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Question DetailsAsked on 2/13/2017

Can I use foam board hung from the roof joists as sophet baffels alont the botton 4 feet of the roof?

I'm trying to prevent ice dams, can I use the foam board against the roof joists and then blow in insulation against them?

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2 Answers


Sure you can - but almost certainly won't work and will probably make it a lot worse - especially since as I read your thought you would be packing the eave area full of insulation - cutting off attic ventilation in the process.

Your ice damming is almost certainly (unless caused by sun exposure on the wall of the house causing hot air rising under the overhanging roof, which proper angled soffits can mitigate), is not due to the overhang getting too warm, it is because the overhang is cold (below freezing) that is causing the ice damming - and insulating the underside of the roof in the soffit area as you suggest will make the overhang even colder by blocking off attic and wall warming heat from getting to it and melting the ice.

The problem is almsot certainly due to melting of snow on a warmish roof surface (above freezing), rain on snowpack, snowpack melting due to sunlight heating the surface and melting it, or rainfall on the roof, staying liquid UNTIL it hits the cold overhang, then freezes there as icicles then eventully building up to an ice dam - or in very cold weather sometimes freeing back from the edge of the roof at the melted-back front edge of the snowpack, or rarely due to rain or snowmelt from further up the roof freezing almost as soon as it hits a very cold overhang (much more common with metal roofs). Sometimes this is aggravated by metal gutters freezing up or filling with icicles from the edge of the roof forming a dam below and working back up at the edge of the roof, which then causes backup or water and more freezing back up the overhang toward (and sometimes in sevgere cases reaching) the point on the roof over or a bit inboard of the house wall (where the roof starts being warmer due to attic heating). Google"ice dam formation" for more info and diagrams/videos on this process.

The normal cause of this issue (probably 80-90% of the time or so) is either rain on snow or freeze-thaw conditions (liek daily afternoon sunn with snowpack on the roof) or excessive heat in the attic due to infiltration of warm house air through walls and ceilings (or open-to-attic vents and fans from the house rather than through the roof like they should be) up into the attic, warming the underside of the roof and causing melting of the snow on the main field of the roof, which then runs down the roof (usually under the snowpack) and freezes when it hits the cold overhang. If your roof is properly built, it can take a few inches of ice dam at the edge OK - usually in cold areas you have a 3 or sometimes 6 foot strip of ice and water shield (a sticky black or gray thick rubbery membrane) along the bottom edge of the roof under the water barrier, so to back up wate behind a dam about 3 feet up the roof on a 4:12 slope say takes a dam pushing a foot high if right at the roof edge, though if that high the water weight on the overhang gets to be a problem unless in a quite high snow load design area. On a 12:12 (45 degree) slope, would take a 3 foot high ice dam - which would almost certainly cause a roof collapse at that point, but with that steep a roof ice damming is rare because the waer starts melting through the dam at spots and eroding a channel out - one reason for steep roofs in high snow areas (along with self-shedding to reduce the load). However - on a 2:12 or 3:12 low-slope roof those numbers drop down to 6 to 9 inches or less to get the backup to a poinit above the ice and water shield protection - not an usual thickness of ice to build up in freeze-thaw or rain-on-snow conditions even if the roof is not overly warm.

Of course, any water backup under the shingles or tiles or such is not good - weathers them faster, causes ice jacking deformation, long-term soaking is bad for the roofing materials, and of course the chance of leakage though seams or nail holes in the waster barrrier and ice and water shield (though the latter is fairly self-healing).

Another common cause is localized icing due to warm vents or exhaust ducts penetrating the roof - either melting around a furnace exhaust pipe due to heat radiation say, or from warm air blowing out from kitchen or bathroom exhaust fans, causing locaiized melting of the snowpack. Generally, that does not get serious until you either get repetitive snows that mostly melt around the warm spots, or have a solid 1-2 feet of snowpack on the roof that effectively blocks the warm air flow and causes localized melting and subsequent refreeze icing buildup. This type of icing usually occurs from the warm source (or within a foot or so) all the way down to the edge of the roof, forming an ice bulge or "glacier" directly downhill of the heat source, and can build up many inches to feet thick in a few short weeks commonly if there is enough snow to melt and the outside air temp is well into the freezing range. Is commonly only a few feet to maybe 6 feet wide below a vent or cluster of vents.

Whether your problem is due to too warm an attic due to household air getting up into it, inadequate insulation in the attic floor to keep the house heat from radiating into the attic, inadequate ventilation in the attic (normally best done with eave and ridge vents on peaked or gabled roofs), rain on snowpack freezing on a truly cold roof (my problem on the shady side of the house), or solar heating or alternating thawing days and freezing nights over many days (commonly in the spring - occurs for several months on my west-facing side) causing ice buildup under the snow is something that you or a roofer would have to investigate.

Or occasionally, with a warm roof house (rarely found in normal heavy snow areas), the problem can be not ENOUGH heat on the underside of the roof - so the snowpack is melting aggressively on the bottom due to the warm roof but not totally melting soon after falling, then freezing downslope where exposed to very cold air - common when the snowpack has partly melted back from the eave area but still covers the main field or upper part of the roof, or sometimes in drifted or sheltered roof valleys or along gables or stepped roof sections, causing glaciering downhill from the lower limit of the snow to the edge of the roof.

