Rigid Foam Insulation

[wd4lc]

Am I over-analyzing this?

I need to build dog house walls. Thanks to help in another thread I'm going to use BC plywood (11/32) for the interior walls and T1-11 (either 11/32 or 15/32) for the exterior walls (which will be painted).

According to the plans, the frame of the walls are made up of 2x2x8. You place 1-1/2" styrofoam insulation within the frame pieces between the walls (the frame is basically sectioned off into 3 spaces for the insulation).

We don't have that size of insulation sheathing down here, so my thought was to double stack two pieces of 3/4" of polyisocyanurate or a polystyrene.

Since I've worked with this lumber and insulation in the past I have some spare pieces on hand. I decided to lay them out for a quick test. Laying the 2x2x8 pieces out and placing two pieces of the 3/4" sheathing in between (of which I took the paper off each side of the sheathing), the sheathing is actually about 1/16"-1/8" higher than the post pieces. Therefore the plywood walls would need to give a bit and flex over the insulation in order to be fastened into the posts.

Any thoughts on whether this would pose a problem? I'm trying to figure out if the bow in the walls would be noticeable and/or if the tight fit would at all affect the plywood in different weather conditions.

I had thought that maybe I could use one piece of 3/4" and one piece of 1/2" sheathing. However, this would cause about a 1/4" air pocket within the wall. Any idea if this would affect the insulation properties....would it defeat the purpose and not keep the inside of the shelter as warm?

Maybe I'm overthinking it. If it were for me, I probably wouldn't consider it too much. However since it is for someone else I'm always a bit overly concerned about making sure things are right.

Thanks for any tips.

[nz5d]

R Values

I think you are worrying too much. The R value of polystyyrene is about 4 per inch--polyurethane is about 6 per inch. So the R value of a 3/4 plus 1/2 inch polystyrene would be about 5 vs 6 for two 3/4 pieces. I don't think the dog will see much difference. If you won't be able to sleep at night worrying about the doggie and worrying about the wall bowing and......., why not just cut some strips of 1/8 inch something (masonite, etc. and tack onto the 2x2s and go ahead and use two 3/4 sheets? Good luck, Charlie

[parnelli]

I don't think an air pocket is going to reduce any warmth inside. Air is pretty much the insulator anyhow...that's why you don't cram fiberglass into a space- you lose R value.

If you didn't want to use 3/4 plus 1/2 why not get 1 plus 1/2?

[wd4lc]

Thanks parnelli and nz5d. Since it sounds like the pocket of air with 3/4 + 1/2 won't matter, I'm going to go that route.

Two last questions please. Can I place a polyisocyanunate next to a polystyrene piece?

Also, does it matter which direction the paper is facing (or does that only matter when its exposed to the elements)? The polysicyanurate that I have has silver, reflective paper glued on one side and a white sheet on the other end.

Thanks.

[dbhost]

Sounds like you are on the right track...

[wd4lc]

Oddly enough, I haven't found anything on the net in terms of which direction to place the reflective paper. On one site it did indicate that reflective stand alone material (something you would add to existing insulation) could be used anywhere that you want "to reflect heat back to its source". Sounds like a good idea to face the reflective paper towards the exterior wall....at least during the summer. During the winter I imagine you'd prefer to let the sun heat up the shelter.

I ended up asking a box store associate about double stacking the sheathing and which direction to face the reflective facers. His suggest was to have the reflective portions of both sheathing pieces to face each other within the wall. However, he didn't sound very confident about his answer or really the reason why it should be that way.

I'll continue researching. If I come up with a solid answer, I'll post it here for any others who may need to know the same info in the future (no matter how slim those chances may be ).

Thanks parnelli and nz5d. Since it sounds like the pocket of air with 3/4 + 1/2 won't matter, I'm going to go that route.

Two last questions please. Can I place a polyisocyanunate next to a polystyrene piece?

Also, does it matter which direction the paper is facing (or does that only matter when its exposed to the elements)? The polysicyanurate that I have has silver, reflective paper glued on one side and a white sheet on the other end.

Thanks.

[parnelli]

Two last questions please. Can I place a polyisocyanunate next to a polystyrene piece?

Also, does it matter which direction the paper is facing (or does that only matter when its exposed to the elements)? The polysicyanurate that I have has silver, reflective paper glued on one side and a white sheet on the other end.

Thanks.

Interesting- the only polyiso sheets I've ever seen/used have the foil on both sides.

[wd4lc]

Interesting- the only polyiso sheets I've ever seen/used have the foil on both sides.

The polyiso that we have down here at the box stores look like this (reflective one side and blue/white on the other)...


I looked at the styrene sheathing as well. That had reflective on one side and a clear material on the other.

Maybe our less severe climate in the south doesn't require it on both sides or something. Not too sure.

