Spray insulation for timber framed new build?

[davejura]

Hi folks. With the cost of insulation seemingly rising monthly, I'm just wondering if anyone has used or looked into this option? At the moment, our kit is specified at 100mm with another 25mm under the plasterboard. We also have attic trusses with 140mm specified there. Any thoughts / advice appreciated!

[ProDave]

100mm is woefully inadequate.  I have not heard of a 100mm tinber frame for nearly 20 years.  The standard has been 140mm for a long time, with 200mm now a better target to aim for.

 

Before you proceed any further what are the predicted U values of the walls and roof of your proposed build.

[the_r_sole]

where are you building @davejura? You really need to work through a SAP calc for the build to figure out what you need where.

I've done a couple of sprayfoam kits (client choice, not mine!) but they do give good air tightness, the uvalues I've seen aren't actually much better than your rigid insulation boards and the costs aren't hugely different, no matter what the reps tell you

[davejura]

I have all the figures, just need to look them up when I get a chance after home schooling! I was planning on upping the insulation anyway, to at least 120mm plus the 25mm. Going to be doing a lot of the work myself, so cost is an issue. It is scary how much prices are rising! It is looking like deferring the build by a year to save more money is going to get wiped out!

 

[Russell griffiths]

Spray foam installed is very expensive, with other types of insulation you can install yourself so save on the labour. 

Look at a deeper frame so you can use more of a cheaper form of insulation. 

Start building as soon as you can you will never save enough to match rising costs. 

[Conor]

Roof and floor insulation are two ways to save lots of cash for a job that can be done at relative leisure and doesn't need much skill or tools. You'b be wanting 100mm PIR between joists and another 50mm under them, as a minimum. Aim for a U value of 0.15-0.1.

 

Spray foam is an attractive option, but pricey, and can be done very badly and inconsistently.

 

[Iceverge]

Frametherm 35 or similar is about the best bang for your buck with insulation. 

 

If I was to DIY a timber frame I'd look at a 225mm frame with a 50mm service cavity filled with blown cellulose and then mineral wool respectively. 

 

I would swap out the cellulose for mineral wool if cost was really an issue and your own labour is "free". There after reduce the thickness of the outer stud . Cutting and fitting PIR in timberframe is a PITA and given the shrinkage of timber and PIR overtime its like to be loose after a few years. 

Edited March 9, 2021 by Iceverge

[Gone West]

5 hours ago, davejura said:

Hi folks. With the cost of insulation seemingly rising monthly, I'm just wondering if anyone has used or looked into this option? At the moment, our kit is specified at 100mm with another 25mm under the plasterboard. We also have attic trusses with 140mm specified there. Any thoughts / advice appreciated!

I have a 350mm I-beam timber frame with Icynene spray foam. It's very good for heat insulation and airtightness but it's not cheap and there's a lot of tidying up required afterwards. 130m2 floor area, two storey, and it was just under £10,000 for walls and roof about five years ago. I can look up the details if you're interested.

[davejura]

I have all the carcassing for the timber frame which I got hold of before the January price rise, and it's 145mm. With the 100mm + 25mm insulation, the walls are showing a U value of 0.21, and the roof 0.15. Was planning on beefing the walls up anyway to at least 120, if not 140mm, which should help matters.

I'll phone a couple of the Icynene installers and see what they say.

 

[davejura]

So their prices haven't moved since I first talked to them at Self Build Scotland show a couple of years ago. Basically £20 per square M for 100mm plus £1 for every extra 10mm. 

[Gone West]

16 hours ago, davejura said:

I have all the carcassing for the timber frame which I got hold of before the January price rise, and it's 145mm. With the 100mm + 25mm insulation, the walls are showing a U value of 0.21, and the roof 0.15.

I'm sorry, I'm obviously missing something but if the studs are 145mm deep and with external racking why not 145mm of insulation?

[davejura]

Just going by what the architect specified to reach the required U-value. Will almost certainly go to 120mm or possibly 140mm if budget allows.

[ProDave]

8 minutes ago, davejura said:

Just going by what the architect specified to reach the required U-value. Will almost certainly go to 120mm or possibly 140mm if budget allows.

Think in terms of achieving the best U value you can afford, rather than just the bare minimum.

[davejura]

Absolutely Dave. I'll report back comparisons on here wen I get a quote from the spray company. Not looking like much in it at the moment.

