Ice dams are not just created by the lack of proper insulation depth (R-Factor). Most often they are caused by bypasses or the ability of the heat to pass quickly through the insulation package. With a lack of proper ventilation or attic air flow to control it you have the perfect recipe for ice dam formation
Snow does have an insulation value of approximately R-1 per inch. As snow accumulates the R-factor grows accordingly. Just 12″ of snow has an R-12. Yes, that 12″ of snow has approximately the same amount of insulation value as the 3 1/2 inches of fiberglass (R-13) you normally have in a 2 x 4 wall system.
Attic systems are designed to be as cold as the air outside through an engineered ventilation design and calculated CFM transfer. Any time we disrupt that plan, we increase the likelihood of creating the dreaded ice dams. Ventilation is supplied through intake (soffits, vents) and exhaust (roof vents). You need one “ventilation” package, a combination of 1 Roof vent and two Soffit Vents.*Roof vent near the ridge and the two soffit vents, one either side of the house as seen here.
Take the amount of heated sq footage (condition air space) of the main floor, then divide by 350, is the quick formula for the amount of vent packages you would need. Suppose you have a 1400 sq foot house, you divide the 1400 by 350 or 4 units. (4 roof vents and 8 soffit vents) Now that’s the bare minimum, and code to ventilate the system adequately. it can be improved.
When you use a continuous soffit venting system you can improve you attics overall performance and balance, but be sure that air is allowed to pass over the exterior wall plate and successfully exist the roof vents. (see picture) We accomplish that by inserting quality insulation chutes thereby keeping that airway open to the roof vent. Fallen or blocked chutes, dirty soffit vents, skylights, roof dormers and any recent additions to an existing structure can disrupt this passage of air. With all these things corrected, the code compliant ventilation of the system should be adequate to cool the attic, unless you have excessive bypasses.
Bypasses allow much more heat to escape into that attic than the system can expel. Issues can start right at the wall plate. In earlier construction this wall plate has the least amount of insulation covering it due to the heights of the rafter tails being cut to fit over the wall plate. Insulation can spill out into the soffit and off the wall to be blown around by wind pressure. This wind pressure movement can also take place inside the attic, making it look like drifts, if “wind wash” was not installed. Wind wash could be as little as cardboard stapled into place but is usually a piece of fiberglass that prevents the blown (loose insulation) from moving around inside the attic. (see picture) These high and low areas can cause greater heat loss into the attic.
Remember, insulating of homes wasn’t even in the building code for walls and attics until sometime in the early 1960’s. Currently we use higher “energy heals” on our trusses and spray foam the top plates to bring an overall R rating evenly across the home. With existing houses the R-rating figured as an average due to the low slopes and style of construction.
The next series of bypasses are caused by “mechanicals”. These are created by trades: electricians, HVAC, and cable guys, and are created during construction when wires, pipes and duct work goes through interior top plates and drywall ceilings, into the attic. Openings from the basement to the attics exist as plumbing and chimney flue march to the outside. Sometimes HVAC’s heat ducts can run over the rafters or they have added an additional “Space Pac’s heating units ” located in the unconditioned space just add to the bypass problems. Over the years of “service men” in and out of the attic many more problems occur, as the insulation is disrupted by trades moving around in existing attics insulation and not raking or fluffed back into place.
An average of 3 sq in of “opening” gaps occur around the lighting, wiring and cable in each room. The average house having 8 to 12 rooms, so the small gaps sure add up. But much MORE occurs in kitchens and bathrooms, where the areas around the vents and pipes, passing into the attic, have to be made bigger to install the pipe and are often never sealed. “Recess” lighting cans add 2 to 3 sq inches for each unit with just a 1/4 inc gap around the light units. When turned on the heat radiates off the unit itself into the attic. Just add this all up, it’s like leaving your front AND back door open 3/4 to a full inch or more 24 hours a day. Many houses are even worse.
Don’t fret they are all correctable with qualified and knowledgeable personal and contractors.
Recently we corrected a home that had continued to have huge ice after being repeatedly insulated by others. This house actually had insulation blown so deep it was impossible to get into the attic, yes it was blown right to the rafters. Our installers would be under the insulation as they tried to move around the inside looking to find and seal any of bypasses. Not safe for the installers and had to be removed. The insulation extraction took two full days just to vacuum the material into two 30 yard dumpster it took to dispose of the extra material. We remover the roofing, opened the roof deck over the wall plates and spray foamed the first 3 ft of the cavity. Than added 6′ chutes and re insulated the balance after sealing the 23 can lights, two chimney’s, two bathroom vents and plumbing stacks, fixed the venting in the kitchen as well. Gaps were found around the 4 ceiling fan units and all the surface mounted bedroom and hall lights We also sealed every electrical wire hole that came into the attic. Then re-roofed then replace all 20 of the dirty soffit vents with brand new.
This improved system actually has less of an R-factor then when we arrived but performs beautifully after fixing the real cause of their ice dams, it’s Bypass.
Your Roofing Professionals in the Twin Cities
Based out of the Twin Cities area, Iron River is a trusted name among Minneapolis and Minnesota roofing contractors. With a name that represents integrity and the highest quality, Iron Rivers provides many roofing services. We invite you to get started with our process by scheduling a free roofing consultation.
