Dormer Insulation Advice

tophurler

I would be grateful for any knowledge shared on what route would prove the biggest improvement for us. House is an early 90s dormer, wool insulation from wall plate to ridge (but obviously not stopping the draught coming in first floor level leaving the house roasting in summer and cold in winter).

A few opinions have been provided by various insulation companies and would appreciate anyone's thoughts on them:

1) Mineral wool insulation from wall plate, along crawl space floor and up the knee walls to the slope. Increase insulation in the ceiling. (Space behind sloped ceilings extremely tight to roof, preference is to avoid removing those if possible). Cavity wall pumped with bonded bead.

2) Spray foam from wall plate, along crawl space floor and up the knee walls to the slope. Increase insulation in the ceiling with mineral wool. Nothing done with sloped section. Cavity wall pumped with bonded bead.

3) Spray foam from wall plate, up the rafters to sloped area. Removal of the current insulation in the slope and spray foam onto felt in that area (assured the icynene foam can be sprayed to felt). Spray continued in upper ceiling to the ridge. Do nothing to the cavity walls for the time being to assess the difference.

Option 1 and 2 are roughly equal in cost, with 3 being slightly higher.

I would be interested to hear the thoughts of anyone with knowledge of retrofitting insulation, particularly in dormers.

Thanks!

Calahonda52

You have the answer in the first line: not stopping the draught coming in first floor level

This is an uncontrolled ventilation issue, not an insulation issue, yet.

the house roasting in summer is from the type of insulation chosen: google decrement delay

What insulation is in the ceiling behind the knee wall and on the knee wall?

Don't proceed with any of these ideas till we get the uncontrolled ventilation issue sorted.

Are the soffits ventilated?

tophurler

Thanks Calahonda,

So currently there is zero insulation on the floor of the crawl space, or up the knee walls.

There are soffit vents, but they appear to have been someway blocked up with timber.

I was putting the heat upstairs during summer down to the warm air getting in up there?

Calahonda52

Thanks, is the sheets wool faced with any sort of a membrane?
is the floor off the crawl space board out?

tophurler

I'm not sure about the membrane but I think not.

The floor of the crawl space is not boarded, the joists are exposed

Calahonda52

Post some pictures please, including where the joists disappear under the knee wall and where the rafters meet the wall plate

tophurler

Thanks Cala, some images uploaded, appreciate it

1 and 3 are the wall plate photos.
2 is the joists going under the knee walls.

Calahonda52

Really excellent pictures, really good when someone actually posts what was asked. Thank you.

First up, that is fibre glass, (FG), unless it came from Dolly, the GM'd sheep.

Its a very neat looking job but its not doing a lot.
.
Next question: what sort of felt is at the back of the slates?
The second question is, is there an air gap between the FG and the felt?

Before we get into any suggestions, the power cables in shot 2 will need to be sleeved with an air gap before any insulation is added.
We can get back to that later.

While there is some evidence of bridging between the ceiling joists, however step one is to stop the wind passing through under the floor.
Its a tedious job but it involves cutting pieces of ply the exact size of the openings, screwed to the side of the sole plate and sealing them all around where the ply touches the ceiling joist and the plaster board.

The cables can be accommodated using slots on the ply, and airtight tape

This will stop air passing through under the floor and calling both the room below and the floor above.

More later, day job calls

tophurler

My baaaaad on the FG/Sheep mix up! Trying to educate myself as I go along!

As regards the felt I will have to figure that one out tomorrow for you.

I would highly doubt that theres any air gap between the FG and felt, in particular the area behind the sloped ceiling is so tight.

The suggestion for using ply, what would you use to seal around it's edges? Silicone? Foam?

Calahonda52

tophurler wrote: »

My baaaaad on the FG/Sheep mix up! Trying to educate myself as I go along!

As regards the felt I will have to figure that one out tomorrow for you.

I would highly doubt that theres any air gap between the FG and felt, in particular the area behind the sloped ceiling is so tight.

The suggestion for using ply, what would you use to seal around it's edges? Silicone? Foam?

https://www.ecologicalbuildingsystems.com/product/orcon-f
is the proper stuff, will go a long way if the ply is cut neatly and not sloppy and then rely on the stuff to fill massive gaps

Lumen

Calahonda52 wrote: »

https://www.ecologicalbuildingsystems.com/product/orcon-f
is the proper stuff, will go along way if the ply is cut neatly and not sloppy and then rely on the stuff to fill massive gaps

I'm not sure that's the right product for this application. It's called sealant but isn't designed for gap filling, but for flexible adhesion, e.g. you run a bead along a block wall floor junction to bond membrane to it.

That said I have used it to fill small butt joint cracks where I didn't want to use tape for aesthetic reasons, but it's not ideal.

To bridge gaps between butted timber I'd use airtight tape like Tescon Vana.

tophurler

I hope to get some photos up of the felt this evening

Calahonda52

Lumen wrote: »

I'm not sure that's the right product for this application. It's called sealant but isn't designed for gap filling, but for flexible adhesion, e.g. you run a bead along a block wall floor junction to bond membrane to it.

That said I have used it to fill small butt joint cracks where I didn't want to use tape for aesthetic reasons, but it's not ideal.

To bridge gaps between butted timber I'd use airtight tape like Tescon Vana.

Cant disagree, if the timber is cut a good tight fit then its quicker than the tape which can be tough to use.
I usually put a bevel on the edges to allow for a good bead.

tophurler

Cala, my apologies for the delay but I have gotten a picture of the felt under the insulation. Also, it doesn't seem like there is any air gap between the current insulation and the felt.

Calahonda52

So, IMO, you have a problem mere is so far as the felt is not breathable and the insulation touches it so risk of the rafters wetting from condensation.
You should look at this
https://www.nsai.ie/about/news/publication-of-sr-542014-code-of-practice

tophurler

So I read through the document you sent, and from what I can see in the roof timbers here there isn't any indicator of rot/water/mould on the timbers.

What approach do you think I should take once the gaps beneath the sole plate is blocked?

Calahonda52

One of the reasons that there is no damp is because the insulation is so ineffective that the air passes through it.
The 2" gap in the document i sent you, combined with properly fitted ventilation baffles at the eaves, and then a suitable barrier on the inside face of the FG between the rafters.
However that does not solve the problem where the sloped ceiling lines the roof, which is inaccessible from the inside