Roof/Attic Insulation Comments and Thoughts...

Started by MountainDon, September 21, 2009, 07:37:25 PM

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Those who have followed my 15.75 x 30 Jemez Cabin topic may recall that we added some blown in cellulose to the attic this summer. We bult the ceiling with R13 batts last fall as an expedient way to get something up there along with a vapor barrier. We now have about R30 near the walls and that increases to R45 over most of the ceiling area.

We immediately noticed the cabin stayed cooler on the hot summer days, 80-88 F.

We also noted some other things, like this fall we have built fewer wood stove fires than last fall and this past spring. 

I'm going to compare two cabins, Cabin A, our cabin and cabin B, another in the same mountain locale.

Cabin A and B both have fiberglass batt insulated walls and floors to basically the same specs. Cabin A has an 8 foot ceiling with the above mentioned R30 - 45 attic insulation. Cabin B is a little larger, being a 14 x 24 with an addition on one long wall. The addition is approx 12 x 24. Cabin B has a 12:12 roof with loft over the main section. 2x6 rafters were used just like our cabin. The shed roof addition on Cabin B also uses 2x6 rafters. Both the lofted roof and the shed roof have the insulation between the rafters for a value of approx R19.

We've been in cabin B several times over the summer. We both noted that it was almost always warm to very warm (too warm for us) inside. Our cabin used to be warm to too warm in the afternoons until we increased the attic insulation.

This past Friday evening we arrived at our cabin about 7 PM. Inside the temperature was 54 F. I built a fire in the VC Aspen. Within an hour or two the interior temperature was up to 72 F. A side note: the RCH 24 VDC ceiling fan proved it's worth in assisting to circulate the heat.  We allowed the fire to die out overnight. The outside temperature fell to 42 degrees F. Inside in the morning the temperature was 69 F.

Saturday the maximum outdoors temperature was 66 F. The cabin interior temperature rose to 72 degrees through the day. We added heat to the interior with cooking. As well there is a small amount opf heat from the propane refrigerator. Saturday night the temperature ell to 41 F. Inside it only dropped to 67 degrees.

The weather was more or less ditto through Sunday night to Monday morning. I never had to build a fire in the wood stove after the Friday night burn. Cabin B is close by. I noticed that they had a wood fire going every morning. It must have cooled off more than us. Both cabins have comparable propane refrigerators.

A couple other differences in the cabins. Cabin A is situated on a ridge while Cabin B is in a small valley. We get more wind. We also placed our cabin with the long walls across the prevailing wind pattern. In summer we have nice breezes moving through the large windows. Down in the valley they do not. The windows in Cabin A are positioned under 25" deep eves. The summer sun does not touch any of the glass. Cabin B has more or less standard eves and some windows are struck fully by the summer sun; the winter sun too, to be fair.

In winter the windows exposed to the winds could result in greater heat loss I suppose. In winter the sun's lower angle peeks under the eve overhang and hits the glass. The sun now strikes the lower half of the south windows. Where the sun hits the window sill and a table top we can feel the warmth.
Cabin A's windows are low-e types with a pretty good u-factor of .30.

So what's it all mean? Well, we are pleased, that our cabin stays cooler in summer and warmer in winter in this simple non scientific comparison. Part of our success came from a long exploration phase as we developed our plan. The cabin changed in orientation as well as in location on our property. The eve overhang grew as we watched the sun the summer before we built. And high R values in the roof or ceiling make a huge difference.

Perhaps some of this information may help someone else in the planning stages of their cabin.

Just because something has been done and has not failed, doesn't mean it is good design.


Cabin A has around 3780 (15.75x30x8) cu ft of area to heat and keep cool.  Cabin B has 2688 (14x24x8) + 1176 ((14x24x7)/2) + 2304 (12x24x8) or 6178 cu ft of area to heat and cool.
Lesson, build smaller and use less resources.

Cold air falls into valleys at night.  If Cabin B had a lot of thermal mass and could cool itself overnight, it could probably stay cooler than it normally does in the summer.

