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mixing rigid polyurethane board with spray foam SR in existing wall cavity

mixing rigid polyurethane board with spray foam SR in existing wall cavity

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Old 08-31-09, 08:48 AM
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mixing rigid polyurethane board with spray foam SR in existing wall cavity

In the unfinished second floor attic of this 1920s built house, I can see right down the inside wall to the ground: the plaster walls in the first floor have absolutely no insulation. So there's a 3.5 inch cavity 8 feet deep (and 15 inches wide) between the studs.

Since we live in a cold climate (WI), I am planning on filling this space with spray foam SR (slow rise) insulation. Handi-foam is apparently safe for filling existing cavities. (Other brands warn against using SR foam to fill existing wall cavities.)

However:
(1) I would rather not drill holes in the walls. I'm hoping I can pour foam in from the 2nd floor (since the hole is so large), perhaps using nylon tubing to help it down.

(2) (Good = R6-7/inch) Spray foam is expensive (over $1/boardfoot) even in a DIY kit. I was thinking of buying rigid 1" thick PU boards (also R-7/inch), cutting into 12"x8' slices, and putting in one or two boards in the cavity before pouring in the foam. It seems to me that the spray foam would fill in the gaps around the board(s) and the result would be about as good as "solid" spray foam.

Does anyone see problems with these measures?
 
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  #2  
Old 08-31-09, 10:27 AM
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Hi boyland, it sounds like balloon construction and may be interior and exterior walls. The exterior will need the insulation, but the interior will also need to at least be air sealed and fire blocked. I have no real experience with filling these with foam, everyone I have dealt with has chosen cellulose, much less expensive.

As for filling existing cavities, I would be concerned about the foam going places I would not want, outside or inside. There may also be spaces you will not reach from the attic/2nd floor. Cellulose can be a DIY, but get a quote from a contractor just to get some ideas.

Bud
 
  #3  
Old 08-31-09, 01:08 PM
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Blown-in cellulose has a much lower R-value (R-3/inch ?). Anything is better than nothing (air), I suppose. What about blowing in cellulose after putting in rigid foam boards (in the cavity)?
 
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Old 08-31-09, 06:50 PM
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Not knowing where you are, makes it hard to talk about savings etc..
Where I am we have an amazing program that pays some pretty good money towards insulation.

You need an energy audit before and after, then you will get a nice government cheque.

Not sure what is in place in your state / province
Make sure you get the audit done first if you need one...
a) Look seriously at professionally applied foaming. The insulation value is well worth the cost.
b) The amount of energy you lose just thru infiltration is quite amazing actually. The foam will improve this quite a bit.
Caulk the windows and weather strip the doors.

Right know those walls are all chimneys of sort, and all they do is carry all you heating money up to the attic all thru convection. You need to seal these all up some how. Foam will obviously do that, blown in insulation may not as well.
You can plate the top with plywood or solid wood, or even some tyvek / typar in the hard spots.
 
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Old 08-31-09, 07:14 PM
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TO Heating:
Yes, here is Wisconsin, insulation is important. I was wondering if you knew any problem with (1) pouring SR foam from the upper floor and/or (2) using a mix of rigid foam boards + spray foam (SR).
 
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Old 09-01-09, 09:35 AM
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When foam expands and sets, it creates heat.
The manufacturers warn about this issue.
You may find that pouring from the top will work but, falling foam is unpredictable and it may attache to the sides and create holes.
I have used polystyrene balls in the same situation, but a masonry wall, poured from the top, however, they tend to stick together and its hard work to be sure the space is full.
I think the best idea is to strip off the surface and tightly fit polystyrene sheets between the wood, making good any holes with cans of foam.
Note: Humidity from water vapour produced in the home will immediately make a bee line for the wood, which will be the weak link, being colder than the insulation. This will start mould in the wall and wood rot.
You must fix sheets of plastic against the wood, under the drywall, carefully sealed to prevent water vapour getting into the wall.
 
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Old 09-01-09, 01:08 PM
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Perry,

Your reply seems to speak against using foam at all in this situation unless I want to strip out the plaster walls in the entire first floor, which is a much bigger project than I want to do (and may not be worth the insulation improvement).

