Softboard to Insulate Attic Floor?

Aeneas

I want to improve insulation and put in flooring in an attic space over a living room extension to my home. I have looked at three options
1) put polyiso over the existing 6in fibreglass and install the flooring (chipboard or plywood) over this. I have been advised against this in another thread because of the risk of condensation.
2) put additional fibre glass between batons screwed to the existing joists and install the flooring on this. The problem with this is that the batons would need to be about 6in high and installing them would be cumbersome - like putting in a new set of joists.
3) install softboard on the existing joists over the fibreglass.
I am attracted to the third idea but before I go ahead could anyone advise me:
a) is softboard permeable to water vapour
b) how does its insulation properties (eg u-value) compare with other rigid board insulation like polyiso
c) does softboard come in different thicknesses and what thickness would be desirable to top up the existing 6in of fibreglass
d) is it suitable on its own for flooring or would I still need to cover it with chipboard or plywood?

Mellor

Aeneas wrote: »

3) install softboard on the existing joists over the fibreglass.
I am attracted to the third idea but before I go ahead could anyone advise me:
a) is softboard permeable to water vapour
b) how does its insulation properties (eg u-value) compare with other rigid board insulation like polyiso

Softboard is a decent insulator compared to ply or OSB, but it doesn't have similar resistance to polyiso, or fibreglass.
You mentioned not liking the fibreglass route because the it would have to go to 150mm. It you went softboard it would have to be be greater than this, I have seen various values, but i'd say at least 200mm, maybe 250mm.
Or and softboard would be one of the more permeable materials

Aeneas

Thanks Mellor. I see the problem with softboard. I suppose there is an issue too with weight. The joists in this attic are the normal attic ones and as far as I know not as strong as flooring joists. If I go down the baton, fibreglass, plywood floor route, would a layer of 6in batons over the existing joists be too heavy?

AckwelFoley

After 10 years in the Building supply business im not familiar with the term "softboard" what are you referring to mellor?

Is this it?

http://www.viking-house.ie/softboard

Mellor

Yeah thats one, its a fibre board, similar to MDF but far less dense

10-10-20

Can I ask as to what you have installed between the rafters, under the tiles/sarking/felt?
This may be an idea place to put insulation as you require storage areas underneath which should have some level of insulation ABOVE it also.
As Mellor previously pointed out to me in another thread, you can insulate both under and over the storage space and not have issues of condensation on the stored items.

If there is already insulation between the rafters, what about installing a softboard across the top of this again? I'm considering this as the Kingspan rigid boards which were fitted to my attic were poorly secured so are loose and allow airflow around them!

A final note, if your sarking isn't breathable, you'll need to maintain a 50mm (??) distance between the insulation and the sarking/felt to allow airflow from the soffit vents to the ridge vents.

Aeneas

The roof space is ventilated and I am advised that putting insulation in the rafters would be pointless because of the need to maintain an airflow in the attic space. I have 6in fibreglass between the joists. I use the attic for storage so the boxes need to be above the insulation. At the moment they rest on the joists. My original intention was to install Kingspan or Extratherm across the joists and put flooring on top of this. But this presents condensation problems because the polyiso is less permeable than the fibreglass - although I have seen on US forums that some people try to get over this problem by slashing or piercing the foil-backed polyiso boards to allow water vapour through. This has led me to the idea you mention of fixing softboard across the joists. But I gather from what Mellor says above that I would need quite a thick layer of softboard to get any appreciable insulation. The alternative is to install fibreglas between 6in batons; hence my question above about weight.

10-10-20

Have a look at the Kingspan Product & Application Details for the Thermapitch TP10.
http://www.insulation.kingspan.com/ireland/tp10.htm#
http://www.insulation.kingspan.com/ireland/pdf/tp10.pdf

Fig 2a & 2b may be of interest in terms of providing ventalation of the sarking board or sarking felt. Fig 5a deals with the method of sealing the storage space from the ventalated space.

Hope this helps.

bro'

I was looking at insulating my storage area and came across this

http://www.space-insulation.com/board.html

I think from memory you needed two layers of board over the fibreglass insulation between joists to meet requirements.

I never got around to doing it yet but wonder what comments there are on this setup

Aeneas

bro' wrote: »

I was looking at insulating my storage area and came across this

http://www.space-insulation.com/board.html

I think from memory you needed two layers of board over the fibreglass insulation between joists to meet requirements.

I never got around to doing it yet but wonder what comments there are on this setup

I had a look at this stuff in B&Q in Dublin recently and thought it might solve my problem. I have e-mailed Knauf Insulation about the potential condensation problems and it will be interesting to see what they say.

