Polyseal/MCGA Attic Spray Foam Insulation
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Spray Foam Insulation in the Attic
Alaska is an extreme environment compared to the rest of the US, and as such a lot of the conventional building science rules for insulation in attics do not apply to our area, and can even harm your roof if it's insulated to a warm environment standard (Florida, Arizona) instead of a proper cold environment standard.
Standard Attics
An attic that has a standard attic truss is one that you see all the time. The roof pitch typically ranges from 4/12 on the low end, and 8/12 to sometimes even 12/12 on the high end. The general recommended approach for our area is to install baffles at the eaves for ventilation, and install loose blown fiberglass or cellulose to the attic deck since cost can be so much cheaper than spray foam.
For a typical attic, we at Polyseal strongly recommend this approach. It is cost effective, and the system works well with this truss design. It is imperative to have a class="fontcolorlight">poly vapor barrier installed as well if you go with loose blown insulation, but overall, the system works great through-out a typical Alaskan year.
Cathedral/Vaulted Attics
This is a roof design where the interior ceiling is vaulted - this is common in great room/living room areas of new construction, and typically involves a scissor chord or parallel chord truss design. With this design, space is much more limited to properly insulate so spray foam can have much better value than fiberglass alternatives. If there is enough space, it's still generally best practice in Alaska to install baffles with a 1.5-2" minimum airspace to the underside of the roof deck, and then install spray foam from there for the overall best design.
Vented Vs. Non-Vented Roofs
This is where things get tricky with our climate. Generally speaking, in a typical Alaskan winter a ventilated attic assembly is the best route to go, barring extreme conditions like ice fog that Fairbanks area can be prone to. (in this case, the fog can accumulate in the ventilation space, melt, freeze again at the eaves, and cause ice-damming issues). Most of the rest of Alaska wont typically experience this so it is still best to ventilate when there is the space and design for it.
The reason its best to ventilate is non-ventilated "hot roof" assemblies can be much more prone to ice damming if high snow loads occur, since the snow itself acts as an insulator, and creates a melting/freezing zone at the exterior side of the roof deck. In some cases, vapor drive will cause moisture to get trapped in the assembly and cause roof rot - two insulation types where this can happen are fiberglass applications, and high density closed cell applications - more on that later.
What if you can't ventilate?
In the case of steel buildings where the roof is a steel purlin "ladder truss" design, ventilation isn't possible since airflow is blocked off. Other structures can have this same design or very limited space - cabins, hand framed 2x6 rafters, etc. In these cases, if one uses fiberglass, the necessity of installing a poly vapor barrier can easily allow for water to get trapped in the insulation assembly through weak spots in the barrier; mostly around light fixtures or plugs. This can easily lead to roof rot and serious sweating/condensation issues. TRY TO AVOID A HOT ROOF WITH FIBERGLASS!
If one decides to use closed cell foam instead, some of the same issues can occur. Closed cell foam is amazingly impermeable to water, but vapor drive can still cause moisture to drive up along the rafter studs and accumulate between the foam and roof deck - because closed cell is so good at stopping water, this essentially traps the water and can lead to roof rotting in some cases as well. Because of this, we also don't recommend a closed cell hot roof - it's better than fiberglass, but can still cause major headaches and expensive tear-outs.
So what is the best option for a non-vented hot roof?
The best option is a very expensive ventilated roof overbuild, which is in few people's budgets. The next best option is an open cell foam. More specifically, a 3/4lb open cell hybrid foam; its higher R-value/overall better than the standard 1/2lb, but still retains all of the beneficial properties. If 3/4lb foam is applied directly to the roof deck, the vapor drive that happens with closed cell CAN and will still happen. But this foam has the unique properties of not only being a really good insulator, but also breathable, like almost akin to a gor-tex membrane.
Because of this, the moisture won't stay trapped in the foam and the assembly will dry out during the freeze/thaw cycle and be good as new. Of course, if you don't have enough insulation and a heavy snow load, ice damming can still very much occur. With the sheer amount of extra snow in the past few winters we have experienced, we have been increasing the amount of spray foam in steel building roofs to combat that melting point. In general, if one does have ice damming its still always a good idea to shovel the snow and expose the ice – it will evaporate on its own without any need to chip it and potentially damage your roof.
Energy Savings
A vented attic situation will become approximately 130-140 degrees in the summer. There is no reason for your air-conditioning and vent-ductwork to have to work in that type of severe conditions. There is also an opportunity for moisture to form due to condensation on these appliances.
By applying spray foam directly to the underside of the roof deck, it now insulates the attic space from the extreme heat that once radiated through the hot shingles sheathing and roof. The severe temperatures no longer exist in the attic. In short, the attic now becomes a passively "conditioned" space of the house that is just as comfortable as any other room in the home.
A roof system insulated with Polyseal/MCGA spray foam reduces energy several ways. Energy loss from ducts located in the attic is essentially eliminated. The top of the building is much tighter resulting in less infiltration and exfiltration, so excess moisture isn't pulled into the attic. Infiltration through the ceiling is also reduced. In addition, the attic temperature is remarkably lower, which further reduces energy loads.
Conduction, Convection, and Radiant Heat Transfer
In a standard insulation system, ceiling insulation reduces the transfer of heat from the attic to the living space (in the summer). Attic temperatures can often approach 140 degrees or more during the day. Most of this heat enters the attic space through a multi-step process.
First, solar energy warms the shingles and sheathing. The hot sheathing then transfers heat to the rest of the attic through conduction, convection, and radiant heat transfer. The 140-degree temperature of the underside roof surface drives the heat transfer process that pulls air in from all the openings in a house, costing you a lot of money.
By having Polyseal/MCGA insulate the roof surface with spray foam, the surface temperature exposed to the attic (the temperature driving the heat transfer) is reduced by as much as 40 degrees.
Both conduction and convection heat transfer are proportional to a temperature difference, so that heat transfer will be reduced proportional to the drop in surface temperature.
Sealing the attic is the single most cost-effective solution for lower energy bills and a comfortable home.
Standard Home Vented attic graphic showing the natural convection process that allows air to be 'pulled' up through the house and escaping outside.
Unvented Attic System Unvented attic graphic showing an insulation roof deck and airtight attic that does not allow air to escape the building.
The benefits of including the attic in the insulated space are:
Building Science Experts
Nobody understands the science of building better than Polyseal/MCGA.
If you want to build a thermally efficient and airtight home, it's not enough just to insulate. You must choose the right insulation and air sealing system for the application, and be sure it's properly installed.
Did you know.. An improperly insulated and sealed home delivers 4500 lbs. of excess greenhouse gases into the air each year? It can also waste 20 percent or more of the energy used to heat and cool the home.
Our Building Science experts understand the principles of heat, air and moisture flow, and how the building envelope interacts with a building's mechanical systems as well as its occupants.
N.A.S.A. applied one-inch of spray-on foam insulation on the Space Shuttle's External Tank that serves to insulate the tank. The closed-cell foam used on the tank was developed to keep the Shuttle's liquid hydrogen fuel at minus 423 degrees Fahrenheit and the liquid oxygen tank at near minus 297 degrees Fahrenheit even as the tank sits under the hot Florida sun while preventing a buildup of ice on the outside of the tank.
Properly insulating and sealing your homes with Spray Foam is a greener alternative.
We can help you in specifying the right insulation system for our local climate. Call or Fill Out Our Contact Form to Get in Touch with our Building Science experts.
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