cedar shake vs. asphalt shingles

We have two homes where we tore off cedar shake installed directly on the OSB sheathing and we installed lifetime dimensional shingles. Each house never had ice dams or icicles with the cedar but now both have ice dams and icicles EVERYWHERE! Ventilation is balanced and up to NFA standards. Can anyone tell me why removing the cedar and installing the asphalt has created these issues? Thanks.

We didn't add insulation to either home. Home "A" was fine at R49 (he could use some air sealing though) but Home "B" is really only R29 even though the homeowner thinks it's more and is very defensive as he built the home. He also put two furnaces in and installed the second furnace in the 2nd story attic. So it's pretty warm up there. He isn't opposed to heat sealing and adding insulation he's just freaked as to why it changed so much from having the cedar to the new lifetime laminates. Oh well, like you said maybe the cedar does absorb that much moisture. They were the heavys so they were pretty thick. Oh, and Home "B" started leaking this weekend. All the way from the second story to the basement down an interior wall. Leak is a 14" ice dam on the gutter up along a side wall of a dormer. We have ice and water shield 6' on the eave and 3' along the dormer wall but we can see the ice dripping on an outer wall below the dormer between the cedar siding now. It's a terrible design! I'm sure there's a foot of water just sitting on that roof with the 14" on the gutter. I am really beginning to hate ice dams...and some architects who design these fancy roof lines with no rain/ice performance thought going into them. Ugh.

ice dams are a hot topic right now in the roofing industry. this blizzard that just passed has tested not only how well your roof was installed but also ventilation,insulation, and drainage.so as to why you have ice dams there are other guys on here more qualified to tell you than i, but ill throw my 2 cents in.lol i think the cedar roof breathed better since wood is pourous. ill bet you had an even blanket of snow on roof which held snow longer than the new asphalt roof.the asphalt shingles especially the darker the color soak in the sun and melts faster than cedar. which isnt all bad if like i said if all the other issues have been addressed.the faster you can get water or snow off your roof the better less chances of leaks. now you didnt say it was leaking so i assume not yet at least. dont get freaked out it may not assuming ice and water underlayment was installed in proper areas,but still there is a chance. some companies will warranty ice dam leaks others wont. it depends on their policies and what was causing the leak.so i think your problem is in insulation if roof is properly vented and vents and soffits properly installed. id talk with your roofer and a home insulation contractor about what can be done to remedy your situation. some key questions would be is the ventilation sufficent,is my insulation sufficent,ridge venting, and gutters sometimes larger gutters are needed where multiple roof slopes drain to one area. good luck to you and maybe some other guys here will have more/better advice. jj