Aeneas

Aeneas wrote: »

I had a look at this stuff in B&Q in Dublin recently and thought it might solve my problem. I have e-mailed Knauf Insulation about the potential condensation problems and it will be interesting to see what they say.

The product is called Space Board and is an extruded polystyrene board manufactured by Knauf Insulation. Knauf tell me that condensation is not a problem because water vapour can escape in the gaps between the boards which are not foil backed and are not tongued and grooved. Apparently two boards installed over 100mm of fibreglass will meet building regulations by providing a u-value of 0.16 or R-value of 6.25. The manufacturers rate each board at a R-value of 2.4 which gives a u-value of 0.42. They also say that when installing them it is important to keep the polystyrene away from electrical cables. If the attic is being used for storage the Space Boards should be covered with chipboard. B&Q have them but they are not cheap at approx €30 per pack of 4 boards. They are not ideal but probably the easiest solution to my problem.

Mellor

A problem with alot of manufacturers of this stuff, is that they show you examples of how they work in best case situation. Spaceboard is stadard extruded polystyrene. Thermal conductivities for each board is not relevant, it should be taken reletive to a uniform thickness, a metre is standard.
Space board is 0.029 W/m2K
Polyiso/Phenolic is 0.022
Fibre glass is 0.035-0.04
Rockwool is 0.042-0.044
Softboards are 0.044-0.048 (from memory)

The thicknesses required will be reletive to this.

Aeneas

Mellor wrote: »

A problem with alot of manufacturers of this stuff, is that they show you examples of how they work in best case situation. Spaceboard is stadard extruded polystyrene. Thermal conductivities for each board is not relevant, it should be taken reletive to a uniform thickness, a metre is standard.
Space board is 0.029 W/m2K
Polyiso/Phenolic is 0.022
Fibre glass is 0.035-0.04
Rockwool is 0.042-0.044
Softboards are 0.044-0.048 (from memory)

The thicknesses required will be reletive to this.

I take your point about manufacturers' specs - they are after all trying to flog the stuff. And of course a lot must depend on the quality of installation. But am I right in believing that if they say a product has a particular R-value and meets a British Standard then this should be accurate for a best estimate?
I am not really knowledgeable about any of this but if I understand you correctly
a) the thermal conductivity measurement relates to the inherent effectiveness of a particular material at a constant thickness . This allows us to compare the relative insulation effectiveness of different materials. Therefore extruded polystyrene (at 0.029) is a better insulator than fibreglass (at 0.035) which in turn is better than softboard (at 0.044). As a consequence you need more softboard and fibreglass to achieve the same insulation effect as extruded polystyrene. But the question for the DIY novice like me is how much of any particular product do I need?
b) my understanding was that unlike thermal conductivity the R-value does take account of the thickness of the material in question. I notice for example that manufacturers quote different R-values for the same polystyrene product depending on its thickness. Knauf say that the R-value of 50mm spaceboard is 2.4 so I presumed that would be double for two boards, on the presumption, which may be wrong, that you can add R-values to determine the overall insulation effect of a number of materials, eg 100mm fibreglass + 50mm spaceboard + another 50mm spaceboard gives an overall R-value rating of 2.5+2.4+2.4 = 7.3. This equates to a u-value of 0.14 which is somewhat better than the Building Regulations.
My working assumption is that, even if this is not deadly accurate, it is probably the best way for the DIYer of estimating how much of a particular material, or combination of materials, might be needed to come near to the Building Standard when retrofitting his house. Otherwise I am just guessing and hoping it will be right. But maybe the working assumption and method are wrong and I need to go back to the drawing board?

Mellor

Aeneas wrote: »

But am I right in believing that if they say a product has a particular R-value and meets a British Standard then this should be accurate for a best estimate?

A paticular R-value means nothing, as any material can reach any R value, depending on thickness. Obviously for concrete or similar a huge thickness is needed.

a) the thermal conductivity measurement relates to the inherent effectiveness of a particular material at a constant thickness

b) my understanding was that unlike thermal conductivity the R-value does take account of the thickness of the material in question. I notice for example that manufacturers quote different R-values for the same polystyrene product depending on its thickness.

The two above are linked really. The Thermal conductivity pretty much what you said, it shows how good or bad materials are in relation to each other regardless thickness.
The R value is the derived from the above. It is [thickness] / [conductivity]. They can be added like you said. The problem with R-values is, they should be used to calculate total insulation, U-Values. There shouldn't be used to sell a product. most uk and irieland manufacturers quote conductivity, this is far better, R-values are often quoted in the USA.
The problem with quoting R-values is that is can be misleading
If they say a 50mm board has an R-value of 2.2, thats fine.
But saying they their product can achieve R-values of 2.2...this is not, as it doesn't mention thickness and as I said any material can reach this if thick enough.