Is the danger of moisture an argument for leaving the stud cavity empty despite the terrible way it sucks out heat in the Wisconsin winter? Or do I want a hygroscopic (water attracting) insulation such as blown-in cellulose?

If it helps any: our winters tend to be very dry here (due to forced air heat especially). Perhaps I don't need to worry about moisture settling in the studs.
 
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Old 09-01-09, 03:17 PM
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The concerns about mixing rigid and spray and the problems with getting a proper fill are valid. However, moisture and cellulose have not been an issue in the northern cold climates. Dense pack cellulose performs well as an air barrier and thus limits the movement of moisture laden air into the wall cavity. Here is an article on vapor barriers to help.
http://www.energysavers.gov/your_hom.../mytopic=11810

You definately want to insulate and air seal those cavities, along with a list of other standard wx improvements. There is no sense having the heat half of Wisconsin .

Bud
 
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Old 09-01-09, 03:19 PM
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Moisture will be a big problem. It's not the out door conditions that dictate a moisture problem since it will be the indoor air that will contain 40 - 50 % RH condensing with in the layers of fiberous insulation.

There may be spray foam companies that can lower a wand or similar into the cavity to get better coverage.
a) the expanding foam may expand too much and rupture the plaster venier
b) the heat generated by the foam may melt any foam sheet stock placed in the cavity.
c) the heat may also cause combustion with the wall. (I doubt it)

The foam ball idea may work, you could try to use a concrete vibrator to ensure compaction of the balls in to the cavity.

A very good oil based paint CAN act as a vapor barrier, a few coats are required. Caulk the floor / ceiling.

Thats about all I got
 
  #10  
Old 09-02-09, 04:48 AM
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The big problem with insulating and draft proofing is that the water vapour we all create, just by breathing, washing and cooking, has to find somewhere to go.
If it cannot escape to the outside, then it condenses onto and into the nearest cold surface. (normally this will be your window/s, and you will see the condensation. But it will also condense on the walls and work its way towards the cold outside and make the wood rot)

In your situation, the designer and builders understood this very well and built accordingly.
The moisture in the air simply goes through the wall up inside and out into the sky. Perfect!

However, when your system was all the rage, the cost of heating was low and people didn't consider the expense.

Now things are different, heating cost is important and heating (while going through a cheap period at the moment, natural gas prices coming down) they will no doubt go up again.

The solution is to pick the room you use most, strip off the inside of the outer wall/walls and ceiling and and fill the cavities with polystyrene sheet (or foam) and then fix two inch thick polystyrene panels, tightly butted across the walls and ceiling and finish as you like. (do the floor as well to get the best result)
This will pay for itself in two winters and you will be amazed how comfortable the room will be, you will be able to lean against the wall, sit close to the wall and feel comfortable, even on the coldest night.

Once you see and feel the improvement you will want to do the other rooms.

As you write, the winters are cold and dry (mine are all year wet, wet and warm, wet and cold) unfortunately the weather outside is not the problem, it is the moisture you and your family create that's the problem.

Each of us breathes out 1.5 litres of water vapour every 24 hours. Children and animals being more active breath out more, older people a bit less.

Add to this the water from washing and it is no surprise that millions of homes in twelve States suffer from mould and damp.
Avoid at all costs.
 
  #11  
Old 09-02-09, 07:49 AM
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Perry 525, Thank for the reply. Regarding what I said about dry winters: since we used forced-air heat, the air inside is also very dry. In the past, we've kept a pot on low simmer all winter long so we don't get chapped lips while inside. During the winter, the bathroom doesn't steam up when taking a shower, etc.

But your point about moisture is well-taken. I just don't have the budget to rip out the walls of the first story and re-do them. (We do intend to do the second floor in the way you recommend.) We are going to add a vent to the lower bathroom.
 
  #12  
Old 09-09-09, 06:14 PM
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Here is a DIY spray foam insulation that I stumbled upon,
Tiger Foam | DIY Spray Foam Insulation Kits.

They have a low rise formula that may work well for you.
Not exactly cheap, but I am sure it will pay back fast enough.
 
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