TRULY insulate your loft (or attic!) - and save the planet

Step 30: Footnote on R values and radiant barriers

These pics are from the two data sheets on SF19 and SF40 quilt. The crucial point is to distinguish between Conventional R Value  and Comparative Energy Saving Equivalent R Value. As you can see, Conventional R Values do not make allowance for the radiant barrier.

So any question of doubling depth and doubling R value is not relevant. The question - if there is one - is whether increasing the depth of the quilt increases the Comparative Energy Saving Equivalent R Value, and the two data sheets explain the difference between SF19 (19 layers) and SF40 (36 layers).

Essentially, the latter is 50% more effective than the former. Hope this helps.

Note that as explained in earlier steps, I have added quilt to an existing 120mm layer of fibreglass. So I now have 120mm fibreglass (R= about 6), a layer of chipboard (R= about 1), and the SF40 (R= about 2).

But taking into account its radiant properties, the SF40 has an equivalent R value of 9.6.

For more explanation see http://en.wikipedia.org/wiki/R-value_%28insulation%29 (especially 4.3: The limitations of R-values in evaluating radiant barriers).

Also http://www.theyellowhouse.org.uk/themes/insula.html for a chart of R values of various materials.

Comments

[rattle09]

I do not recommend anyone do this to a roof covered with asphalt shingles. There must be an airspace between the underside of the roof sheathing that is clear to the ridge of the roof. At which, there must be some type of ridge vent. Without these the summer sun and heat will literally bake your roof and with in a short couple years the shingles will curl up and be ruined. Your shingle warranty will also be void.

[tcup]

Ah, good tip. How then, does one safely insulate their roof? I have a ridge vent at the peak of my roof. My roof developes huge icecicles during the winter.

[Mike44]

Yeah, as rattle09 said, you probably don't have continuous airflow from your soffit up to your ridge vent. Look up pictures for attic venting - there's some on the Pink Panther insulation site: http://insulation.owenscorning.com/homeowners/

You may have a situation something like this with no air flow: http://www.nylumber.com/images/ice2.JPG

What you want to do is add those styrofoam spacers (raft-r-mate polystyrene attic rafter vents): http://roofing.owenscorning.com/homeowner/accessories/ventilation/raftrmate.aspx

[tcup]

Thanks guys for your advice. The only insulation in my attic is on the floor. I never heard of rafter vents. I'll have to look into them. I'll be doing that job myself so I will have to buy everything peacemeal as I can afford it. Meanwhile I'll be adding calcium chloride to the areas of my roof, that I can reach. I saw something that says to fill knee high panty hose with calcium chloride then lay them on the edge of your roof, that will prevent ice from forming. This will be interesting, I hate hights.

[michaelcozens]

You can also get wiring kits that have you tack a warming wire in an up-and-down curving pattern on the overhanging portion of your roof. You can then plug this wire when you notice ice starting to form, and melt tracks through the ice dam to allow the accumulating water that can back-up under your shingles and into your attic to flow off the roof. It's a longer-term fix, because you can just leave it up year-round and use it as necessary.

[michaelcozens]

One thing you can do with the panty-hose is to tie a long rope to one end, and just throw them up onto your roof while letting the rope dangle to the ground. This way, you can retrieve and re-throw the package until you get the position right, and also pull it down to refill it or put away in spring. Voila! No roof-climbing required.

[tcup]

Last winter, I tried the calcium chloride in the knee high panty hose thing. It worked. Unfortunately, my house had the look of a bordello with all those panty hose all along the roof edge.lol. Then after winter was over, there were black stockings all over the yard, somewhat embarrassing. But the main thing is that the method worked, I had some icicles but not as much as previous.

[rattle09]

There is a styrofoam product made that is designed to fit between the rafters. Its concave shape allows an area of ventilation from the lower roof edge to the ridge vent. You would install these prior to the process that is described here in this instructable. This will allow the heat to escape from below the roofing therefore helping keep it cooler.

[LDW]

Well I don't have rattle09's knowledge of asphalt shingles, but AFAIK you should be safe if you fix the quilt to the underside of the rafters - in which case there will be a gap the same depth as the rafters between the roofing material and the quilt.

Best check with the manufacturers. Someone here has recommended:

www.eshield.net

and they look OK to me.

[LDW]

There must ALWAYS be an airspace between the quilt and the roofing material. That's why it's fixed to the underside of the rafters - although it can be used in certain new builds immediately below the roofing material - see www.ecofoil.com/Applications/Roof-Insulation.

