Pulling the final trigger... to selfbuild or not to selfbuild..

[Mr Punter]

15 hours ago, Adsibob said:

Yes i think so, this site says: "insulation materials offering a high decrement ‘factor’ include cellulose fibre (7.3 hr), wood fibre insulation board (11.3 hr); whereas materials with a low decrement factor would include low-density mineral fibre (3.7 hr) and polyurethane/polystyrene." I appreciate polyisocyanurate is not exactly the same as polyurethane/polystyrene, but they are similar enough for present purposes.

 

This site gives technical specs of wood fibre and of PIR as follows:

 

Wood fibre

 

Thermal conductivity/ λ (lambda)  W / m . K = 0.038

Thermal resistance at 100mm K⋅m²/W = 2.5

Specific Heat Capacity J / (kg . K)= 2100

Density kg / m³ = 160

Thermal diffusivity m²/s  = n/a

Embodied energy MJ/kg = n/a

 

PIR

Thermal conductivity/ λ (lambda)  W / m . K = 0.023–0.026⁽¹⁸⁾

Thermal resistance at 100mm K⋅m2/W = 4.50

Specific Heat Capacity J / (kg . K) = n/a

Density kg / m3 =  30 – 40

Thermal diffusivity m²/s  = n/a

Embodied energy MJ/kg = 101 ⁽¹⁷⁾

Vapour permeable: No

 

That site also has a page which states the following:

 

Thermal Diffusivity

Thermal Diffusivity ties the above factors together into an equation that measures the ability of a material to conduct thermal energy relative to its ability to store thermal energy.  In effect it is a measure of thermal inertia or ‘buffering’.

The equation is:

Thermal diffusivity = thermal conductivity / specific heat capacity x density

Examples:

• Rigid polyurethane insulation has a thermal diffusivity of approximately 4.46 x 10-7 m2/s
• Timber fibre insulation has a thermal diffusivity of approximately 1.07 x 10-7 m2/s
• Copper has a thermal diffusivity of around 1.11 × 10−4 m2/s

In a material with high thermal diffusivity, heat moves rapidly through it because the substance conducts heat quickly relative to its volumetric heat capacity or 'thermal bulk'.

In the above three examples we can see that heat races through copper while it moves more rapidly through rigid polyurethane than it does through timber fibre board." (my emphasis)

 

Again, I appreciate polyisocyanurate is not exactly the same as polyurethane, but similar enough.

 

To do the proper comparison, one would need to do the decrement delay cals. We did them and found that PIR wasn't as good as wood fibre.

 

If you're still in doubt, see this site, which says:

 

"For a given U value, wood fibre has almost twice the decrement layer of mineral wool and at least 65 per cent more than PIR.  So essentially if you have two walls that have the same U value, it will take twice as long for the heat to get a wood fibre wall as it will through a wall using mineral wool.  How that translates to a building is that during the summer you end up with a building that essentially doesn’t overheat.  It maintains its internal temperature and stays very stable internally.

In volume for volume, flexible wood fibre stores about 12 times more heat than fibreglass and rigid wood fibre boards store about 12 times more heat than PIR, and around 15 times more heat than EPS. So again, on every metric the wood fibre is outperforming the synthetic materials.  This creates buildings that are very stable in the summer.  You’re reducing your overheating risk, so you’re making the building much safer for occupants.  Also that heat absorption lowers heat demand.  It can absorb heat during the day and slowly release it during the evening, so again it’s keeping the internal environment much more stable.  It’s ability to manage moisture also preserves timber, so wood fibre is able to buffer humidity in walls and even out humidity so that you don’t get accumulations in existing timber.  It prevents mould growth and it enables safe refurbishment of building."

 

Note that the insulation in my roof has a significant amount of wood fibre, as well as some PIR. My issues though are that I've not yet blocked out the solar gain coming in through glazing - on the to do list.

 

 

I don't think this really backs up your claim that PIR makes a building "sweaty in summer".

 

I appreciate that it does not have the decrement delay of some materials such  as woodfibre or cellulose, but plasterboard can have a similar buffering effect.

 

Regarding breathability, many modern buildups include a vapour resistant plastic layer.

 

Cold stores and freezer rooms often use PIR as thermal insulation.

[Adsibob]

1 hour ago, Mr Punter said:

don't think this really backs up your claim that PIR makes a building "sweaty in summer".