 

[Iceverge]

Have a price up of the below buildup. If you're DIYing it I can't see a cheaper way. 

 

Remember increasing your insulation much beyond building regs isn't worth much unless you're taking care of thermal bridging and airtightness. 

[Iceverge]

alternatively  put your continuous layer of PIR on the outside like the americans do. 

 

it leaves you a nice large stud to run wires and pipes and cuts thermal bridging at all the openings and studs.

 

 

[the_r_sole]

is Glasswool 35 the same as frametherm 35? I had real difficulty getting uvalues worse than about 0.17/0.18 on the walls through a sap calc recently so it really comes down to what else the op has to pass that

[the_r_sole]

1 minute ago, Iceverge said:

alternatively  put your continuous layer of PIR on the outside like the americans do. 

 

it leaves you a nice large stud to run wires and pipes and cuts thermal bridging at all the openings and studs.

 

 

 

If you go this route, make sure you've done a detailed condensation risk analysis - there's something I always worry about by insulating between and then outside studs, you've got the structure surrounded in insulation and not giving moisture anywhere to go, it's just the same as a hybrid roof construction where you risk condensation forming on the critical bits of structure...

[Iceverge]

7 minutes ago, the_r_sole said:

is Glasswool 35 the same as frametherm 35? I had real difficulty getting uvalues worse than about 0.17/0.18 on the walls through a sap calc recently so it really comes down to what else the op has to pass that

Yup,

 

I just picked something in that list that had a thermal conductivity of 0.035W/mK. 

 

For me SAP is a useful guide but unfortunately can mean little to the comfort of a house once built. I’d work on getting good enough  U values and excellent detailing. 

[the_r_sole]

1 minute ago, Iceverge said:

Yup,

 

I just picked something in that list that had a thermal conductivity of 0.035W/mK. 

 

For me SAP is a useful guide but unfortunately can mean little to the comfort of a house once built. I’d work on getting good enough  U values and excellent detailing. 

 

SAP is a pretty blunt tool but you need to pass it before you can start building in Scotland, so it becomes a critical part of the process

[Iceverge]

2 minutes ago, the_r_sole said:

 

If you go this route, make sure you've done a detailed condensation risk analysis - there's something I always worry about by insulating between and then outside studs, you've got the structure surrounded in insulation and not giving moisture anywhere to go, it's just the same as a hybrid roof construction where you risk condensation forming on the critical bits of structure...

 

I’ve pondered this over and over and wondered why we don’t do it at this side of the atlantic. I can’t imagine that the physics change at 30deg west, although they would claim everything is bigger there!

 

As long as you’re not trapping moisture in the wall and you’re controlling bulk water outside and humidity inside i doubt there’d ever be a problem.

[Iceverge]

1 minute ago, the_r_sole said:

 

SAP is a pretty blunt tool but you need to pass it before you can start building in Scotland, so it becomes a critical part of the process

In my experience of DEAP (based on SAP) here in ireland so long as you pass the backstop values and add enough PV pretty much anything would get through.

 

 

[the_r_sole]

1 minute ago, Iceverge said:

In my experience of DEAP (based on SAP) here in ireland so long as you pass the backstop values and add enough PV pretty much anything would get through.

 

 

 

yeah, but that's a bit like saying, you don't need any PV as long as you bang in loads of insulation and take out all the cold bridging! 

Everything has a cost and a practical consideration to go along with it so it's all about balancing what can be done vs what's a good way to solve the issue

[SimonD]

 

6 hours ago, Iceverge said:

alternatively  put your continuous layer of PIR on the outside like the americans do. 

 

it leaves you a nice large stud to run wires and pipes and cuts thermal bridging at all the openings and studs.

 

 

Would you not want a vcl to the inside of those studs for this buildup?

 

6 hours ago, the_r_sole said:

 

there's something I always worry about by insulating between and then outside studs, you've got the structure surrounded in insulation and not giving moisture anywhere to go

 

That's one area breathable wall systems have the advantage although certainly not the budget option.

 

 

 

[Iceverge]

7 hours ago, the_r_sole said:

 

yeah, but that's a bit like saying, you don't need any PV as long as you bang in loads of insulation and take out all the cold bridging! 

Everything has a cost and a practical consideration to go along with it so it's all about balancing what can be done vs what's a good way to solve the issue

I agree.