[nz5d]

Both polystyrene and isocyanurate are essentially inert at the temperatures the house will see so there shouldn't be any problem with mixing and i don't think for what you are doing, it makes any difference which sides face in or out.

[DrChas]

For isocyanurate the silver side goes towards the heat. If you live in the north it goes to the inside, if you live in the south it goes towards the outside. It really doesn't do much unless there is a slight air space in front of it. It reflects back radiant heat and provides a vapor barrier, and apparently the reflecting properties work best if there is an air space in front of it. The reflective surface actually works VERY well (much like a space blanket) for radiating heat sources such as warm bodies, but not so well for keeping in warm air. Thus, you need both the reflective surface and the insulation to capture both.

I insulated an attic with rigid foam, and it worked very will. I packed between the studs with polystyrene cut to fit, and then sealed with foaming goo. After that I put a layer of isocyanurate over the studs, taped them with metal duct tape, and then secured the dry wall with super long screws driven through the insulation into the studs. It seems to have worked very well. The one problem is that stud finders don't work so well through the metalized paper on the insulation. . .

[capncarl]

Do your dogs know to close the door when they go in or out? if not then all the insulation in the world will not keep it warm inside.
capncarl

[LCHIEN]

You don't say how big a span the ply covers... being 2x2x8 I'm sure you cut down the 8' dimension. How big are the panels. If less than 3 feet probably the ply will be able to compress the insulation 1/16th inch. If not then add an internal cross brace to nail the ply to as necessary to close the ply to an acceptably unbowed state.

An air gap is significantly less insulative than most stuff sold as insulation materials... otherwise we would leave our walls empty of insulation and full of air.

Another option is to Rip 2x4x8s or 2x6x8s to slightly larger than 2x2 (call it a 2x2+ which is 1-3/4 x 1-13/16) esp. if you have a thin kerf rip blade.. You can probably get the extra 1/16th to 1/8th you want and still get 3 2x2+ out of a 2x6.
Then your wall sections will be perfectly fitted for the insulation you have.
Who follows directions anyway?

[wd4lc]

For isocyanurate the silver side goes towards the heat. If you live in the north it goes to the inside, if you live in the south it goes towards the outside. It really doesn't do much unless there is a slight air space in front of it. It reflects back radiant heat and provides a vapor barrier, and apparently the reflecting properties work best if there is an air space in front of it. The reflective surface actually works VERY well (much like a space blanket) for radiating heat sources such as warm bodies, but not so well for keeping in warm air. Thus, you need both the reflective surface and the insulation to capture both.

I insulated an attic with rigid foam, and it worked very will. I packed between the studs with polystyrene cut to fit, and then sealed with foaming goo. After that I put a layer of isocyanurate over the studs, taped them with metal duct tape, and then secured the dry wall with super long screws driven through the insulation into the studs. It seems to have worked very well. The one problem is that stud finders don't work so well through the metalized paper on the insulation. . .

This dog house will be for here in the south. Our winters are not brutal but our humid summers can be really bad.

I checked out some videos last night of folks putting up iso. At one point I saw double stacked 1" with the foil facing outwards (although I couldn't see the inside and don't know if the other side of the iso had the same reflective barrier).

I'm going to do as you say for here in the South and face both foil barriers towards the exterior walls. That way in the summer it reflects the summer rays. In the winter, the insulation itself will do the duty of trapping in the body heat of the doggie within its living quarters.

Thanks.

[wd4lc]

You don't say how big a span the ply covers... being 2x2x8 I'm sure you cut down the 8' dimension. How big are the panels. If less than 3 feet probably the ply will be able to compress the insulation 1/16th inch. If not then add an internal cross brace to nail the ply to as necessary to close the ply to an acceptably unbowed state.

An air gap is significantly less insulative than most stuff sold as insulation materials... otherwise we would leave our walls empty of insulation and full of air.

Another option is to Rip 2x4x8s or 2x6x8s to slightly larger than 2x2 (call it a 2x2+ which is 1-3/4 x 1-13/16) esp. if you have a thin kerf rip blade.. You can probably get the extra 1/16th to 1/8th you want and still get 3 2x2+ out of a 2x6.
Then your wall sections will be perfectly fitted for the insulation you have.
Who follows directions anyway?

Thanks, LCHIEN. Yes, I definitely don't mind deviating from instructions so long as I know I'm doing it right.

I had actually thought about doing the rip idea that you mentioned (or adding thin strips as was mentioned previously) but this really adds on extra time to the project. More importantly, the less number of steps, the less probability of me messing something up!

As for the dimensions/panels, the base is about 40"x40" with the height about 40". (I've got 20, 2x2x8 for the project).

[pelligrini]

I don't think there would be much bow to the plywood. I probably wouldn't bother shimming it either. It's pretty rigid.

I used white bathroom wallboard on the interior of my doghouse rather than plywood. It was cheap, and has a cleanable waterproof surface.