I am wanting to install radiant Barrier foli onto the underside of my roof rafters. I am also planning on adding 2 inch polyiso foam board in the same cavity, leaving a inch and a half airspace. My question is ...they say I must have an air space on one side of the RB. If I install rigid foam board between the rafters, should I push it up tight to the undrside of the roof deck or at the inner edge of the 2x4 rafter and then staple the RB to the underside of the rafter. Also, shud the RB droop between rafters or be pulled taught. Any other advise for RB or polyiso wud be appreciated

Use foam baffles to form an air space. There sold in any home center in the insulation area.

Do not use foam baffles full length unless you love soaking wet insulation. They will form a partial vapor retarder in the wrong place.

Few thoughts on this.

Richard is right with this. If you use the foam boards between the rafters you’re going to create a 2nd vapor barrier along the roof line. This is not the place that this should be. It should be where it currently is located under the insulation on the ceiling rafters. I am assuming your doing this to seal the attic from heat loss. Not a good idea unless you plan to condition the space as you would if its living space in the rest of the house.

Ideally the attic should be as close to exterior air temps as possible. This prevents condensation, mold, and allows the roof to last far beyond its typical life span.
The metal radiant barrier that you’re thinking about will help keep the attic cooler in summer and reflect heat loss from the living areas below in winter. But if the house was properly insulated and properly vented there is really no need to spend that money on the foil.

Also if you still decide to use this foil barrier be sure that vapor can pass through it. This is critical. If it does not allow for vapor transmission it will cause condensation and cause many issues. Also remember this product only works when it’s clean. Once it gets dirty the shiny surface gets dusty as it will its reflective properties will also begin to fail. Lots of hard earned money for something that will not last a long time.

Please understand. There are a lot of gadgets and bells to help save energy. Just like the ones they are selling to save gas in your car.
All of these products claim to save you 20, 30, and up to 50% on your energy bills. If that was the case builders now would be using this in their energy star rated homes.
For the most part their all gimmicks playing on peoples concern about saving energy.
Add insulation, add proper ventilation and enjoy the savings you will see on your energy bill.

One last thought. Not sure how old you are, but several years ago (early 70s) we had an energy crunch much like this one going on. People purchased all types of things to save energy. As time went on all of these products began to fail. As an inspector I have found some of the worse mold, rot and insect activity in attics that had these gadgets and widgets in place.

Joe, the foam baffles that you talking about are used to keep the blown in and batt insulation off of the roof sheathing just along the soffit area. Not what this person was talking about.

Unfortunately, I see many insulation contractors installing those styrofoam baffles full length in vaulted ceiling rafters, and of course, they are nowhere to be found when the poor homeowners discover their wet insulation, and possibly lots of mold, and even perhaps structural decay. The baffles should only be used at the "tight spot" at the intersection of roof and celing, and only for the minimum length necessary to maintain the required air space past that tight spot.