, even with insulation board.

by the sheer size of the insulation board you will stop
a lot of infiltration and heat gain. this is one of the benefit of sheet materials.

install foil faced foam sheathing..I'd put it over tyvek.
use button cap nails to secure to 2x's.

caulk at top and bottom plates. sole plates are never sealed in these areas, caulking will stop air leakage.

make sure that foam covers both plates.
use foil tape to seal all seams holes and joints.

use a razor knife to trim sheathing boards to fit
around any penetrations or wires and caulk to seal holes.

I use caulk more than cans of foam, just tested too many great stuff seals & have seen it deteriorate over the years. caulk conforms to gaps and stays flexible to allow for movement of house. you can buy 25, 35 and 50 year caulks. Dap Alex brand is what I use. (buy the case)

using a foil faced foam board with foil facing into attic space will act as an air, moisture & radiant barrier which will help reduce heat gain thru walls into living space.

I have done this type of install before. if you caulk and air seal as you go it works well. sharp razor knife, button cap nails, straight edge, hammer, caulk & sealants
are all you need. and good light and tunes of your choice.

best of luck.

Thanks Energy rater. This is perfect, exactly what I was looking for. If I use the 3/4" foil back foam, what length button cap nail should I use. Also, these areas have a pretty steep pitch so it is going to be quite difficult to get between the floor and toe boards to nail. Can I use and adhesive in this area and nail the rest? If so, what kind of adhesive caulk should I use?

Thanks again for your response. Greatly appreciated.

Regards,
Paul

1 1/2" button cap nails.
Use construction adhesive, liquid nail or like
product. caulk foam boards at floor..tape seams
caulk everything!
and have fun.

Got it. Thanks much. I thought of liquid nail but thought maybe the solvents in it would dissolve the insulation board. I already have a case of it so that works.

Thanks again. I very much appreciate your advice.

Regards,
Paul

Welcome Paul.
Next we'll have you sealing your return air chase
with ductboard & mastic. LOL!

Liquid nails will eat right through most foams as will many other caulks. Home retailers should have foam compatible caulk.

energy_rater, quick question, albeit maybe a dumb one...

Today, the building supply delivered my 1" Foam insulation board. It is Dow TUFF R and it is foiled on both sides. One side, with the Dow printing and disclaimer, is a dull foil while the other side is a bright and shiny foil, similar to aluminum foil you might cook with this.

Since I am putting this up in interior space, I am assuming when you say "foil towards the attic" you are meaning that I put the bright shiny side of the foam board facing toward the exeterior (against the insulation) and the side with the Dow Tuff R printing will be what I will see when I go into the storage areas? Is this correct or should it be turned the other way?

Thanks again for all your advice. Its much appreciated.

shinny foil faces you, not the studs. this will allow the foil to reflect heat out of the interior walls like a radiant barrier. if you put the foil in contact with the insulation you lose the radiant quality.
makes it easy to light up the space too.

best of luck.

Thanks. Just sent you an email.

Regards,
Paul