The other side of that is heat rises.  R-19 is a horrible insulation value for ceilings. For Code Require ment R-38 is the minimum for fossil fuel in my area and R-49 the minimum for electric. And we don't even have snow yet. (BTW, nice pictures)

Why would they build R-30 walls and R-19 ceilings?


I witnessed a dramatic change in a building first-hand. Our old house here in FL was small block structure with no insulation. The roof pitch was only about 2/12. When I re-did the roof, the pitch was too flat for anything but a built-up roof (tar and gravel) The quoted price was so high, I decided to do a roof-over, and built a 6/12 roof right over the old one. The new roof was vented to let the heat out. As an experiment, I measure the temperature in the original attic both before and after the roof-over. Before, it would peak at 135 degrees. After, it peaked at 92 degrees, a 43 degree drop! It sure made a huge difference in the summer time comfort of that house. The house we have now has no wall insulation, but has R-19 fiberglass, plus a few inches of cellulose in the attic, and it's pretty comfortable. I can't wait to see what a tight, properly insulated house will be like when I build my new place.

I'm not poor- I'm financially underpowered.



 :) I now have a full year's heating experience for my 24x24 1 1/2 story cabin in Alaska. I have 11" of closed cell foam in the cathedral ceiling, R-21 FG in the walls and floors. Lots of window area. I burned a total of 300 gallons of fuel in a TOYO 44,000 btu heater. Heat was set at 54 when gone and 70 on almost every weekend and several weekday stays. It took about 20 minutes from 54 to 70 on -40 f days to warm it up. Temperatures in area commonly hit -35 to -45 in winter. At current fuel oil prices it will cost me an average of $50 a month for heat.  [cool]
Rwanders lived in Southcentral Alaska since 1967
Now lives in St Augustine, Florida


Just because something has been done and has not failed, doesn't mean it is good design.


I encourage folks to consider incorporating insulation with a reflextive foil face into their heating and cooling plans.  The foil will reflect considerable long-wave heat radiation, as long as there is at least 3/4" of dead air space directly over (or under) the foil.  Since I will be using my cabin much more in the hot summers than cold winters, I put the foil-faced sheets above additional fiberglass bats in the vaulted ceiling.  If I were building in Alaska for year-round use, It would go below to help keep heat from a furnace, woodstove, etc. from escaping.  I recall specs for the insulation indicating a gain in R-value of a couple points, just from the foil.  In my cabin, with day-long sun, no shade, and outside temps approaching 100 F, the interior only gets to about 82 F.


There may be some merit to the use of reflective foils in some instances of insulating. I had read controversial information on the subject. We here on the Country Plans Forum have had a number of discussions. A search on the word reflective brought up the following


jraabe:  Reflective insulations have a checked past and a suspect future. They have seldom lived up to the hype and the calculated R-values they are supposed to impart.

To be effective, radiant (reflective) barriers must work across a true dead air space where there are no convection currents. Then the reflective surface must stay shiny and clean for the life of the house or the dust covered mirror does nothing. Both of those conditions are very hard to meet in the real world.

I would not suggest using such insulations in floors or walls. Radiant barrier insulations are used with some frequency for reducing attic temps in high heat areas such a FL and the SE.


jraabe: Every few years there seems to be a new reflective barrier product that touts high energy savings.

As far as I know the only place that reflective barriers have actually turned out to be cost effective in real world testing is when used in vented attics in Florida (and probably other hot summer locations). These were films placed under the rafters to reflect the radiant heat back out. They worked as long as the reflective surface could be kept clean. Dust and grime quickly degrades performance.

In other locations (such as walls and floors) and other products, the savings, if any, did not justify the cost.

The one place I have used reflective barriers is in beam and plank roofs where the insulation is above the decking. If you can find it, make the upper layer Thermax foil faced isocyanurate. If you strap the roof for a vent cavity it gives you an air space for the foil to work in. The foil face comes free with the expensive insulation so you might as well have it work for you. After many years of being up there it probably has little reflective value left.


Just because something has been done and has not failed, doesn't mean it is good design.