[Mike44]

This is a great idea, especially with our energy conscious society now. Just to clarify though, this shouldn't be done on the roof of ATTIC though with the blown (or bat) insulation on the floor, right? Only for a LOFT?

[LDW]

Erm... sorry Mike444 - I don't think I know the difference between an attic and a loft. AFAIK they're the same thing. What did you mean?

[mephit]

I think we've run into a bit of a language issue. I'm guessing that Mike44 is a fellow American and therefore confused by your use of the word loft. In the US, loft can often mean just the upper habitable storey of a building or even just large, open living spaces usually converted from commercial property and often having very high ceilings (i.e., they're "lofty"), whereas the attic is the area just under the roof often used for storage. This means you can actually have an attic over a loft. Confusing, I know. In answer to Mike44's question, though, this is being done to what we here in the States would call an attic. In this case, a floor has been planked in on top of the insulation covering the joists of the attic making it appear to be a more finished space than many Americans are used to seeing in their attics.

[LDW]

Ah - thank you, mephit. Tricky animal that English language...

You are in the right of it: my attic (we use that word interchangeably) has floorboards in it - as explained in step 5.

The title of my 'ible has been amended accordingly.

[mephit]

As that wit Shaw said, we are "two people separated by a common language." I have found it handy on more than one occasion to be an American with a large number of UK relatives.

[Mike44]

Ha true true, well, not quite American....I'm Canadian, but us North American's probably use a little bit different lingo than what you use 'across the pond' :)

Attic: mainly an UNLIVABLE space at the top of your house; has roof trusses throughout the support the roof, so you can't really put a 'livable' space up there; insulation along FLOOR of attic keeps building heat inside (http://www.arrow-insulation.com/img/photos/beforeandafter/large/attic_insulation_after_3.jpg)

Loft: a LIVABLE space where the roof trusses are placed more off to the side in such a way to provide a livable space in between; the insulation is placed along the ROOF/WALLS of the loft; thus there may be no true 'attic' space above (http://i37.tinypic.com/ax1da1.jpg)

I guess my question was just getting to the idea that by installing this on the roof of an ATTIC which already has insulation on the floor would kind of be defeating the blown in floor insulation. Then the heat would be trapped within the metal blanket and blown in insulation, leading to problems.

Also, yes, it's absolutely critical to maintain an airspace between the roof proper (like the plywood) and insulation from the soffit at the bottom of the roof up to the space where the ridge vent is:

http://www.inspectapedia.com/BestPractices/Figure2-57s.jpg
http://www.nachi.org/images10/ridge-vent-baffles.jpg
http://education.nachi.org/images/upload/soffit-ridge_vent.jpg

[mephit]

Ah! excuse me for the incorrect assumption, then. I imagine ya'll get tired of being mistaken for us. :D

I'm not a contractor or roofer or insulation specialist or anything, so please excuse me if I say something stupid, but aren't you assuming there's a ridge and soffit vent system? Not every building has them. I know my house doesn't have either. It's got an old-style rotary attic vent, but no soffit vents. My house was built between 1946 and 1949. There are plenty of other buildings around of a similar age that might not have a modern vent setup. Would they still need the airgap between the roof decking and the insulation?

[Mike44]

Oops, yes, sorry, I should have clarified! :) I'm assuming a more 'recent' house. With an older house like yours though, you probably don't have to be as concerned to get this airflow going because they were built "less airtight" back then when compared to today's houses (vapour barrior, polyseal, acoutiseal, etc. the works nowadays!). There will probably be some natural airflow somewhere there! Oh, and I do believe that those revolving roof vents are (*I think*) about equivalent to the ridge vent. At least that's what I've been told!! Don't quote me though :) Seems logical though. With that being said though, they are much less airtight, so you may experience heat loss through the attic. Do you get any icicles forming on your roof at all?

[mephit]

Don't know yet. I only bought the house in May of this year and I live in the Southeastern US. We're currently having a cold-snap, so I'll keep an eye on it, but I seriously doubt they'll form. My house has nothing but the original inch or so of insulation in the attic floor and no roof insulation of any kind. The heating system seems to mostly be heating the attic and not much the rest of the house. The roommate currently living in the attic room says it gets surprisingly hot up there while our downstairs area is still pretty chilly. I'm wasting dollars and energy I'd rather not waste, so that's one of the reasons I'm interested in this 'ible. Unfortunately, the entire attic space has been thoroughly and very cleanly floored with the inadequate insulation between the ceiling below and the floor above. Factor in the additional fact that more than half of the attic space has been converted into a room (just under the ridgeline, of course!) and it basically means I'd have to gut my attic to re-insulate it. Whee......