Insulating for cold weather... Skip the Reflectix?

[One Awesome Inch]

I want to insulate according to the climate that I live in... which is the PNW. It might get hot here maybe 1 month out of the year. I am much more concerned about retaining heat than blocking out the heat of the sun.

That said, perhaps it would best to skip the reflectix altogether? If its mainly going to be used as a vapor barrier in my case, why not just use some heavy plastic which is much cheaper?

Basically I am trying to figure out which method is best.

Method A:

Bare metal wall > reflectix > 2.5 inches of foamboard > heavy plastic sheet > wood panelling

Method B:

Bare metal wall > heavy plastic sheet > 2.5 inches of foamboard > heavy plastic sheet > wood panelling.

Method C (something different altogether?)

Bare Metal wall > ?

 

[Bdog1]

I believe the moisture barrier should be on the "living" side of the insulation. So A with or w/o the Refletix.

 

[akrvbob]

You want to go with B, that's a very good plan. Reflectix is great in windows but it's very poor insulation, it's R-1 which is nothing. Polyiso is R-6 so that's what you want.

Don't skip on the roof, it's more important than the walls. On the roof 2 1/2 inches is a very good plan but you could probably use less on the walls. I just put 1 inch on my trailer and 2 inches in Alaska. The more the better though! That damp cold is the worst!
Bob

 

[Willy]

Depending on whether they stock it, there's white styrofoam (polystyrene) 4'x8' sheets available that have a plastic film on one side and a reflective (aluminium/plastic) film on the other. A LOT cheaper than polyiso (1/2 the price or less). They have an r-value of abt 3.8, plus whatever the reflective layer confers. ..Willy.

 

[akrvbob]

The Polyiso has almost double the R-Value R-6 and also has very heavy reflective layer on one side. It is also much tougher. The white stuff is a mess to work with and the edges fray easily.

Polyiso is one of the few times that paying more really does pay off. If money is tight, get Plyiso half the thickness, it's R value will be nearly as good and you'll lose less room. It'll also hold up much better.
Bob

 

[One Awesome Inch]

Thanks guys. I'm strongly leaning towards Method B. In my climate I just dont need the heat protection of reflectix. Plus I'd rather put another layer of foamboard in than build in the 1/2 inch air gap the reflectix requires. Now that I have the van I can see that there really is limited headroom and I want to use the insulation as effectively as possible.

How should I fasten the heavy plastic sheet? I really would rather avoid spray on adhesive as that stuff really smells strong and up here in Home Depot its $17cdn a can... which doesnt go far.

When the seller sold me the van he told me I can keep whatever was in it. There is about 8 tubes of silicone which I will use to seal all holes in the van I might. I'll also use it to connect all the seams of the walls making it quite air tight hopefully.

What about just using the silicone to adhere the plastic sheeting?

 

[Willy]

Huh.. you guys are lucky insofar as the polyiso I've seen up here doesn't have the reflective layer. ..Willy.

 

[GotSmart]

I just bought a roll of bubble reflextix. I cut a pair of covers to go inside the windshield (one from each side around the mirror) to replace the one piece I had been using. With a little extending around the door side, it kept the light out, and made the van much warmer.

I have about 18 ft left, so if anyone needs it at RTR>>>

 

[cdiggy]

Huh.. you guys are lucky insofar as the polyiso I've seen up here doesn't have the reflective layer. ..Willy.

dont know where you are at Willy but the poly down here (NJ) comes as just poly (pink and blue color) and RMaxx thermasheath which has aluminum on both sides (one shiny reflective and one side dull) just like the two sides of aluminum foil wrap for the kitchen.

 

[StarEcho]

Please forgive this probably stupid question...

I thought Reflectix bubble wrap roll was insulation as well as radiant barrier? I had read that it had an R of 8 if used with air space.

This made sense to me because if it keeps the heat out then wouldn't it also help to keep the heat in? So I was thinking one could put reflectix down, then a 1/2" strip of furring around the edges then lay the reflectix over that and then cover the whole thing with whatever... That would give an air space both ways and keep heat in in the winter and out in the summer.

I saw one product that was selling for my jeep windshield that had mylar on one side and black something or other on the other side and the selling point was that in the summer you would put the silver side out to reflect the sun and keep it cooler inside and then in the winter you would put the black side out to absorb all the sun and keep your windshield from icing over.

Anyway, I guess my question is why did I read on the reflectix website that if used correctly it can have an r value of up to 8 but everyone here is saying that it does not insulate?

 

[akrvbob]

Heat and insulation is a much more complicated subject than it first appears and I'm going to over-simplify it here. There are different kinds of heat and one of them is Radiant heat. The suns heat is radiant because heat radiates out from it and when it strikes something it warms it up. For example, if you stand in the sun on a summer day your skin will get hot, but if you move over behind a tree your skin won't be as hot. Reflectix works extremely well against radiant heat because it not only shades, it reflects the heat away. So if you want to keep the suns heat out, it's a good choice. But only if you put in a air gap.

Reflectix has a shiny metal to reflect heat, but metal is an extremely poor insulator, which is why our vans get hot in the summer and cold in the winter. The metal skin on reflectix easily passes heat through it. The tiny amount of bubble wrap has some insulation value, but very little and the metal is sucking heat through really fast. The net affect is that it has almost no insulation R-value.

If you put Reflectix in a window, the radiant heat from the sun easily passes through the glass, hits the shiny metal and is easily passed back outside through the glass. Reflectix is the perfect thing to put in a window!

On the other hand, in the winter, the heat inside your van is not radiant and so Reflectix does almost zero good. That heat loss is through conduction and the way to stop it with high R-values. Compared to a high R-value insulation like Polyiso, it's money thrown away.

Here i a discussion on Wukipedia on the confusion caused by trying to apply the concpet of R-value to a reflective barrier:

The limitations of R-values in evaluating radiant barriers[edit]
See also: Cool roofs
Unlike bulk insulators, radiant barriers resist conducted heat poorly. Materials such as reflective foil have a high thermal conductivity and would function poorly as a conductive insulator. Radiant barriers retard heat transfer by two means - by reflecting radiant energy away from its surface or by reducing the emission of radiation from its opposite side.

The question of how to quantify performance of other systems such as radiant barriers has resulted in controversy and confusion in the building industry with the use of R-values or 'equivalent R-values' for products which have entirely different systems of inhibiting heat transfer. (In the U.S., the federal government's R-Value Rule establishes a legal definition for the R-value of a building material; the term 'equivalent R-value' has no legal definition and is therefore meaningless.) According to current standards, R-values are most reliably stated for bulk insulation materials.
Bob

 

[VanLifeCrisis]

I have the foil backed styrofoam panels. i have reflectix in a few spots too, with panels over both (air space for the reflectix, for keeping heat out). in the summer i can feel the panelling getting hot where i have reflectix, but not where the styrofoam is. good enough for me to use styrofoam exlusively next time. it does fray at the cut ends, and messy to install but once its up behind paneling where it cant be damaged its fine. even on my slide with it exposed the main problem is the film peeling.