In a cathedral roof situation where you wish to attach foam insulation to the underside of the rafters with strapping perpendicular to the rafters, is the drywall installed perpendicular or parallel to the strapping?
Secondly, what kind and length of screws do you use to run through the 3/4 inch strapping, 1 1/2 or 2 inch foam and into the rafters?. I suppose I should ask, should the stapping be on 16 or 24 OC? Any problems with this technique leading to sagging ceilings and or trapped moisture in the air space between the drywall and the foam?
Any other thoughts?
Thanks everyone.
Replies
The drywall runs perpendicular to the strapping. End joints are the most difficult to hide and it helps if they are solidly attached, landing on a strapping. If the drywall is tight against the foam board, there isn't a moisture issue. The foam is impermeable. It is important to allow breathing space continuously along the top of the rafter bays with propa vents or similar when using fiberglass batts. We normally go 16" OC, it's only a couple more rows. Screws should penetrate the strapping about 1/4". We strap the ceiling first, add the foam board, then the drywall. T&G foam sheets make a pretty good vapor barrier. Structural ridge beams along with deep enough rafters for adequate insulation is important on cathedral construction.
Extruded polystryrene is about R5 per inch, fiberglass batts are available in 9 7/8" R35. We like to use R38 as a minimum in ceilings so 1" foam and 9 7/8" batts should give you R 40 using 12" deep rafters. The foam board also gives you a homogeneous surface, eliminating thermal breaks at framing members, it's also great at abating sound transmission. My own place, built in 1980, uses this method and the house is tight and quiet, no problems with sagging. Walls are the same, you just need longer nails and screws.
Ok, clarification
I was thinking you would place the foam againt the underside of the rafters and then screw strapping on top of the foam through the foam and into the rafters. Sounds like you are suggesting attaching the strapping to the rafters and then eitehr foam BETWEEN the strapping or on top of the strapping (which is it?). And if the foam is on top of the strapping why have strapping at all, just screw the drywall up throught the foam and into the rafters?
You say "screws should penetrate the strapping 1/4 inc ". Do you mean go through plus 1/4 inch or just penetrate 1/4 inch?
Sounds like you don't use vent chutes. I think current thinking is to use chutes and to air seal them, so warm air in the insulation moves more slowly into the vents. ?????
When I say the screws penetrate the strapping, I mean they go all the way through and stick out the back side. Just going in 1/4" won't give you reliable holding power.
I used the term "Propa Vents" which is another name for vent chutes. We don't seal them. You are bound to have some moisture accumulation through condensation as warm air hits cold temperatures when heating in the winter. You need some ventilation to help expell this moisture.
There is a difference between using 1" extruded polystyrene sheets and 2". We typically use 1" which means we use 2 1/4" screws to attach the drywall. It's not very difficult when strapping goes up first, then the foam sheets and then the drywall. 2" sheets step up the difficulty level and if using them, strapping on top of the sheets might make more sense. The foam sheets are not cut to fit between strappings, it's placed on top to form a continuous homogeneous surface.
We use strapping for a number of reasons, although, it's more typical in New England than other parts of the country. It helps keep framing members on their intended centers and prevents them from warping or twisting. It's placed before raising interior partitions which eliminates the need for blocking. It can help when running electrical wiring as well as placing boxes. It holds unfaced batts in place without using other aids. It provides a wider nailing surface for drywall and can easily be shimmed to get a flat, straight ceiling line. It helps distribute the effects of truss lift and framing member crowning. More nailing and support surface when butt joints land on the 1x3 strapping.