Really? Why do you say that? The conclusion I form when I read the links I’ve posted is that PIR is far inferior to wood fibre for keeping a building cool. And my real life experience of the two materials, albeit anecdotal, matches that: Our loft is warmer than ideal for use as a study in the summer, but compared to my parents’ place, where the loft is only insulated with PIR and no wood fibre, it’s completely different. Ignoring heat wave type temperatures, are when the maximum temp outside is 28c, I only start to get a little uncomfortable at about 4pm or 5pm, depending on the day, whereas in my parents’ loft it tends to be uncomfortable all the time, even outside of summer. Both houses are 1930s semis with the same orientation, only 2 miles apart, so pretty good comparables, although my loft was done this year, whereas theirs was done in 2016. Not sure if building regs changed in that time. My skylight is also quite a bit bigger than theirs, which would expose mine to more solar gain. I’m hoping that once I install external roller blinds on my large Velux, and duette blinds on the west facing window, it will be much cooler. 

[Russdl]

On 02/08/2022 at 23:11, puntloos said:

I always thought passivhaus was assumed to be airtight all year round?

Anything but (in our case at least).
 

Right now nearly every window is tilted open and will stay that way until morning whilst the house takes a big long breath of cool air.

 

In the morning when the outside temperature gets to that of the inside temperature then everything will be closed up and the house will keep its cool until tomorrow night when we will rinse and repeat.
 

We also turn the MVHR down to its lowest setting when it’s properly hot so that it’s not breathing in more hot air than absolutely necessary. It’s worked a treat - no overheating at all. 

[puntloos]

24 minutes ago, Russdl said:

Anything but (in our case at least).
 

Right now nearly every window is tilted open and will stay that way until morning whilst the house takes a big long breath of cool air.

 

In the morning when the outside temperature gets to that of the inside temperature then everything will be closed up and the house will keep its cool until tomorrow night when we will rinse and repeat.
 

We also turn the MVHR down to its lowest setting when it’s properly hot so that it’s not breathing in more hot air than absolutely necessary. It’s worked a treat - no overheating at all. 

 

Yeah, makes sense. I am planning to do 'all the things' - I do have active cooling designed, but also a house design around a central hallway(chimney) with a skylight that can vent, and even in theory I could reverse the UFH ashp to cool a bit in an emergency.. 

 

We'll see.. overheating is the #1 problem with passivhaus I've heard on here.. so it feels worth it not to assume it'll be OK...

[Adsibob]

2 hours ago, Russdl said:

 

We also turn the MVHR down to its lowest setting when it’s properly hot so that it’s not breathing in more hot air than absolutely necessary. It’s worked a treat - no overheating at all. 

That seems so obvious, I can’t believe I didn’t think to do it!

[Mr Punter]

On 13/08/2022 at 12:31, Adsibob said:

Really? Why do you say that? The conclusion I form when I read the links I’ve posted is that PIR is far inferior to wood fibre for keeping a building cool. And my real life experience of the two materials, albeit anecdotal, matches that: Our loft is warmer than ideal for use as a study in the summer, but compared to my parents’ place, where the loft is only insulated with PIR and no wood fibre, it’s completely different. Ignoring heat wave type temperatures, are when the maximum temp outside is 28c, I only start to get a little uncomfortable at about 4pm or 5pm, depending on the day, whereas in my parents’ loft it tends to be uncomfortable all the time, even outside of summer. Both houses are 1930s semis with the same orientation, only 2 miles apart, so pretty good comparables, although my loft was done this year, whereas theirs was done in 2016. Not sure if building regs changed in that time. My skylight is also quite a bit bigger than theirs, which would expose mine to more solar gain. I’m hoping that once I install external roller blinds on my large Velux, and duette blinds on the west facing window, it will be much cooler. 

 

The links you posted seemed to be from vested interests so I take them with a pinch.

 

I have a WC on the top (fourth) floor of my house.  It has PIR in the roof and walls but no window.  South facing wall.  The door connecting to the landing is normally closed.  In spite of it being baking elsewhere in the house, the top floor WC has remained pretty stable.  Anywhere with a window (which are triple glazed), even when the sun is not on them, gets considerable heat gain.

 

I also have a similar space but below a sloping roof with PIR and it is used to house the boiler.  It is always fairly warm but not noticably more so in the hot weather.

 

I really don't think that PIR is the cause of your troubles.  More likely solar gain.

[Adsibob]

31 minutes ago, Mr Punter said:

really don't think that PIR is the cause of your troubles.  More likely solar gain.

Yes, I agree with you. Particularly as I have very little PIR as part of my roof makeup.