re-roof; with increased R value

Guest User
8/9/2017
Hello, I am looking to get some feedback and or input on a residential home from the 70’s. I have a home that was initially a summer cottage and is now lived in year round in Wisconsin. So the concern is that the home is under insulated on the roof. The shingles are respectable, but the heating and cooling bills are “out of the roof”, literally! The current materials present are five 4x10 beams which run horizontally the length of the house, and 2x6 T&G that runs vertical on top of the horizontal beams. Inside the home is a cathedral ceiling throughout, so no attic space or insulation inside, the 2x6 T&G viewed from inside is also the roof planking. On the outside is 1 rigid foam, felt underlayment and asphalt shingles. I would like to add R value with 3 or 4 inches of rigid insulation on top off existing roof. I have done quite a bit of research and believe the extruded closed cell polystyrene would be the way to go, which would get about R5/in. I would also like to avoid tear off and try to use present materials to help with R value, have less clean up, and not have to worry about timing and weather. Also, it seems that venting between top of new rigid insulation and new roofing material would be best practice to avoid ice dams and bridging, creating a “cold roof”. So far the plan would be to run 2x4’s flat 24 in OC vertically on top of rigid foam, then put down substrate, and new roof material. Venting would be from soffit to ridge. Another question revolves around new roofing material. I would like to go with metal roofing, probably standing seam. I am trying to decide if I should go with a solid substrate (5/8 OSB), or flat 1x4’s every 24 in OC, either would lay on top of flat vertical 2x4’s used for venting. I still need to choose metal supplier, and hopefully will have a choice of substrate, it would be nice to keep weight of solid substrate down by using spaced 1x4's. The home has large overhangs on all sides, 43 in @ the eves, and 50 in @ the ends, so I was planning to only add insulation over the living space and block up outer perimeter with flat lumber to match same height. So the question is if there should be concern for shrinkage and changing thickness over the area with rigid insulation, versus remaining perimeter spaces that will hold size with built up lumber? Lastly, would there be a need to put roofing felt or a barrier over existing shingles before putting rigid insulation down? In conclusion, after consulting with several roofers and insulation companies, it seems that this is a unique situation, and each has their own Idea and approach, which is kind of scary. Roof pitch is 3:12, metal panel lengths would be apex 16'5", total roof area is apex 17 square (1700 sq Ft) Thank you for your time and consideration, I look forward to your reply, Tony K. Concerned homeowner Tony K
Todd Miller
Isaiah Industries, Inc.
8/9/2017
Your home's situation seems prime for issues with ice dams and also with condensation. So, you are doing the right thing by researching and working on a solution. Generally, vapor barriers, insulation, and ventilation / thermal breaks are called for in this situation. I would prefer to see the standing seam on top of solid decking, and then vented airspace beneath that with the insulation beneath that. I would do the exact same thing over the overhangs as you do over the living space. It just seems simplest as far as venting, etc. As long as you have underlayment beneath the metal roofing, I do not feel it is essential over the old shingles. However, adding a non breathable vapor barrier over the old shingles could help prevent moisture migration into the roof assembly, which will help guard against condensation. I hope this helps.
Eric Novotny
An informed customer is the Best Customer!
8/9/2017
You have got a pretty good strategy in this case. Couple of points. 1. You are going to be spending good money on this roof on the foam alone. I would not leave the shingles in place. They serve no benefit here and will only interfere will the foam layers getting seamless interface with another. 2. I am not sure the vent space above deck is needed here. While it is ultimately a benefit from a standpoint of keeping the roof closer to ambient temperatures, with 5" of foam, that roof is not going to really allow much in the way of heat out of it to where it would ice dam. Keep in mind, you can ice dam on a roof in the right conditions over an open air roof so that potential always exists to some extent. That said, probably better than 95% of ice dams are from poorly insulated or vented roofs. A roof with 5" of foam will be neither of those things. 3. Depending on the condition of the foam in that first 1", you might want to see about removing that to make sure all of the decking is screwed down and nailed down sufficiently prior to adding all of these layers. My thought process here is that you just want to make sure that everything you are going to be attaching that very expensive foam and roof too is well secured itself. 4. You are going to need to devise a method for attaching these layers of foam and instead of running a 5" screw and plate, I would like to see some intermediary layers of wood (i.e. 2x4s) to clamp everything down and then give you a better position to locate your fasteners to for additional layers and ultimately the roof. 5. If you want to go the above deck venting route, you are definitely going to spend a little bit more on wood, labor (that is critical to get someone that knows what they are doing), ridge cap fabrication, and vent screening to keep the bugs out of the airspace. I think you are in good shape regardless but you are going to need to find a good labor resource to be certain. Start with the MRA contractors in your area and go from there. Where in WI are you located?
Eric Novotny
An informed customer is the Best Customer!
8/9/2017
Forgot to add...if you pull the old roof and foam, seal the entire T&G deck with Ice/water to serve as a vapor/air barrier. That will stop any moisture or air migration to the outside at that point. Then do all your foam details and roof installation.
Guest User
8/11/2017
First of all, thanks to both replies. At a minimum I have a better feeling with the plan, less a couple of tweaks here and there. Location is SE Wisconsin. Only reason I was looking @ going with sturdy metal (22/24 ga) with 1x4 flat horizontally 24" OC for substrate was to keep additional weight off home, solid decking would call for 5/8 OSB minimum per code (17 sq, or 54 sheets). And I agree, entire deck insulated would be much easier to construct, but I was hoping to shave a few $'s off insulation cost, and additionally use open space around perimeter to add support to long overhangs. I am considering a complete tear off, although I do feel confident T&G boards are sound as seen and inspected from below. I will surely look for a MRA contractor to ensure proper construction. Thank you again for your time and consideration. Tony K.
tony kaczmarek
8/12/2017
A few more questions, pros & cons that may help me seal up the plans are: Comparing standing seem metal panels to metal shingles with locking system? Steel versus aluminum, concerns for storm/hail damage and durability? Extruded polystyrene sheets; will it be necessary and or cost worthy to use higher density insulation board for compressive strength 25 PSI versus 15 PSI, with R value being equal for both? Thank you again for your time and consideration
Todd Miller
Isaiah Industries, Inc.
8/13/2017
Thanks. Both metal shingles and standing seam pass the same uplift tests. You typically have more interlocks with shingles than standing seam but, again, they pass the same tests. Metal shingles do require a 3:12 minimum pitch ... if your roof may be below that, then that fact needs to guide your decision. Metal shingles typically provide a more "traditional" look for a 1970s home. Standing seam, due to its flat surfaces, is more likely than metal shingles often to show cosmetic hail damage. Severe hall will destroy any product. There is a point though if the hail is bad enough that steel will do better than aluminum as resisting indentations, if all other things are equal. Because of your marginal roof pitch, you do not want foam that compresses badly. I would use the higher density for certain.

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