Tips for properly venting an attic
Learn the difference between gable, soffit, and ridge vents.
Q. I am having my roof reshingled and am weighing the option of having a ridge vent installed at the same time. I have gable vents that seem to function well; one end seems to allow hot air out, and the other apparently has a good intake of air. I will be installing soffit vents along the entire span. Will a ridge vent have any adverse effects on the gable vents? Should I leave well enough alone?
THOMAS BRIGHAM
A. I always turn to Joseph Lstiburek, founding principal of Building Science Corp., for guidance on ventilation and insulation issues. “Doc L.’’ (he has a doctorate of philosophy in building science) is awesome, and he has forgotten more than I’ll ever know about this stuff. Here are his rules of thumb:
1) The ceiling plane MUST be airtight;
2) The entire perimeter of the roof needs to have air inlets, meaning continuous soffit ventilation. What’s important is that you have continuous air entry at the perimeter of the roof down low;
3) Put more entry vents down low than exit vents up high. The reason is, if you construct a house with a leaky attic ceiling and you have lots of ridge vents (or lots of vents up high), the makeup air is going to be pulled from the house rather than from the outside. Unbalanced ventilation should be in favor of the lower vents (gable and soffit), because you don’t want to depressurize the attic;
4) Put more insulation on top of the wall than inside it; if you have R-20-value insulation in your wall, you want at least R-20 on your top plate. The higher the R-value the better.
Q. We have to replace our 30-plus-year-old second-floor windows. Does it make more sense to stick with the maker of the original windows? Would the original dealer be able to replace size for size? My understanding is that if we went with a different manufacturer, it would probably involve more work, including replacing the siding.
MARY ELLEN DOWNEY, Reading
A. I would not stick with the original manufacturer unless you absolutely like what it offers now for features. Given the age of your house, I’m betting that the rough-opening sizes for your windows are pretty standard. I’d look at window manufacturers for features, cost, and other factors first and then see what you can fit in the rough opening. If the window you want is slightly larger, the installer can always cut away the siding. It’s when windows get smaller that you end up removing and patching siding and interior wallboard.
Rob Robillard is a general contractor, carpenter, editor of AConcordCarpenter.com, and principal of a carpentry and renovation business. Send your questions to [email protected] or tweet them to @robertrobillard.
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