Also happens commonly in spring as the snowpack is melting back in the daytime but still getting cold enough at night or cololing enough from clear-sky radiation for the runoff to refreeze at night - commonly causing edgeof roof icicling and edge ice damming.

Quite a few prior questions about the ice damming and eave roof leakage issues can be found in the Home > Roofing link in Browse Projects, at lower left. Also a lot of discussions about roof/attic insulation and ventilation in the Home > Insulation category same place.

Solutions - depending on cause, can include:

1) vapor barrier and penetration sealing in the attic to prevent warm air from infiltrating into the attic

2) increased attic ventilation - eave to vent much preferred though with some designs powered ventilation fans or eave-to gable or gable-to-gable venting is necessary.

3) attic floor insulation, or (rarely desireable except in "warm roof" houses) underside of roof insulation

4) removing the snow load when it reaches a certain critical thickness and starts causing problems - or sometimes at a certain time of the year when melting starts causing glaciering or ice damming (generally in the spring, or sometimes in mind-winter thaws)

5) changing type or raising vent hoods so they do not blow into/down onto the snow pack and melt it

6) heat tapes or heated strips on the roof overhang (though energy-inefficient, sometimes necessary - particularly at times over entry doors or large windows that leak heat and cause overhead roof icing

7) upgraded water barrier on the lower portion of the roof - though this along without solving the ice damming results in much shorter shingle life from the repeated exposure to water and freezing and thawing

8) localized melting of the ice dam with hot air or water (not good to hammer on it - damages roofing easily) to relieve the load and drain out the water. I do this 1-2 times a year on my west-facing roof where it melts in the daytime because of a 50 foot wide wall directly exposed to the sun causing eave area heating which will start snowpack melting even at 20-some degree air temps= in the spring after the sun starts having some strengh again.

Note one thing people commonly do that is NOT good - raking/shoveling the snow off the overhang or eave area only. They think removing this snow will eliminate the problem because most or all the water that is freezing up and causing the problem is coming from the snow melting, especially near the edge of the roof (due to solar exposure at the front of the snowpack, due to more attic heating near the exterior walls where attic insulation is thinner, or due to exterior wall heat (through the wall or solar heating) rising up under the overhang. Simple huh - but wrong. Remove that snow, which normally insulates the water running off from cold night air and promotes drainage at the roof surface, all of a sudden creates a cold face at the limit of snow removal - meaning you form an ice dam right at the edge of the snowpack, which would normally be ABOVE the limit of the ice and water shield (which is a pretty good water barrier) near the edge of the roof. Hence, any roofing damage from the freezing water woudl now be in the main field of the roof rather than on the overhang where a leak is normally not so important. Also, the water backing up behind the ice dam is now in the field area also - uphill of the ice and water shield, so as the water backs up under the shingles it now has nothing but the water barrier (which is water resistant, not waterproof) to keep it from leaking down into the attic. Under these conditions, leakage intop the attic is almost guaranteed - sometimes at a pretty substantial rate - I have seen flows of probably 10-30 gallons per hour in this sort of case - that is a LOT of water inside the house, especially if it is not noticed for 8-12 hours overnight or while at work - or several days when away from the house on a trip. Bottom line on clearing snow - if you need to do it (or want to do it preventatively) avoid scraping down to the roofing material to minimize damge to it, and remove it all the way to the peak of the roof. Notnecessarily full width - you may only need to do it around vents and such for instance - but do it peak to eave to eliminate the water source entirely. And if the snow load is more than about a foot in low-snow areas or about 1.5-2 feet deep in high snow design load areas, remove evenly on both sides of the ridge - not just one side of the roof, so you don't but excessive uneven load on the roofing framing. Some free-spanning designs do not like tolerate unbalanced loads well.

Generally, a roofer or insulation contractor will push his favorite product or solution, which might not be the best for your case. For an independent opinion and remedy design an Energy Auditor with attic insulation and ventilation experience, or an Architect or Civil Engineer with thermodynamics and energy efficiency/insulation training is your best bet - typically $200-500 for a visit and recommendations. In many cases, a progressive approach is best unless your ice damming/glaciering is real bad - commonly starting with sealing off penetrations into the attic and any venting that is discharging in the attic, and maybe changing types and elevations of outside vents if causing localized glaciering. Ridge venting connecting with eave venting to move out attic heat is usually tops or at leat high on the economic and effectvie remedy list if you don't already have that - easy in most peaked/gabled roofs, tougher with flat or zero-overhang roofs. Ditto with eave baffles if your eave vents are blocked with insulation, or if you have an attic roof and need to establish rafter-bay ventilation over the top of a "room-within-the-attic" room with "cold" galleries or crawlspaces around the outside on the low sides of the roof. Then if that does not totally solve it, or cut it down to acceptable levels, maybe adding insulation in the attic, thaw cables, or other more aggressive remedies - though sometimes for localized problem areas like over heated entries or bay windows, if weatherstripping or insulated curtains or such will not solve it, localized heat cables to maintain melted paths to the edge of the roof move up in the solutions list.

Answered 3 years ago by LCD



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