As for the vapour issue, the jury is still out on that, I'm not sure is I agree with the state ment that the vapour can escape between the boards.

Aeneas

I think I'm getting the hang of this now. Thermal conductivity allows me to compare the relative insulation qualities of various materials and tells me which particular KIND of insulation material is more efficient. R-value tells me the insulation effectiveness for a GIVEN THICKNESS of material and how, by simply adding the values, it will combine with other materials whose R-value I know (in this respect I think it's important that manufacturers tell us the R-value for various thicknesses of any given material, as most of the good ones like Knauf, Isover and Kingspan appear to do). By converting the total of the R-values into u-value (ie by dividing the R-value total into 1) I can get the overall u-value of the combined insulation and see how it meets Building Standards (which are expressed in u-value). The calculation is probably not necessary for an unused attic as I can simply add more insulation if I feel that what I have is not giving me sufficient warmth. But for a used attic, like the one I want to floor and use for storage, or behind the upstairs walls in my dormer, where the installation is a once-off job, doing the sums has helped me judge the amount and type of insulation I might require. I understand that the vapour issue is still unclear but I think I will take the risk.

slave1

Hope ye all don't mind me expanding this thread with a similar topic.
I have a 35 years old semiD, I already have the floor part of the attic insulated but looking at insulating the roof part itself, at the minute just bare felt.
Is this a worthwhile investment of time/energy?
Is this something that can be tackled considered the assumed need for ventilation "up there"?

If it is worthwhile can anyone recommend a suitable product?

TIA

10-10-20

I'll refer you back to the two links to Kingspan I posted earlier on this topic, not because I'm lazy, but because I used them as a reference when I was doing work on my attic space.
I have storage space which I wanted to retain as such, but I also wanted to insulate UNDER the storage space.

I had Kingspan installed between the rafters and I then placed 150mm of glass-wool between the joists. According to the Kingspan documents you can insulate between the rafters without affecting the ventilation for the roof-space.

In terms of answering your question as to the effectiveness, I don't have or know how to calculate the figures, but I would guess that if your current insulation between the joists is less than 150mm of glass-wool, insulating the rafters will provide a worthwhile investment.
Just be warned, you will need two insulating products to complete the job, one for between the rafters and one to finish off under the rafters such as insulated plasterboard or one of those thermally reflective wraps.

slave1

tks for quick reply

Just be warned, you will need two insulating products to complete the job, one for between the rafters and one to finish off under the rafters such as insulated plasterboard or one of those thermally reflective wraps.

My attic is storage only so will not be bothering with any plasterboard finish, presume this is okay?

10-10-20

Yes, it is, but you may still need to seal off the between-rafter insulation as if it's rigid, it may not seal completely.

Aeneas

I looked at the Kingspan website but I'm not sure that insulating the rafters (ie the roof) is the right approach for me because
1. I think I can get better insulation values from insulating the joists ie the floor of the attic. If I insulate the rafters the heat from the house below will pass into the attic and I am in effect heating the attic space.
2. I really don't need a warm attic for the storage I have.
3. to operate effectively it looks to me as if the rafter installation would need very careful detailing. The conversion of a ventilated attic into a well insulated non-ventilated space is notoriously difficult, as witness the number of people who have cold rooms in their attic space.

Mellor

I think I'm getting the hang of this now. Thermal conductivity allows me to compare the relative insulation qualities of various materials and tells me which particular KIND of insulation material is more efficient. R-value tells me the insulation effectiveness for a GIVEN THICKNESS of material

This is a good way to put it, I have no problem with companys using R-values, as long as thermal conductivity is clearly listed too, and each R-value is is cleary listed alongside its reletive thickness,


As for insulation along the rafters. If the attic is to be used for storage only then the above is pointless. As it partially reduced effectiveness of joist insulation. And creates a more complicated eaves/ventilation detail.

Storage only - Insulate along joists
Habitable space - Along rafters, (joist insulation can be added to improve acoustics and/or zoned heating, but this is a minor improvement)

Aeneas

Knauf will only tell me on the phone that there is no problem with interstisal condensation using Space Board over fibreglass. Despite several e-mails they do not appear to be willing to confirm this in writing. So I have doubts about this method. Moreover it works out quite expensive. I have now gone back to the idea of batons and fibreglass and have a few questions on that which I will take up in the thread " Attic Flooring Question" where some of these issues are discussed.

air

Any recommendations on what to put between joists and plywood to try and reduce sound transmission to the floor below when the attic is being converted to habitable space?