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Question DetailsAsked on 12/20/2016

We used thermostat decking on new cathe ceiling roof.. Can we use rigid foam to make unvented roof? waste therm ?

New addition, valleys we cant vent, hate to put foam or foam board against thermostat decking since we were told it would negate the foil backing facing down

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If you put foam (or almost any other material) under the Thermostat sheathing, in contact with it (for summer case) or even if gapped away from it (in winter case) what little benefit you might be getting from it will be mostly to almost totally eliminated. How much benefit it is giving you now depends on the brand construction - if the foil layer is air-gapped from the board it can reflect a fair amount of heat back to into the roof materials to reradiate to the sky (though the 97% reflectance they claim is hopeful wishing by far because that is the value for foil surfaces facing open sky, which only works with bare metal or metallized paint coatings on the roof surface) - in many of these products the foil is glued directly to the board so it is somewhat ineffectual in keeping heat out of the attic from above because it transmits energy to the attic air through convection, and can actually trap MORE heat in the attic by reflecting heat from the house back into the attic under certain conditions - though the foil being glued is less detrimental in the winter, though still generally not as good as air-gapped. Actually, this type of foil-under-sheathing product is commonly more likely to be energy-saving in cold weather than hot - but the sheathing is generally just not the right place to be putting either reflective or insulation products.

Anyway, if you put rigid foam up under the sheathing, assuming this is an unconditioned space (attic is vented and open to the outside air) that is about the least effective place to put it both for summer and winter conditions, and will raise the roof temperature which is bad for wood or asphaltic or synthetic roofing materials and water barriers. In an outdoor ventilated attic space, the attic "floor" right above the ceiling is the right place for insulation.

As for making the attic unvented - there are a lot of roofs having serious decay problems these days because of people putting in foam board, or spraying insulation onto the undersize of the sheathing. Basically, you get moisture coming into the attic from the moist air in the house leaking into the attic (a LOT of moisture in the normal house where all ceiling/wall/pipe/duct/electrical penetrations have not been sealed and full vapor barrier coverage with sealed seams was not done). You also get moisture coming down from the roof - both from the drying out of the commonly damp or wet lumber and sheathing used in construction or rained on before the roof was put on, through the sheathing from any roofing or roof penetration leaks, and also from vapor diffusion through the water barrier from wet shingles during rain or snow melting on the roof. You also get moisture at times from outside air coming into the attic and either increasing the wood and insulation moisture (humid summers) or coming during the winter and condensing on cold attic surfaces. You are NOT going to get rid of all these sources of moisture - so sealing the attic up will almost always ultimately cause problems unless you either make it part of the "conditioned (heated/cooled/household ventilated) space", or provide the correct type of supplemental ventilation to control moisture - which then means it is not really "unvented".

Putting insulation under the roof sheathing without providing sheathing ventilation is asking for trouble in almost all cases. And don't fall for the blown-in insulation contractors who say they can blow in tight-packed cellulose in low-headroom or cathedral ceiling attic speaces which will insulate and prevent moisture issues. Aside from the fact that tight-packing it dramatically reduces its insulation value and because it settles dramatically with age you will still have an airgap under the sheathing, doing that also largely stops air movements down to negligable ventilation so it promotes water damage anywhere moisture gets into the attic area, and it also absorbs and retains moisture - promoting mildew and mold and wood rot, and in severe cases absorbing enough moisture from leaks or moisture coming through from household air leaks and then condensing/freezing in cold weather (since it can't get out with ventilation air) that you can even have ceiling collapse under the weight.

Plus there is the practical side - other than foam-in-place foam, unless the roof is totally off (sheathing and all) getting a decent insulation job in the typically tight space over cathedral ceilings is an iffy thing in the first place.

You did not say WHY you are considering this - do you have moisture issues, too hot or cold a ceiling, or what ?

Commonly, cathedral and folded ceilings have such issues - generally because of improper design of insulation and ventilation in the first place. The after-the-fact remedy, especially since many of these roofs have minimal (or commonly totally inadequate height for proper insulation in the first place) attic space over them to work with in the first place, is supplemental ventilation - seasonally reversible high-volume hanging fans are commonly used to move warmer air to the ceiling to reduce humidity or move hot air from the high points. Alternatives used at times (though more often in commercial applications) include thermostat and/or humidistat-controlled scavenger fans that pull air from the ceiling area, redistributing it with other household air or sometimes into an otherwise unheated basement or sometimes (with hot summer air only) venting it directly outside when it gets above a certain temperature. HVAC vents directing ventilation into that area from vents high on the wall or even diffusers located in the ceiling are also commonly used to mitigate the temperature difference in that area, though the latter are hard to conceal as retrofits - sometimes you end up having to mount them on the ceiling itself and then conceal them with architectural facades or valences.

Since you say new addition construction, sounds like you may have a design or construction deficiency which should be brought up with your architect, or if you did not hire an architect yourself to do the design, take it up with the contractor under his warranty if he did the design or had it done for him by an architect.

You can also find a lot more discussion on the attic insulation and ventilation and foil barrier issues in the Home > Roofing and Home > Insulation links in Browse Projects, at lower left.

Answered 3 years ago by LCD

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